Understanding the Role of Fisheries and Aquaculture in Carbon Sequestration June 2024 © 2024 International Bank for Reconstruction and Development / The World Bank 1818 H Street NW Washington, DC 20433 Telephone: 202-473-1000 Internet: www.worldbank.org This work is a product of the staff of the World Bank with external contributions. The findings, interpretations, and conclusions expressed in this work do not necessarily reflect the views of The World Bank, its Board of Executive Directors, or the governments they represent. The World Bank does not guarantee the accuracy, completeness, or currency of the data included in this work and does not assume responsibility for any errors, omissions, or discrepancies in the information, or liability with respect to the use of or failure to use the information, methods, processes, or conclusions set forth. The boundaries, colors, denominations, and other information shown on any map in this work do not imply any judgment on the part of The World Bank concerning the legal status of any territory or the endorsement or acceptance of such boundaries. Nothing herein shall constitute or be construed or considered to be a limitation upon or waiver of the privileges and immunities of The World Bank, all of which are specifically reserved. Rights and Permissions The material in this work is subject to copyright. Because The World Bank encourages dissemination of its knowledge, this work may be reproduced, in whole or in part, for noncommercail purposes as long as full attribution to this work is given. Any queries on rights and licences, including subsidary rights, should be addressed to World Bank Publications.,The World Bank, 1818 H Street NW, Washington, DC 20433, USA; fax 202-522-2625; email: pubrights@worldbank.org. Attribution—Please cite the work as follows: World Bank. 2024. Understanding the Role of Fisheries and Aquaculture in Carbon Sequestration. Washington, DC: World Bank. Cover photo: © Sean Stiengenger / Shutterstock.com Used with permission of Shutterstock. Further permission required for reuse. Acknowledgements This work was funded by the PROBLUE Trust Fund, and Miquel Ortega, Villy Christensen, Jeroen Steenbeek and Marta Coll at Ecopath International Initiative (EII), Barcelona, Spain prepared the report. White Paper Understanding the Role of Fisheries and Aquaculture in Carbon Sequestration Understanding the Role of the Fisheries and Aquaculture in Carbon Sequestration Contents EXECUTIVE SUMMARY�������������������������������������������������������������������������������������������������������������������������������������������������������������� 3 GLOSSARY���������������������������������������������������������������������������������������������������������������������������������������������������������������������������������� 4 1. METHODS & SCOPE������������������������������������������������������������������������������������������������������������������������������������������������������������� 5 2. INTRODUCTION TO THE BIOLOGIAL PUMP����������������������������������������������������������������������������������������������������������������������� 6 3. MODELING THE FISH CARBON PUMP�������������������������������������������������������������������������������������������������������������������������������10 3.1 Metabolic 'production' processes�������������������������������������������������������������������������������������������������������������������������������������10 3.3 Definition of key stocks and flows���������������������������������������������������������������������������������������������������������������������������������� 11 3.4 Metabolic 'production'��������������������������������������������������������������������������������������������������������������������������������������������������� 12 3. 4. 1 Living biomass����������������������������������������������������������������������������������������������������������������������������������������������������16 3. 4. 2 Carcass Production��������������������������������������������������������������������������������������������������������������������������������������������� 17 3.5 Biochemical transformations������������������������������������������������������������������������������������������������������������������������������������������18 3.6 Phyical Transports������������������������������������������������������������������������������������������������������������������������������������������������������� 21 3. 6. 1. Fish active vertical transport���������������������������������������������������������������������������������������������������������������������������� 21 3. 6. 2 Water mass flows����������������������������������������������������������������������������������������������������������������������������������������������� 21 3.7 Mobile demersal fishing gears - carbon sediment interactions�������������������������������������������������������������������������������������������23 3.8 Marine benthic producer carbon sequestraton��������������������������������������������������������������������������������������������������������������� 26 4. ICES WORKSHOP ON ASSESSING THE IMPACT OF FISHING ON OCEANIC CARBON���������������������������������������������������������������29 4.1 Carbon Pump����������������������������������������������������������������������������������������������������������������������������������������������������������������� 29 4.2 Fishing fleet emissions����������������������������������������������������������������������������������������������������������������������������������������������������32 4.3 Fishing impacts���������������������������������������������������������������������������������������������������������������������������������������������������������������32 4.4 Further work����������������������������������������������������������������������������������������������������������������������������������������������������������������32 5. RELEVANT POLICY INITIATIVES����������������������������������������������������������������������������������������������������������������������������������������33 5.1 United Kingdom��������������������������������������������������������������������������������������������������������������������������������������������������������������33 5.2 Europena Union�������������������������������������������������������������������������������������������������������������������������������������������������������������33 5.3 International Council for the Exploration of the Seas�����������������������������������������������������������������������������������������������������33 6. CONCLUSIONS���������������������������������������������������������������������������������������������������������������������������������������������������������������������34 7. RECOMMENDATIONS AND NEXT STEPS��������������������������������������������������������������������������������������������������������������������������� 36 7.1 Digital Resarch Platform����������������������������������������������������������������������������������������������������������������������������������������������� 36 7.. 1. 1 Quantify fish and fisheries carbon flows���������������������������������������������������������������������������������������������������������� 36 7. 1. 2 Macroalgal dynamics�������������������������������������������������������������������������������������������������������������������������������������������37 7. 1. 3 Interacting with the environment����������������������������������������������������������������������������������������������������������������������37 7. 1. 4 Development timeline and wider impact����������������������������������������������������������������������������������������������������������37 7.2 Standardizing the dialogue����������������������������������������������������������������������������������������������������������������������������������������������37 REFERENCES��������������������������������������������������������������������������������������������������������������������������������������������������������������������������� 38 1 Understanding the Role of the Fisheries and Aquaculture in Carbon Sequestration Figures Figure 1: Main processes regulating the marine carbon cycling (reproduced from Lutz and Martin, 2014)........................... 6 Figure 2: Main pathways and processes of the biological pump with a specific focus on fish. ...............................................8 Figure 3: Models needed to quantify the biological pump carbon sequestration of marine ecosystems.............................. 9 Figure 4: Direct and indirect impacts of fishing on carbon sinkdead zones ............................................................................11 Figure 5 - Fish and fisheries carbon flux scheme...................................................................................................................... 12 Figure 6 - Conceptual view of the oceanic CaCO3 cycle. ........................................................................................................... 15 Figure 7 - Simplified scheme of the global carbon cycle. N...................................................................................................... 18 Figure 8: Top - Main fluxes and carbon budget; knowledge gaps and research priorities.....................................................24 Figure 9: Conceptual diagram of the pathways for export and sequestration of macroalgal carbon. .................................26 Figure 10: Macroalgal carbon sequestration scheme ............................................................................................................... 27 Tables Table 1: Sequestration and sequestration time of the biological pump pathways................................................................... 7 Table 2: Production table ............................................................................................................................................................13 Table 3: Remineralization rates.................................................................................................................................................. 19 Table 4: Sinking rates reference values for fish and zooplankton fecal pellets and carcasses............................................... 19 Table 5: Biochemical table.......................................................................................................................................................... 22 Table 6: Physical transport table............................................................................................................................................... 22 Table 7: Key resuspension variables........................................................................................................................................... 23 Table 8: Link between seabed sediment organic carbon and mobile demersal fishing......................................................... 25 Table 9 - Marine macroalgae reference values. (Adapted from Krause-Jensen and Duarte, 2016)........................................ 27 Table 10: Summary of key variables of biological carbon pump modelling, and why they matter........................................30 Table 11: Risk assessment and confidence assessment of environmental factors governing seabed carbon remineralization and storage. ....................................................................................................................................31 2 Understanding the Role of the Fisheries and Aquaculture in Carbon Sequestration EXECUTIVE SUMMARY The ocean plays a crucial role in regulating the climate by absorbing excessive heat and carbon dioxide emissions. In addition to solubility and the physical pump, the biological pump is increasingly acknowledged as a vital component of ocean ecosystem services. Marine life contributes to the absorption of atmospheric carbon and sequesters it in the marine environment through various processes, with sequestration occurring for time periods ranging from daily processes to thousands of years. In this context, there is a growing interest among both the scientific community and a wide network of stakeholders to enhance understanding and modeling capabilities concerning fish and fisheries’ carbon sequestration processes. Over the past few decades, significant progress has been made in comprehending and characterizing these processes for the open ocean, resulting in a broad and expanding literature with substantial information on many key parameters. Nonetheless, there are still significant knowledge gaps, especially for coastal and shelf areas, and the scientific community is actively striving to establish a comprehensive consensus on essential elements required for accurately evaluating the effects of fishing activities and policies on marine carbon sequestration. This ASA sought to address the development challenge of comprehensively quantifying the carbon sequestration attributable to improved fisheries management and aquaculture. It focused on delivering a white paper that describes existing and potential GHG accounting methods to inform climate-mitigation co-benefits assessments of improved fisheries management and aquaculture production investments for a more comprehensive estimate. The outcomes of this work culminated in a white paper which relies on a detailed literature and stakeholder consultations to assess the possibility of enhancing existing models to overcome the identified knowledge limitations in order to further our knowledge on how to better capture/estimate climate-mitigation co-benefits from improved fisheries management and seaweed production. This white paper provides the current state of scientific understanding of the field, and suggests next steps forward in terms of how modeling can contribute to filling this gap. The report is structured to support the future development or enhancement of models, featuring a map of the key stocks and flows, an analysis of how fish transform and ‘produce’ carbon, an exploration of the carbon biochemical transformations in the marine environment, and an examination of physical transport within the marine ecosystem. Additionally, it includes dedicated sections on the implications of sediment interactions with mobile demersal fishing gears for carbon sequestration, the significance of marine macroalgae-kelp ecosystems in the biological carbon pump, and noteworthy policy initiatives related to marine carbon sequestration. The research conducted here identified general gaps, such as the need to better characterize how different fish contribute to and consume the various carbon flows identified in the marine realm, or the need to better characterize the interrelation between trawling activities and sedimentary ecosystems. Specific difficulties have been identified arising from the fact that most fishing activities take place on the marine shelf, where key physicochemical and biological processes are often more complex than in the open ocean where carbon flows are better understood, leading to a higher level of complexity in evaluating carbon sequestration times in coastal areas. This white paper concludes with a summary how existing models could be enhanced to overcome some of the identified knowledge limitations. To accomplish this, the development of coupled food-web models with carbon sequestration models is proposed, aiming to attain a more realistic understanding of the implications of fishing activities for carbon sequestration. In the final section, this vision is elaborated on with recommendations for next steps. 3 Understanding the Role of the Fisheries and Aquaculture in Carbon Sequestration GLOSSARY Abbreviation Definition ACMC Amorphous calcium–magnesium carbonate AR Caudal fin aspect ratio CDR Carbon dioxide removal CI Conservation International DIC Dissolved inorganic carbon DVM Diurnal vertical migration DOC Dissolved organic carbon EBFM Ecosystem-based fisheries management EBM Ecosystem-based management EwE Ecopath with Ecosim food web modelling approach HMC High magnesium calcite IPCC Intergovernmental Panel on Climate Change LDOC Labile DOC LMC Low magnesium calcite MHC Monohydrocalcite NPZD Nutrient, Phytoplankton, Zooplankton and Detritus model OC Organic carbon PgC Petagram of carbon (10^15 grams = 1 gigaton) POC Particulate organic carbon RIL Relative intestinal length to body standard length TgC Teragram of carbon (10^12 grams) 4 Understanding the Role of the Fisheries and Aquaculture in Carbon Sequestration Photo credit: The World Bank 1. METHODS & SCOPE The information provided in this report has been obtained from four different sources: • A literature review was conducted using a “Web of Science” database search in March 2023 with two sets of search terms: a) ‘carbon sequestration’ and ‘biological pump’), and b) ‘carbon sequestration’ & ‘fish’. A total of 909 references were identified and after reviewing the title and abstracts, 50 papers were selected for a complete review. • The literature used for the presentations of the online “Fish, Fisheries, and Carbon international workshop”, held last 6th, 8th and 9th of March 2023, organized by the “Ocean Carbon & Biogeochemistry program” and the discussions that took place at the workshop. • Review of the literature used for the presentations of the “Workshop on Assessing the Impact of Fishing on Oceanic Carbon”, held 25-27 April, 2024, at ICES, Copenhagen. Supplemented by results from discussions at the workshop and the draft workshop report. • A roll-on technique was used to review additional scientific papers based on the references of the previous mentioned papers. A total of 90 additional papers were reviewed. This report reviews the current scientific understanding of the role of fish in marine carbon sequestration and how fishing affects sequestration processes. The review excludes carbon sequestration mechanisms associated with coastal vegetation systems such as mangrove forests, tidal marshes and benthic primary producers, which have already been addressed broadly in the literature (Macreadie et al., 2021; Rosentreter et al., 2023; Zhong et al., 2023). Additionally, the review does not include the carbon sequestration processes associated with aquaculture and anthropogenic ocean- based carbon dioxide removal (CDR) projects (Lebling et al., 2022). 5 Understanding the Role of the Fisheries and Aquaculture in Carbon Sequestration 2. INTRODUCTION TO THE BIOLOGIAL PUMP The ocean plays a critical role in regulating the climate by absorbing excess heat and carbon dioxide emissions. Over the last 60 years, the ocean has stored approximately 23 ± 5% of anthropogenic carbon emissions. Furthermore, since the 1970s, the ocean has absorbed over 90% of the Earth’s excess heat, accumulated in the Earth’s system (IPCC, 2021). The amount of carbon dioxide emitted into the atmosphere that remains dissolved in seawater is influenced by three interlinked processes: the solubility pump, which is dependent on physicochemical conditions such as sea water temperature, salinity, and total alkalinity; the physical pump, which involves water fluxes and advective-diffusive transport; and the biological pump (Figure 1). Figure 1: Main processes regulating the marine carbon cycling (reproduced from Lutz and Martin, 2014) Traditionally, ocean-related climate change research has focused on the solubility pump and physical pump. However, in recent decades, there has been a growing recognition of the importance of the biological pump and its interconnections with the solubility and physical pumps. It is now understood that the biological pump can play a significant role in the ocean sink of anthropogenic carbon, particularly in certain regional contexts. As a result, attention on the biological pump has increased and is likely to continue to be a key area of research (IPCC, 2021; Wilson et al., 2022). The biological pump refers to the various processes that transfer organic matter produced by phytoplankton net primary production from the surface ocean to depth, where it can be sequestered for months to millennia. While research is ongoing to quantify the different pathways that contribute to carbon sequestration, their relative importance is not fully understood (Nowicki et al., 2022). 6 Understanding the Role of the Fisheries and Aquaculture in Carbon Sequestration The biological pump refers to the various processes that transfer organic matter produced by phytoplankton net primary production from the surface ocean to depth, where it can be sequestered for months to millennia. While research is ongoing to quantify the different pathways that contribute to carbon sequestration, their relative importance is not fully understood (Nowicki et al., 2022). The three main pathways of the biological pump are the ‘gravitational pump’, the ‘vertical migration pump’ and the ‘mixing pump’ (Figure 1). The ‘gravitational pump’ involves the passive sinking of Particulate Organic Carbon (POC) in the form of aggregates, carcasses, and fecal pellets. The ‘vertical migrant pump’ refers to the active transport of carbon by vertically migrating zooplankton, fish, and other marine animals. Last, the ‘mixing pump’ includes the physical transport of both suspended POC and Dissolved Organic Carbon (DOC) by marine water fluxes. Carbon dioxide is fixed into organic carbon (OC) by phytoplankton in the euphotic zone, where it is partially incorporated into the marine food web. Once consumed, it can be involved in excretion, exudation, respiration, consumption, aggregation, solubilization and grazing processes where bacteria, zooplankton, fish and other marine animals are involved (Figure 2). While most of the organic particles are recycled in surface waters, a small fraction sinks in a proportion that is called ‘export rate’ from the euphotic zone, the mixed surface layer, or across an arbitrary horizon often set around 100 meters. The biological pump exports approximately 10 (PgC yr−1 (Nowicki et al., 2022) to the deeper ocean, with the estimated fish-based contribution being around 1.5 ± 1.2 (PgC yr−1 (Saba et al., 2021). However, it is important to note that there is a high uncertainty in estimation of fish export flux, aggravated by fish having the highest biomasses in coastal and shelf areas where knowledge about carbon sequestration is cursory at best. The sinking flux of POC exported from the surface ocean is rapidly attenuated due to a combination of abiotic and biotic processes. Fragmentation by water turbulence, and zooplankton and microbial action convert large, rapidly settling particles into smaller, more slowly settling particles, thereby reducing the vertical flux of POC. Biotic processes such as microbial degradation and consumption by organisms reduce the amount of sinking POC and so reduce the flux, converting part of it to Dissolved Inorganic Carbon (DIC) through respiration (Countryman et al., 2022). The amount of POC reaching the deep ocean is only between 0.2 and 2% of the exported carbon, that is (0.02-0.2) PgC yr−1, so most of the sequestered oceanic carbon is DIC (Siegel et al., 2023). The depth and location at which organic carbon is transported and remineralized, and the water fluxes in the area, determine how long the carbon is sequestered in the ocean (Saba et al., 2021). In general, carbon is sequestered for longer than a year by particles that penetrate beneath the wintertime mixed layer, and for up to centuries by particles that reach deep water masses below 1000 m (Boyd et al., 2019). According to Nowicki (2022), the annual global carbon sequestration by the biological pump is 1293 (1302-1281) PgC, with an average sequestration time of 127 (133-122) years, which is coherent with the inventory of DIC (Carter et al., 2021) from respired organic matter: 1300 (±230) PgC, being the gravitational pump the more important pathway (Table 1). Table 1: Sequestration and sequestration time of the biological pump pathways Sequestration Sequestration time Source (PgC/year) (years) Mixing pump 102 (100 - 106) 54 (48 - 64) Nowicki et al., 2022 Gravitational pump: aggregate POC 207 (98 - 293) 185 (170 - 202) Nowicki et al., 2022 Gravitational pump: fecal pellet POC 833 (746 - 943) 136 (129 - 143) Nowicki et al., 2022 Migrant pump 150 (83 - 188) 150 (94 - 213) Nowicki et al., 2022 Undifferentiated 1300 (±230) Carter et al., 2021 7 Mixing pump Gravitational pump (passive pump) Vertical migration pump (active pump) Depth Photosynthesis - DIC Respiration - DIC Bacteria Zooplankton Whales Euphotic zone Phytoplankton Zooplankton Fish -meso- sequestration zooplankton time 1-10 y -macro- DOC and POC physical zooplankton DOC and POC production transport sequestration Aggregations Zooplankton fecal pellets Fish fecal pellets time 10-100 y ~ 100m Aggreations and fecal pellets Sinking phytoplankton & Solubilization Consumption zooplankton Mesopelagic & carcases zone disaggregation Zooplankton -fast vertical sequestration DOC and POC physical DOC and POC time 100-1000 y speed transport Fish Respiration DIC Uptake Bacteria Zooplankton Fish fecal pellets fecal pellets ~ 1000m Bathypelagic Fish Solubilization & disaggregation Uptake Deep sea zone Bacteria DOC and POC physical DOC and POC Respiration transport DIC Fish Figure 2: Main pathways and processes of the biological pump with a specific focus on fish. DOC=Dissolved organic carbon. POC=particulate organic carbon. DIC=dissolved inorganic carbon. Yellow=fish related (Adapted from Siegel et al. 2021) Understanding the Role of the Fisheries and Aquaculture in Carbon Sequestration The quantification of carbon fluxes and sequestration processes in the ocean is challenging due to limitations in the available data for every step of the marine carbon cycle. The variability of these processes under different environmental conditions adds further complexity. Additionally, the difficulty of integrating different types of models, which may use different assumptions and input data, can lead to additional uncertainty in the final estimates (Figure 3). Ecosystem Metabolic Biochemical Physical Fishing active model “production” model transport gears interactions model • Partical transformations with sediments • Mutlispecies dynamics in the water column • Water transport and interactions • Partical transformations services • DOC, DIC, POC and • Fish vertical movements • Physical processes carbonates production in the sediments • Ecological implications • Carcasses production • Dissolved transformations Environmental conditions • Temperature • Salinity • ...... Figure 3: Models needed to quantify the biological pump carbon sequestration of marine ecosystems. 9 Understanding the Role of the Fisheries and Aquaculture in Carbon Sequestration 3. MODELING THE FISH CARBON PUMP As described above, phytoplankton, zooplankton and bacteria play a major role in the functioning of the biological pump, but in addition marine vertebrates, and especially fish, through multiple processes also have a role in marine carbon sequestration. 3.1 Metabolic 'production' processes • POC production: Fish eat and repackage food into carbon-rich fecal pellets that sink rapidly through the water column until a part reaches the bottom of the ocean. At the same time, fish also contribute to POC degradation through consumption processes. • DOC and DIC production: Fish, when alive, produce DOC, and through respiration DIC, that is integrated in the water column, where biological and physical processes will determinate their storage time in the ocean. • Carbonate production: Fish produce carbonate (CaCO3)1 as a by-product of the osmoregulation process. • Carcass production: When fish and other large marine vertebrates die, their carcasses sink rapidly to the seafloor, where the carbon biomass is transformed (and partially predated by other organisms) and in part potentially buried. • Living biomass carbon: All living things are partially made of carbon and thus serve as carbon reservoirs through their lifespans. The larger and more long-lived an organism is, and the larger a population is, the more carbon is stored. • Other indirect related processes: ° Fertilizing species: many marine vertebrates, such as whales, have active horizontal and vertical movements. Vertically, whales dive to feed and return to the surface to breathe. While at the surface, they release buoyant fecal plumes that are rich in nutrients that phytoplankton need for growth, stimulating carbon dioxide capture. Horizontally, many whale species undertake seasonal migrations from nutrient-rich feeding grounds to nutrient-poor breeding grounds. At these breeding grounds, whales release nitrogen-rich urea, which can promote phytoplankton growth and stimulate carbon dioxide capture. ° Coastal cascade carbon: Marine predators help maintain the carbon storage function of coastal vegetation by keeping herbivore populations in check. This process can be relevant for coastal blue carbon and seaweed (Atwood et al., 2015). ° Biomixing carbon: The swimming movements of marine animals can stir up nutrients towards surface waters, which phytoplankton can use to grow, absorbing carbon in the process. The processes described have different levels of uncertainty and relevance for carbon sequestration. 3.2 Fisheries Unlike farming, which is generally not considered a carbon sink unless specifically designed to capture excess carbon (e.g., Leifeld 2023), fisheries directly and indirectly influence a number of large-scale, long-term oceanic carbon sequestration pathways (Krabbe et al, 2022) and therefore, fisheries management may aid to provide some level of control on carbon sequestration. Fishing activities interact directly with the processes described above by extracting biomass and changing the food web structure and functioning of ecosystems. As a consequence, fisheries not only affect volumes of living biomass or carcasses, but affect fish POC, DOC and DIC total production and the indirect process described earlier. 1 At long-term carbonate dilutes and becomes dissolved inorganic carbon https://www.nature.com/articles/s41561-021-00743-y 10 Understanding the Role of the Fisheries and Aquaculture in Carbon Sequestration Further, some fishing practices, such as bottom trawling have direct impacts on the seafloor and can potentially reintroduce carbon in the sediment into the water column, thereby impacting carbon sequestration by altering the sea bottom (Figure 4). Figure 4: Direct and indirect impacts of fishing on carbon sink: 1) Fertilizing species, 2) egestion of fast-sinking car- bon-rich fecal pellets, 3) harvesting low-mid trophic level pellet-producing species, 4) removing species living near the seabed where the sink of carbon will be short, 5) sediment disturbance from groundfish harvesting, 6) removing resi- dent or migratory mesopelagic species that contribute to the carbon sink, 7) removing large fish and whales reducing large falls of dead organic matter to the deep sea and sediment. Indirect impacts: 8) causing trophic cascades when removing high trophic level species impacting low trophic level communities that sink carbon, 9) removing prey items for fertilizing species, 10) killing predators that may otherwise fertilize the oceans and maintain a balanced food web, 11) release of discards which could cause localized dead zones (Adapted from Cavan and Hill 2022) The following section explores the main direct processes by which fisheries contribute to carbon sequestration. It describes these processes, reports some key reference values, and points out some of the main discussions and uncertainties. 3.3 Definition of key stocks and flows Figure 5 shows a simplified general scheme of the different sections that a fish-carbon model requires. The scheme assumes the biomass distribution is known for the key fish groups of species in an ecosystem that can be obtained, for example from a food-web model. 11 Understanding the Role of the Fisheries and Aquaculture in Carbon Sequestration Food-web model Metabolic “production” Biochemical Physical Carbon “transfer” interactions Phytoplankton Fish species and Initial Carbon Carbon biomass distribution carbon Zooplankton stocks stocks stocks Iterative process Sea bottom processes Trawling sea bottom sediment interactions Figure 5 - Fish and fisheries carbon flux scheme The ultimate goal of the fish-carbon model is to assess the temporal stock evolution of the different carbon stocks, which collectively determine the carbon sequestration time of the fish pump. The following sections describe and, when possible, parametrize three groups of actions that form the foundation of any model: • Metabolic ‘production’ flows: This section reviews the metabolic inputs that are necessary to understand the carbon web produced by fish including POC production, DOC production, DIC-respiration, carbonate production, carcass production, and the carbon stored in living creatures. • Biochemical interactions: This section includes all biochemical processes that create internal fluxes between stocks, such as the remineralization and changes in the characteristics of the different stocks of organic carbon and carbonates dissolving processes. • Physical flows: This section includes two types of physical flows - those associated with water mass fluxes and those linked to vertical fish movements in the marine column. Bottom trawling sea bottom sediment interactions are also included in the analysis. These interactions are not biological but anthropogenic in nature, involving the reintroduction of organic carbon into the water column from previously stored carbon in sediment stocks when bottom trawling takes place. They also change the benthic ecosystem where carbon is stored and transformed. The following sections will discuss these factors individually, highlighting the available information and evaluate their variability in terms of environmental data. 3.4 Metabolic 'production' While models exist for the productions of the different fish stocks, there is limited field data available on fish POC and DOC production, as well as fish carbonate production. According to a recent expert workshop on “Fish, Fisheries and Carbon”, the study of carbon release rates is the second priority research objective in this field, with only the need to decrease fish biomass uncertainty ranking higher (Ocean Carbon and Biogeochemistry program, 2023). Table 2 summarizes the key parameters and models commonly used to quantify POC production, DOC production, DIC respiration, carbonates production, carcass production, and the carbon stored in living fish. The following paragraphs provide an explanation of the table and supplementary information. 12 Table 2: Production table POC DOC DIC respiration Carbonates Carcasses Living biomass Potentially relevant (Heymans et al., 2016) (Liu et al., 2022) data based (Ikeda, 2016).data (Wilson et al., 2009) model Results from ecosystem (Bianchi et al., 2021) references (Liu et al., 2022) data based on marine medaka Zooplankton: (Ikeda, 2014; (Ghilardi et al., 2023) data models. on marine medaka Steinberg and Landry, 2017). and model Zooplankton: (Ikeda, 2014; Steinberg and Landry, 2017). Proxies and relevant data Liu: 8%–13% of the Approximately 20% of Ikeda has parameterized In the open ocean, Mg Large uncertainty remains ingested food carbon is not ingested food carbon is respiration rates for calcites secreted by fish in estimating total fish assimilated and is released excreted or egested, but different types of fish based could account for up to 15% biomass at a global scale, as particulate carbon. this percentage can vary on more than 90 different of total CaCO3 production. with ranges varying from between functional groups. fish species. These rates less than 1 Gt to more than Groups that predominantly can be directly used by 50 Gt depending not only feed on phytoplankton may modelers. on the models used but also have higher values of up to Zooplankton respiration on the sizes evaluated. 40%. Zooplankton values is typically considered to are typically around 30% be approximately 50% of The carbon content of but vary depending on the carbon absorption, but this various marine species is nutritional quality of their proportion is influenced as follows: i) fish: 11.5%, food. by two main factors: ii) marine mammals: 16%, According to Liu, 20% temperature and body iii) sea turtles: 13.9%, and to 31% of ingested food mass, as well as depth. iv) seabirds: 18.4%. Other carbon is not assimilated studies assume that, on but is released as DOC. average, big fish (such as Protozoan and metazoan tuna, mackerel, billfish, and zooplankton release shark species) have a 12.5% between 10% and 30% of (±2.5%) carbon content POC ingested as DOC. relative to their whole- body wet weight. Key variables and Species, temperature, Production increase Carbon contents : (Mariani tendencies depth and body mass with body mass and et al., 2020) temperature. Other models consider RIL. Fish body mass is the strongest predictor of carbonate excretion rate, RIL - relative intestinal length to body standard length - has a stronger influence on carbonate excretion rates compared to temperature. Caudal fin aspect ratio had a weakest effect. 13 Understanding the Role of the Fisheries and Aquaculture in Carbon Sequestration Respiration data for many fish species indicate that body mass and temperature are the primary predictors of fish respiration rates, with occurrence at depth being an additional factor for mesopelagic and bathypelagic fishes. Ikeda (2016) offers a comprehensive parametrization of respiration rates for various fish types, based on data from more than 90 different fish species, using the previously explained predictors. This information can be directly used by modelers. Many food web models commonly used in fisheries studies show that productivity and flow rates decrease with increased trophic level (Pauly and Christensen, 1995). It is typically assumed that the link between productivity and respiration is largely linear (Clarke, 2019). Therefore, knowing only the amount of fish that is extracted by fisheries is insufficient for estimating the impact on DIC production; the food web structure and response to fish extraction are also important factors. For example, fishing targeting top predators may lead to an overall reduction in fish biomass, but this may trigger a ‘prey-release’ process (Christensen et al., 2014) that boosts the biomass of more productive, lower trophic level prey fish with higher respiration rates. This process can result in significant changes in DIC production compared to direct assumptions of DIC decreasing rates due to direct biomass decrease (Spiers et al., 2016; Stafford et al., 2021), highlighting the importance of considering food web interactions. In many food web models, the portion of ingested food by fish that is not assimilated and is excreted/egested is typically assumed to be around 20% – based on Ivlev (1955). However, this value can vary between functional groups, and is usually higher for groups that predominantly feed on phytoplankton, with values reaching up to 40% (Heymans et al., 2016). Values for zooplankton are often assumed to be around 30%, but this can vary based on the nutritional quality of their food (Steinberg and Landry, 2017). According to available global marine ecosystem models, one-fifth of the food consumption by fish is returned to the environment as fecal pellets (Bianchi et al., 2021; Pinti et al., 2022). Assuming a constant carbon content of organic matter of 10 gC/100gWB, it is possible to estimate fish POC production. Some direct measurements on fish indicate POC production values slightly lower, in the range of 8-16% of consumption (Liu et al., 2022; Pinnegar et al., 2007). In some marine species with high potential impact on the marine carbon cycle, such as krill (Cavan et al., 2019), the egestion rate is very uncertain, with values typically ranging from 0.67 to 6.29 mg C ind-1 d-1 (Belcher et al., 2019). Some studies even suggest lower values approaching 0.1 mg C ind-1 d-1 (Pauli et al., 2021). Regarding DOC, while most of the marine DOC production is associated with phytoplankton photosynthesis, which produce labile DOC (LDOC) that is highly bioavailable and is rapidly metabolized on the ocean surface, fish also contribute to DOC through excretion and defecation. In some species, a significant portion of the excreted or ejected matter is in form of DOC, rather than POC, which affects the sequestration capacity due to differences in transport process and biochemical interactions. DOC is mostly controlled by mixing, in contrast to gravity-controlled export of POC. As a consequence, a better understanding of the relative production of DOC and POC by marine species is important. This can be especially important for mesopelagic contribution to carbon sequestration. According to Liu et al. (2022), there are very few direct fish DOC measurements, and values used in modeling often come from freshwater species or zooplankton. An experiment using marine medaka Oryzias melastigma (a needlefish often used as substitute laboratory species for mesopelagic zooplanktivorous fish), demonstrated that 53%–75% of the ingested food carbon was not assimilated but was released, and that excretion of DOC accounted for 39–42% of the carbon released, compared to 40–45% via respiration of CO2 and 16–18% as particulate carbon. This result suggests that mesopelagic fish may produce a large amount of DOC in the ocean. The same study also suggests that these values can be applied to estimate the general fish-released carbon in the forms of DOC, respiration, and POC for mesopelagic fish (Liu et al., 2022). Global fish carbon models have up to now not explicitly considered DOC production. Bianchi et al. (2021) for example did not include it in their model, while Pinti et al. (2023) only included it indirectly in a global concept of ‘other losses’. 14 Understanding the Role of the Fisheries and Aquaculture in Carbon Sequestration Without going into details about zooplankton carbon ‘production’, some of its key characteristics are as follows. Regarding respiration, it is typically considered to be approximately 50% of the carbon absorption, but this proportion is influenced by two main factors - temperature and body mass, and to some extent, depth. Ikeda (2014) and Steinberg et al. (2017) have designed empirical models that take these elements into consideration. Regarding DOC production, between 10–30% of the ingested POC by protozoan and metazoan zooplankton is released as DOC (Steinberg and Landry, 2017). Zooplankton POC contribution to carbon sequestration includes many different processes and varies widely among regions, seasons, and depths. It depends on factors such as the species composition, size distribution, and of course abundance. A complete discussion with reference values can be found in Steinberg and Landry (2017). Many zooplankton species have daily or seasonal vertical migrations, which should be considered when evaluating the role of zooplankton in carbon sequestration. Finally, other processes such as their ability to transform sinking and suspended particles of POC, by consuming and remineralizing them to CO2, or fragmenting sinking POC into smaller and slower-sinking particles via feeding or swimming activities, are also important functions for understanding their global role in the biological carbon pump. Fish also produce calcium carbonates, although most of the oceanic production of calcium carbonate is attributed to marine plankton. Fish excrete fine-grained carbonates as a by-product of osmoregulation, in the form of mucus coated pellets or in feces. In the open ocean, Mg calcites secreted by fish could account for up to 15% of total CaCO3 production (Wilson et al., 2009), most of which is dissolved in the upper layers due to their relative high solubility2 (Woosley et al., 2012) (Figure 6). Figure 6 - Conceptual view of the oceanic CaCO3 cycle. Fluxes given in Tmol yr−1 are the CaCO3 export flux from the sur- face ocean, the CaCO3 flux reaching the seafloor, and the CaCO3 flux buried in sediments. Green circles indicate the five main mechanisms of dissolution (Reproduced from Sulpis et al., 2021) 2 Carbonates are inorganic, they have transformations to DIC with a different pattern https://www.nature.com/articles/s41561-021-00743-y 15 Understanding the Role of the Fisheries and Aquaculture in Carbon Sequestration Often models use Wilson’s equation to calculate fish production rates (Wilson et al., 2009): = ∙ 9.809 × 108 ∙ −0.25 ∙ −4727(1/(+273)) where C is carbonate production rate (μmol kg-1 h -1), α and ρ are constants, which were set to one to achieve the best fit to data for resting fish in experimental chambers, w is individual body mass (g) and T is temperature (°C). α was included in the equation to account for the ratio between active and resting metabolism because wild fish engage in foraging, digestion, nutrient assimilation and growth, predator avoidance, migration, reproductive, and other behaviors not present when unfed fish are resting in experimental conditions. The constant ρ was also included to account for the differences in resting metabolism between the species that live on the seabed (for which carbonate production rates have been measured experimentally) and midwater and pelagic species that have relatively more metabolically active tissue and specifically red muscle that has a much higher metabolic demand than white muscle. (Wilson et al., 2009) use α = 2.5 as the constant α when using the equation to calculate carbonate production by wild fish, which is considered to be a conservative estimation in terms of production. However, they recognized that for many pelagic species, α should be greater, as species like tunas and cod may have metabolic rates 10-20 times greater than some benthic species. They also assigned the value of 2.4 to ρ. Ghilardi et al. (2023) recently developed a model based on 71 species and 21 families of tropical reef fisheries to explore the relationship between total carbonate excretion rate and various factors, including body mass, caudal fin aspect ratio (AR), relative intestinal length to body standard length (RIL), temperature, and fish family. The study found a strong relationship between observed and predicted excretion rate. The results showed that fish body mass was the strongest predictor of carbonate excretion rate, followed by RIL and temperature, while AR had the weakest effects. In addition, taxonomic identity explained only a small proportion of the variance in the dataset (~5%). The model was also able to accurately predict the excretion rate of five major carbonate polymorphs produced by fishes, including Low Magnesium calcite (LMC), High Magnesium Calcite (HMC), aragonite, Monohydrocalcite (MHC), and amorphous calcium–magnesium carbonate (ACMC). The study also investigated whether temperature, RIL, and taxonomic identity influence carbonate mineralogy. Although the production of these carbonates is primarily based on data from warm-water reef fishes, recent studies have shown that carbonate products were mineralogically, compositionally, and morphologically similar across a broader thermal range. In most cases, there were no differences within species (18 vs. 24 °C) or families (10 vs. 25 °C) (Salter et al., 2019). However, caution should be taken when extrapolating to unsampled families as carbonate composition is strongly conserved at the family level. It is important to note that the taxonomic scope of the existing carbonate database remains limited to only 35 reef-associated fish families. HMC comprises more than 75% of carbonates produced by nearly two-thirds of the fish families for which they have been characterized (Salter et al., 2019). Thus, available evidence suggests HMC is likely to be a major fish carbonate product globally. Due to the uniformity of carbonate contents over a large thermal gradient ranging from 10 to 25 oC, HMC from a given fish family is expected to be less stable at higher latitudes, dissolving more rapidly and/or at shallower depths, and playing a different role in the marine inorganic carbon cycle than in warm-water regions. While there are several estimates of total marine CaCO3 and some data-based regional profiles of CaCO3 settling fluxes, e.g., Sulpis et al. (2021), many basic questions, such as which CaCO3 polymorphs are dissolving and what the controlling factors are, remain unanswered. Some estimations of solubility have been done for fishes but much uncertainty remains, as solubilities and rates of dissolution may vary by fish species and/or environmental factors, and clearly additional work is needed (Woosley et al., 2012). 3. 4. 1 Living biomass The carbon stock linked to fish biomass depends on the total fish biomass and its composition of organic carbon. At the global scale, large uncertainty remains on the total fish biomass, ranges can be found from less than 1 Gt to more than 16 Understanding the Role of the Fisheries and Aquaculture in Carbon Sequestration 50 Gt not only depending on the models used, but also by the sizes evaluated (Bianchi et al., 2021). Even for regional studies, absolute biomasses are also difficult to assess, since the most common situation is that only partial information is available for the key commercial species. Ecosystem models are, however, useful as they can be used to evaluate population trends across an ecosystem and use mass-balance criteria to restrict potential outcomes and thereby reduce uncertainty and provide best-possible evaluations of biomasses across the ecosystem (e.g., Christensen and Walters, 2011). The marine carbon cycle is unevenly represented by modelling approaches. Broadly applied lower trophic level models such as ERSEM (European Regional Seas Ecosystem Model; Butenschön et al. 2016) and COBALT (Stock et al. 2014) typically focus on the interplay between climate, biochemistry, geography and circulation (thus covering, at least in part, the solubility pump and the physical pump), and may include so-called NPZD (nutrient, phytoplankton, zooplankton and detritus) modules that represent the carbon flows within the biological pump reliably up to phytoplankton – the foundation for living food webs. To date, only two higher trophic level models have attempted to mechanistically address carbon flows in living marine food webs: the BiOeconomic mArine Trophic Size-spectrum (BOATS) model; Carozza et al. 2017); and the FishErIes Size and functional TYpe model (FEISTY; Petrik et al. 2019). Once the biomass is determined, the amount of carbon within the living populations can be estimated. According to Pearson (2021), based on different sources, some usual values for carbon content for marine animals are fish: 11.5%, marine mammals: 16%, sea turtles: 13.9%, and seabirds: 18.4%. Other studies have assumed that large fish (tuna, mackerel, bill fish, shark species) have an average carbon content of 12.5% (±2.5%) relative to their whole-body wet weight (Mariani et al., 2020). Detailed studies have shown that values depend on the species, for example Scomber scombrus (Atlantic mackerel, Scombridae) has a carbon content of 12.3% relative to its wet weight, while S. japonicus (Spanish mackerel, Scombridae) has a carbon content of 11%, and mesopelagic fisheries have a carbon content of 15% (Mariani et al., 2020). When fishing occurs, carbon is extracted from the sea. According to Mariani et al. (2020), between 1950 and 2014, the world’s fishing fleets have extracted 318.4 Mt of large fish from the ocean (tuna, mackerel, billfish and shark species), equivalent to 37.5 ± 7.4 Mt of carbon (MtC). According to Bianchi et al. (2021), the pre-exploitation global biomass of fish (10 g to 100 kg) was 6.9 ± 3.6 Gt, and by the time of the global peak catch, the biomass had decreased to 47 ± 20% of preindustrial values. The model applied did, however, not consider prey-release mechanisms, and other models have found that while large fish have declined with around 2/3 over the last century, small fish may have more than doubled over the same period (Christensen et al., 2015) due to trophic cascades. It is worth noting that carbon stored in marine animals constitutes, for long-lived species, a carbon storage of the order of decades, as opposed to hundreds of years or more typically associated with other sequestration mechanisms (Vanderklift et al., 2022). Therefore, it is not directly comparable. Additionally, it is important to recognize that the carbon stock of marine biota is relatively minor when compared to other marine and terrestrial carbon stocks (Figure 7). 3. 4. 2 Carcass Production Another relevant element in carbon sequestration is the relationship between biomass and the number of marine organisms that die and naturally sink to the bottom, where the carbon may stay sequestered for many years. This relationship is determined by the mortality rate from senescence and disease (often assumed to equal the unexplained ‘other’ natural mortality in ecosystem models). Usually, natural mortality is considered to be a species- or stock- specific constant, which means that the estimate can be applied to all exploited ages and sizes of the species or stock in question, taking into consideration the temperature. Values can be estimated by specific ecosystem models. Some experts point out that the global impact of natural senescence in fishes on carbon sequestration is presumed to be small (Saba et al., 2021). Other studies, such as Pinti et al. (2023), suggest that this pathway may be more relevant than previously thought. The impact of the fall of large fishes and mammals on carbon sequestration has been populations to pre-fishing period would remove 1.6×105 tons of carbon each year through sinking whale carcasses3. 3 The role of whales in carbon sequestration has been studied from other perspectives as well, such as their ability to bring nutrients up to the euphotic zone through the ‘whale pump’. This process involves the consumption of planktivores at depth, followed by the release of liquid excretion with dissolved nutrients that can fuel phytoplankton production at the surface (Pearson et al., 2023). 17 Understanding the Role of the Fisheries and Aquaculture in Carbon Sequestration Figure 7 - Simplified scheme of the global carbon cycle. Numbers represent ‘carbon stocks’ in PgC and annual carbon exchange fluxes (in PgC yr–1). Black numbers and arrows indicate reservoir mass and exchange fluxes estimated for the time prior to the Industrial Era, about 1750. The sediment storage is a sum of 150 PgC of the organic carbon in the mixed layer and 1600 PgC of the deep-sea CaCO3 sediments available to neutralize fossil fuel CO2. Red arrows and numbers indicate annual ‘anthropogenic’ fluxes averaged over the 2000–2009 time period (Reproduced from Ciais et al. 2013) The practice of fisheries’ discarding is also a source of fish carcasses, with very different discard rates depending on the fishery and region. Globally, an estimated 9.1 Mt are estimated to be discarded annually (95% uncertainty interval: 7–16 Mt), which is equivalent to 10.8% of the global catch (95% UI: 10–12%) (Gilman et al., 2020). Therefore, fisheries contribute to a carbon carcass ‘production’ of approximately 0.91 Mt C yr−1. A large proportion of these carcasses will be eaten by scavengers, but a fraction of the carbon released may be sequestered at the bottom. 3.5 Biochemical transformations The degradation of POC and DOC depends on various biological, chemical and physical parameters. While DOC is primarily degraded by microbes and is affected by complex biochemical processes (Wagner et al., 2020), both microbes and larger organisms such as zooplankton can degrade the POC pool. This is especially significant in terms of carbon sequestration. The literature provides a wide range of values for key parameters to assess POC evolution in the marine water column such as remineralization rates (Table 3), that depends on environmental variables, such as temperature, oxygen concentration, stratification, community composition and the mineral context of the sinking particles (Baumas et al., 2023), and sinking rates (Table 4). One of the widely used approaches for modelling POC evolution in the twilight water column (under the euphotic zone up to 1000 m) is the ‘Martin curve’ model (Martin et al., 1987). This model is based on an empirical relation between POC fluxes at different depths in the water column from different regions of the world. The model proposes that POC flux follows a power law in the water column, where the flux at depth z is related to the export flux at zo (the euphotic zone, 100 meters in the original paper) by the equation: J(z)=Jo(z/zo)-b. The attenuation coefficient b reflects the magnitude of the particle-specific remineralization rate and their sinking speed. Faster sinking speeds and slower rates results in smaller attenuation coefficients. Several studies have estimated the b coefficient, with values ranging between 0.27 and 1.29 (the original value was 0.858), depending on the region and data used (Buesseler and Boyd, 2009; Primeau, 2006a). However, values around 0.7 are commonly used (Primeau, 2006b). 18 Understanding the Role of the Fisheries and Aquaculture in Carbon Sequestration Table 3: Remineralization rates Value Units Source Base remineralization rate of fast-sinking POC 0.32 ± 0.15 d-1 DeVries and Weber, 2017 Base remineralization rate of slow-sinking POC 0.16 ± 0.06 d-1 DeVries and Weber, 2017 Remineralization rate of labile DOC 10 ± 0.9 d-1 DeVries and Weber, 2017 Remineralization rate of semi-labile DOC 1.9 ± 1.1 year-1 DeVries and Weber, 2017 Remineralization rate of semi-refractory DOC 200 ± 130 year-1 DeVries and Weber, 2017 Remineralization rate of refractory DOC 5500 ± 5100 year-1 DeVries and Weber, 2017 Copepod remineralization rate 0.12 d-1 Serra-Pompei et al., 2022 Table 4: Sinking rates reference values for fish and zooplankton fecal pellets and carcasses (m d−1) Source Northern anchovy 787 (485–1370) Saba and Steinberg, 2012 Anchoveta, Engraulis ringens 1100 (691–1987) Staresinic et al., 1983 Stenobrachius leucopsarus, Triphoturus mexicanus, Leuroglossus 1028 Robison and Bailey, 1981 stilbius, Lampanyctus ritteri, Argyropelecus affinis and Parvilux ingens (mean) Blacksmith (Chromis punctipinnis) 2635 ± 121 Bray et al., 1981 Mesozooplankton 100 Pinti et al., 2023 Macrozooplankton 150 Pinti et al., 2023 Mesopelagic fish 300 Pinti et al., 2023 Forage fish 400 Pinti et al., 2023 Large pelagic fish 800 Pinti et al., 2023 Tractile predators 500 Pinti et al., 2023 Fast-sinking POC 85±11 DeVries and Weber, 2017 Slow-sinking POC 11±3 DeVries and Weber, 2017 Fast-sinking detritus 52 (10-500) Stukel et al., 2022 Slow-sinking detritus 0.88 (0.5-10) Stukel et al., 2022 Mesozooplankto carcasses sinking speed 200 Pinti et al., 2023 Macrozooplankton carcasses sinking speed 400 Pinti et al., 2023 Mesopelagic carcasses sinking speed 600 Pinti et al., 2023 Forage carcasses sinking speed 800 Pinti et al., 2023 Large pelagic carcasses sinking speed 1500 Pinti et al., 2023 Tractile predator carcasses sinking speed 800 Pinti et al., 2023 Copepods 5-220 Eduardo Menschel and González, 2019 Euphausiids 16-862 Eduardo Menschel and González, 2019 Doliolids 41-504 Eduardo Menschel and González, 2019 Appendicularians 25-166 Eduardo Menschel and González, 2019 Chaetognaths 27-1313 Eduardo Menschel and González, 2019 Pteropods 120-1800 Eduardo Menschel and González, 2019 Heteropods 120-646 Eduardo Menschel and González, 2019 Salps 43-368 Eduardo Menschel and González, 2019 Krill (Euphausia superba) 233.3 ±154.3 Pauli et al., 2021 Salp (Salpa thompsoni) 586.0 ±692.0 Pauli et al., 2021 19 Understanding the Role of the Fisheries and Aquaculture in Carbon Sequestration Recently, Bianchi et al. (2021) used this approach to estimate the degradation of fish POC. To account for the difference in sinking speed of fish fecal pellets compared to the general POC stock (that includes phytoplankton and zooplankton), a much smaller b value of 0.07 was used. It is important to note that in this case, neither temperature nor oxygen variations were considered, so remineralization is not geographically dependent. Nevertheless, these two factors appeared to be important in explaining remineralization processes. In case of studies on Antarctic krill, a b Martin’s value of 0.32, is often utilized (Belcher et al., 2019). Other models attempt to incorporate differences in remineralization rates associated with temperature, oxygen concentration, and particles’ chemicals characteristics and/or size (Anderson and Tang, 2010; DeVries and Weber, 2017; Galí et al., 2022; Kwon et al., 2009). Validation is, however, a main problem for these models, as disaggregated data available is limited. Typically, the dynamics of particulate organic carbon (POC) is determined using ‘box models’ that contain one or more pools of different POC stocks, based on their remineralization and sinking speeds. The key parameters can be summarized in this equation (DeVries and Weber, 2017): dPOCF #prod POCF # POCF ∂ POCF) - b WF POCF dt = APOCF + - rem + ∂Z (WF iZbot POCF Where APOC is the matrix transport (advection-diffusion) operator derived from an ocean model, f prod is the production rate, f POC rem F the remineralization rate (sometimes the equation also includes a POC consumption factor to take into consideration consumption by other species), w the sinking speed, ∆Zbot the thickness of the model cell, and bF the proportion of POC with characteristics F. For instance, in some biochemical models such as the TRIM model (DeVries and Weber, 2017) POC and DOC produced (in this case, by phytoplankton and zooplankton) are divided in stocks with different sinking speeds (fast-slow, Table 4) and different remineralization rates for the DOC pools (labile, semi-labile, semi-refractory and refractory) (Table 3). For DOC pools, the remineralization rates are considered to be constant for each pool, while for POC, the remineralization varies spatially and depends on the ambient temperature and oxygen concentration according to the following formula (DeVries and Weber, 2017): − 2, = 10 10 ( ) 2, + 2 Where γ is the base remineralization rate (Table 3), Tref is 20o, O2,obs is the observed dissolved O2, and q10 and KO2 are adjustable parameters governing the temperature sensitivity and oxygen sensitivity of POC remineralization. DeVries found a q10 value of 2.6 ± 0.2 for the temperature dependence, which is larger than the q10 typically used in most ocean biogeochemical models (usually around 2), but falls below some estimates with values between 3.3 and 3.7 for some geographical areas (DeVries and Weber, 2017). KO2, which is the half-saturation constant for O2, was optimized with a value of 19 ± 9. This model has been used recently in Pinti et al. (2022) model to estimate metazoan’s contributions to the biological carbon pump. Other biochemical models, such as the Ocean Carbon Model Intercomparison Project (OCMIP) biotic model, use a similar approach, also taking into consideration oxygen and temperatures to establish variable remineralization rates, but using different stocks structures, based on the differentiated compositions (Lima et al., 2014), resulting in different parameters after optimization (Cram et al., 2018; Dinauer et al., 2022). Another commonly used model is the Carbon, Ocean Biogeochemistry, and Lower Trophics (COBALT, Laufkötter et al., 2017) model, which differentiates between ballast particles, where part of the organic content is aggregated and remineralized according to the length scale of the ballast materials, while the other particles remineralize using an 20 Understanding the Role of the Fisheries and Aquaculture in Carbon Sequestration exponential decay function. Again, temperature and oxygen seem to be key elements in the definition of remineralization rates. However, the model cannot determine whether the relationship between water temperature and particle flux attenuation is driven by faster bacterial degradation and remineralization, by viscosity-induced differences in sinking speed, or by indirect temperature effects such as differences in particle composition and/or plankton community structure in different temperature regimes. The attenuation of POC flux depends on temperature with a q10 value between 1.5 and 2.01, and on oxygen with a half-saturation constant between 4 and 12 μmol/L, assuming an average sinking speed of 100 m d-1 (Laufkötter et al., 2017). More complex mechanistic models are trying to model the steady-state flux of the sinking particles, including the combinations of factors such as the excess particle density, the change in seawater density with depth, different remineralization rates, and spectral slopes of the particle size distribution among other elements. However, data availability remains a challenge in evaluating the driving processes and under which circumstances they occur (Omand et al., 2020). As a consequence of the biochemical processes discussed earlier, the total amount of POC production by fish is much lower than by zooplankton, but their higher sinking speeds lead to an increased significance in terms of carbon fixation with depth. According to some models, POC contribution by fish can account for more than 20% of the deep ocean respiration and carbon sequestration driven by the biological pump (Bianchi et al., 2021) (Table 5). 3.6 Phyical Transports 3. 6. 1. Fish active vertical transport Diurnal vertical migration (DVM) is a behavior observed in many marine zooplankton species, including copepods, euphausiids (krill) and gelatinous zooplankton such as jellyfish, pelagic tunicates, chaetognaths, pteropods. These species are in turn followed by fish, cephalopods and other higher-order predators. The migratory amplitudes of DVM vary widely, from a few tens or hundreds of meters for jellyfish to mesopelagic depths (200–1000 m) for many salps, squid, and fish. Metazoan consumers actively collect and transport surface-generated productivity to depth, where it is off-loaded in the form of respiration, DOC secretion, feces, exuviae, and carcasses (Butterfield, 2018). Recently, the role of daily vertical migrations of fish and other metazoans in carbon sequestration has been estimated to contribute to more than 50% of the estimated global total biological pump sequestration (Pinti et al., 2023). Nevertheless, there is crucial uncertainty about these results. The biggest uncertainty is the global mesopelagic biomass, estimates of which show huge variation, ranging from 1 and 20 Gt (Proud et al., 2019). Therefore, the uncertainty about fish carbon sequestration is also significant. Two more factors are relevant in modelling of fish vertical active transport: the diel vertical migration behavior (i.e., the depth and the time spent at each depth) and the fraction of populations taking part in DVM, as not all mesopelagic organisms carry out DVM. Experimental estimations for both characteristics have been provided by Klevjer et al. (2016) for the most relevant geographic regions (Indian Ocean, North Atlantic, East Pacific, West Pacific, and other areas), along with a model based on environmental variables such as oxygen, turbidity, temperature, chlorophyll and sea surface temperature, which can be used for estimating regional fish vertical movements (Table 6). 3. 6. 2 Water mass flows Although it is beyond the scope of this report to review the characteristics of available water mass flow models, it is important to note that a coupling of such models is required in order to model carbon sequestration as an integral part of a fish model. This coupling is necessary not only to model the evolution of particulate organic carbon, but also, more importantly, to understand the evolution of dissolved organic carbon and dissolved inorganic carbon stocks and their retention time. Ocean Circulation Inverse Models (OCIM) have been used in the past (Devries, 2014; DeVries and Primeau 2011; Pinti et al., 2023). 21 Understanding the Role of the Fisheries and Aquaculture in Carbon Sequestration Table 5: Biochemical table Model Parameters Reference values References Comments “Martin curve” based Single parameter not B=0.07 for fish (Bianchi et (Martin et al., 1987) Very simple verifiable with indirect al., 2021) (Bianchi et al., 2021) Does not take into dependence on sinking B=0.7 (around) consideration T or O speed (Primeau, 2006b). “Biochemical” based Basic remineralization Remineralization levels: (DeVries and Weber, More reliable models levels Table 3 and modelled 2017) Requires more Sinking speeds estimations based in (Pinti et al., 2022) information differentiated stocks, There is no agreement Temperature temperature and oxygen (Cram et al., 2018). on the different Oxygen Sinking speeds: Table 4 (Laufkötter et al., 2017) “remineralization boxes” Temperature and oxygen profiles (regional dependent) Table 6: Physical transport table Model Parameters Reference values References Comments Fish vertical active Mesopelagic biomass Diel vertical migration (Klevjer et al., 2016) Very simple transport Diel vertical migra- behaviour and fraction Does not take into tion behaviour of population obtained consideration T or O Fraction of popula- from Malaspina expe- tions taking part in dition. Data available DVM for geographic regions (Indian Ocean, North Atlantic, East Pacific, West Pacific, Other areas). Model based on oxygen, turbidity, temperature and chlo- rophyll and sea surface temperature Water mass flow Regional specific Detailed information transport Several global mod- may be required if els of transport are coastal or shelf se- available questration studies wants to be used, especially when water flows are strong. 22 Understanding the Role of the Fisheries and Aquaculture in Carbon Sequestration The need for a water mass flow model adds a layer of complexity especially when evaluating carbon sequestration in coastal and shelf areas, where lateral fluxes or complex topographies present challenges in modelling water movement that can be significant in terms of carbon transport and sequestration. These challenges are also relevant when analyzing processes such as the interactions between mobile demersal fishing gears and sediments in near-canyon areas, where horizontal POC fluxes may be important (Arjona-Camas et al., 2021; Mengual et al., 2019; Paradis et al., 2017), in upwelling-downwelling and eddy areas, or in kelp studies. 3.7 Mobile demersal fishing gears - carbon sediment interactions Subtidal marine sediments contain the ocean's biggest organic carbon stock, with 87 ± 43 PgC in the upper 5 cm (Lee et al., 2019), 64 PgC in the continental margin mixed layer (20 cm), and 83 PgC in the Open Ocean mixed layer (10 cm). Global estimates of annual burial rates in sediments range from 0.12–0.23 PgC year−1, with coastal habitats, continental shelves and slopes at depths above 1000 m accounting for up to 86% of all the organic carbon that is buried annually in global subtidal sediments (Epstein et al., 2022). Recently, Atwood et al. (2020) and Epstein et al. (2019) provided global maps of sedimentary carbon sediments estimates. Organic carbon buried in the sediments of the ocean can remain there for thousands to millions of years if left undisturbed (Estes et al., 2019). However, bottom trawling and dredging for fish and shellfish are important disturbances in many areas (Puig et al., 2012). Around 4.9 million km2 or 1.3% of the global ocean is trawled each year (Sala et al., 2021). Trawling is mostly concentrated in subtidal areas in coastal habitats and offshore on continental shelves and slopes, at depths above 1000 m. In total, these non-abyssal/basin areas cover around 9% of the global seabed, and contain around 20% of the stocked sedimentary carbon (Atwood et al., 2020). Detailed estimates of carbon sedimentary budgets and fluxes exists for shelfs seas in some regions, such as the northwest European Seas (Legge et al., 2020). The quantification of carbon budgets and fluxes in shelf areas is an ongoing active research field worldwide, with significant knowledge gaps remaining due to challenges in quantifying inputs, fluxes, and biochemical transformations that occur (Figure 8). While the impacts of trawling on sediments have been studied for decades, a recent publication of a global estimation of its impact in terms of carbon storage in 2021 has drawn attention to this theme (Sala et al., 2021). The study suggested that trawl fisheries’ sediment disturbance can led to remineralization of organic carbon that was previously sequestered in the sediments, causing an increase in DIC fluxes into the water column. A global first-order estimate suggests that mobile demersal fishing activities may cause 0.16–0.4 PgC of organic carbon to be remineralized annually from seabed sediment carbon stocks (Sala et al., 2021). Table 7 shows the main variables used in the study. Table 7: Key resuspension variables Resuspension variable Sources or values Carbon stock geographical and vertical distribution Atwood et al., 2020 Fishing activity Global Fishing Watch Swept volume ratio = average penetration depth of gear type * SAR Pcrd = is the proportion that resettles after disturbance 0.87 Plab = Fraction of carbon that is labile 0.7 ‘fine’ sediments 0.286 ‘coarse’ sediments K = first-order degradation rate constant 1 year SAR = swept area = tradable distance* vessel size footprint Pdepth = average penetration depth of gear otter trawls (2.44 cm), beam trawls (2.72 cm), towed dredges (5.47 cm) and hydraulic dredges (16.11 cm). 23 Understanding the Role of the Fisheries and Aquaculture in Carbon Sequestration Figure 8: Top - Main fluxes and carbon budget; and (bottom) knowledge gaps and research priorities in the northwest European shelf. (Reproduced from Ciais et al., 2013). This estimation has received criticism from various perspectives, suggesting that the study overestimates the impact of trawling on carbon sequestration. Some critics have pointed out that the paper overestimates the biodegradable labile fraction of sediments (Smeaton and Austin, 2022) and the volume of sediment mobilized by trawling activities (Hiddink et al., 2021). They also noted that the paper overlooks the impacts on benthic fauna, which may compensate for some of the resuspension impacts (Epstein et al., 2022). Others noted that the paper does not distinguish between high and lower reactive OC and assumes that OC mineralization does not occur in non-trawled areas. A review of 49 studies into 24 Understanding the Role of the Fisheries and Aquaculture in Carbon Sequestration OC stocks after trawl disturbance showed highly mixed results, underpinning that more research is needed before reliable global estimates can be made (Hiddink et al. 2023). Finally, some critics have warned about the accuracy of the trawling distribution data (Hilborn and Kaiser, 2022). Moreover, the Sala et al., (2021) paper itself recognizes that the hypothesis that carbon distribution in the first meter is equally distributed, is known not to occur. While in stable accreting sediments, organic carbon concentrations are generally highest at the surface and reduce with depth until a steady-state burial rate is reached, the distribution is more homogeneous if persistent trawling or natural mixing takes place (Epstein et al., 2022). Regarding the lability, studies conducted in UK indicate that organic matter in inshore sediments is considerably more labile than that found offshore, resulting in a higher potential for the organic carbon in these sediments to be released as CO2 through remineralization. However, the highest trawling intensities are observed in offshore areas (>5 km from land), where sediments contain relatively small amounts of OC and are dominated by recalcitrant or refractory organic matter, limiting the potential impact of trawling on remineralization. In highly dynamic environments with low deposition rates and significant oxygen penetration, the additional impact of fishing-related disturbance on sediment organic carbon may be limited. These studies emphasize the need to pay attention not only to the amount of organic matter stored in sediments but also to its vulnerability, that is, its labile content, in modelling the effects of fishing on sediment carbon. Furthermore, the studies support the idea that in general, inshore and coastal sediments are more relevant in terms of interactions between fishing and sediment carbon sequestration (Smeaton and Austin, 2022). Recent research suggests that a comprehensive understanding of the effects of mobile gears on the seafloor necessitates the consideration of both the resuspension process and the biological and hydrological interactions that take place (Epstein et al., 2022; Paradis et al., 2021). Additionally, this understanding critically depends on factors such as carbon abundance, provenance, and recalcitrance (Graves et al., 2022). The cycling and storage of organic carbon at the seabed are influenced by a variety of factors, including sediment fauna, flora and microbes; seabed lithology and granulometry; and the chemistry, hydrology and biology. Bottom trawling can reduce organic carbon in sediments through increased resuspension, decreased flora and fauna production, the loss of fine flocculant material, mixing and transport, and increased oxygen exposure. However, these reductions can be offset to varying degrees by reduced faunal bioturbation and community respiration, increased off-shelf transport, and increases in primary production resulting from nutrients resuspension (Table 8). The relative importance of these processes depends on the context where fishing activities take place. Consequently, studies investigating the effects of demersal fishing on organic carbon stocks have produced mixed results, with 61% of 49 investigations finding no significant effect; 29% reporting lower organic carbon due to fishing activities, and 10% reporting higher organic carbon (Epstein et al., 2022). Overall, the conditions under which alterations to the processes identified as being produced by fish demersal fishing gears activities contribute significantly to changes in carbon sequestration of sediments remain highly uncertain. Table 8: Link between seabed sediment organic carbon and mobile demersal fishing (Adapted from Epstein et al., 2022) Main processes Alteration of production of benthic micro-and macroalgae Benthic faunal production and processing of organic carbon Alteration to sediment composition Sediment resuspension and transport Alteration in pelagic primary production The contribution of vertebrate fauna to organic carbon storage There are very few sedimentary ecological and biochemical coupled models available to assess the long-term carbon sequestration impact of trawling. A recent study utilizing North Sea data from 1950 to 2020, estimated that trawling- induced resuspension and the reduction of bioturbation collectively and progressively diminish the regional capacity for sedimentary organic carbon sequestration by 21-67%, equivalent to 0.58-1.84 Mt CO2yr-1 (Zhang et al., 2023) 25 Understanding the Role of the Fisheries and Aquaculture in Carbon Sequestration Of special relevance in shelf areas are the potential direct and indirect impacts of demersal fishing gears on seagrass- es. Seagrass meadows have the capacity to sequester substantial amounts of organic carbon, which depends not only on their biological characteristics (Duarte et al., 2010), but also on the physical and hydrological properties of seagrass meadows (Miyajima and Hamaguchi, 2019). However, if degraded, seagrass meadows can release stored carbon into the atmosphere, thereby accelerating global warming (Macreadie et al., 2014). Consequently, the direct impact of demersal fishing gears in these areas is a matter of concern. An indirect and relatively less studied aspect pertains to the impact on carbon storage resulting from the potential degradation of benthic primary producers such as seagrass meadows and kelp forests, which are crucial as habitat-forming species. Aside from their common role as food, benthic primary producers offer areas for spawning and nurseries (Dahlgren et al. 2006) and may act as climate refuges (Geraldo-Ospi- na et al. 2020). Their degradation can have repercussions on the entire fish food web, thus influencing the fish carbon pump. Although methodologies to quantify habitat degradation due to fishing is not straight forward (Auster 2008, Suka et al. 2020), the cassation of destructive fishing practices can lead to marked ecosystem recoveries (e.g., Bejarnaro et al. 2019, Lambert et al. 2014). 3.8 Marine benthic producer carbon sequestraton Although the topics of macroalgae carbon sequestration and aquaculture were not supposed to be included in this review, some key aspects are cursorily provided for important context. The role of macroalgae in carbon sequestration, and in particular, seaweed farming, has received increasing attention in recent years (DeAngelo et al., 2022; Fujita et al., 2022; Ross et al., 2022). An increasing number of studies are pointing out that seaweed can make a relevant contribution to coastal carbon sequestration at the regional and global levels (Bayley et al., 2021; Eger et al., 2023; Filbee-Dexter and Wernberg, 2020; Moura et al., 2019; Watanabe et al., 2020). Most macroalgae grow on rocks where burial is precluded but some grow on sandy sediments. When macroalgae detach from the benthos, a fraction of the carbon fixed during growth may sink to the deep ocean where it can be long-term sequestered. The extent to which this detrital carbon is remineralized - through grazing and microbial decomposition before reaching these sinks at depth - remains an active field of research. Figure 9 (Pedersen et al., 2021) shows these main pathways and Figure 10 shows a macroalgal seques- tration scheme. Figure 9: Conceptual diagram of the pathways for export and sequestration of macroalgal carbon. Each step of the carbon flow from global macroalgal net primary production (NPP) to carbon sequestration (in blue) is supported by the literature or inferred by a difference between a total and subcomponents supported by literature (Table 1). The means (with 25 to 75% quartile ranges in parentheses) shown are derived from an uncertainty propagation analysis (Methods), except for those fluxes not conducive to carbon sequestration (all values are in TgC yr–1). As the estimates have been derived independently, their total does not necessarily match to the mean global NPP estimate. Grazing (33.6% of the NPP) and remineralization (37.3% of the NPP) in the algal bed are adopted from a previous budget (Reproduced from Krause-Jensen and Duarte, 2016). 26 Understanding the Role of the Fisheries and Aquaculture in Carbon Sequestration Remineralized Buried in shelf NPP Grazed Exported Exported as DOC Exported as POC Remineralized in shelf Exported before mixed layer Retained in shelf Exported to deep sea Remineralized, grazed, Buried in shelf transported to the beach Figure 10: Macroalgal carbon sequestration scheme. Blue boxes indicate stocks that can contribute to carbon sequestra- tion (Reproduced from Krause-Jensen and Duarte, 2016). Since harvested macroalgae can be used as food, they may replace foods with higher carbon footprints, so their harvest can also provide an indirect benefit for carbon mitigation. Krause-Jensen and Duarte (2016) estimate that macroalgae could sequester about 173 TgC yr–1 (ranging 61–268 TgC yr–1). Globally, 90% of this sequestration occurs through export to the deep sea, and the rest through burial in coastal sediments (Krause-Jensen and Duarte, 2016), although the relative relevance may vary depending on the context studied. Figure 9 shows a general scheme on a macroalgae sequestration model following the same structure used in (Krause-Jensen and Duarte, 2016). Specific figures will depend on the context, but Table 9 shows some reference values based in global data obtained from Krause-Jensen and Duarte (2016). Table 9 - Marine macroalgae reference values. (Adapted from Krause-Jensen and Duarte, 2016) Minimum Maximum Mean Standard deviation NPP (gC m–2 yr–1) 91 750 420 165 Percentage of NPP buried in algae bed 0.4 0.54 Percentage of NPP exported from algal beds 43.5 48 DOC exported from algal beds (gC m–2 yr–1) 101 55 Percentage of DOC exported below the mixed layer 30 9 Percentage of POC exported to the deep sea 11 1.7 POC buried in shelf sediments (gC m–2 yr–1) 4.65 2.47 Macroalgae exhibit a broad phylogenetic and ecological diversity, comprising four phyla (Rhodophyta, Phaeophyta, Chlorophyta and Cyanophyta) and about 60 orders. Red and green algae are categorized within the plant kingdom, brown algae within the Chromista kingdom, and blue–green algae in the bacteria kingdom. The huge diversity in forms and size affects the fate of macroalgal carbon. For example, in kelp communities, on average, 82% of the local primary production is exported to adjacent communities, compared to the previously indicated 43.5% average. The differences between macroalgae extends to other key parameters that determines exports such buoyancy (Krause-Jensen et al., 2018), so specific information by species is required to evaluate carbon potentials, taking into consideration both the specific biological and physical characteristics. Moreover, findings suggest that it does not depend only on the macroalgae forms and size; it also depends on water exchange rates and local topographies (Harrold et al., 1998). Macroalgal beds in habitats associated with high water exchange rates can create significant CO2 sinks around them and export a substantial amount of DOC to offshore areas (Watanabe et al., 2020). Biotic interactions also play a significant role in their sequestration capacities (Wernberg and Filbee-Dexter, 2018). 27 Understanding the Role of the Fisheries and Aquaculture in Carbon Sequestration Caution should be exercised when globally evaluating macroalgae-kelp carbon sequestration estimates, as the inte- gration of all ecological interactions within seaweed ecosystems may impact the global carbon balance. Under certain circumstances, seaweed ecosystems may even become a carbon source to the water column (Gallagher et al., 2022). 28 Understanding the Role of the Fisheries and Aquaculture in Carbon Sequestration 4. ICES WORKSHOP ON ASSESSING THE IMPACT OF FISHING ON OCEANIC CARBON A Workshop on Assessing the Impact of Fishing on Oceanic Carbon (WKFISHCARBON) was organized and held by the International Council for Exploration of the Sea (ICES), Copenhagen, 25-27 April 2023. The workshop had participation of more ~35 experts in-person and an additional 25-30 remotely. Participation was mainly by European experts, but it should be clear that research on benthic processes and impact of fisheries related to carbon sequestration is most advanced in Europe. The workshop focused on: (1) Review and consolidate the existing knowledge, and identify knowledge gaps, on the functioning of the oceanic carbon pump in terms of the role of fish in carbon fluxes in the open ocean, including the extent of oceanic carbon released into the atmosphere due to the removal of fish; (2) Review and consolidate the existing knowledge on direct emissions from fishing fleets using different extraction methods, and indirect emissions from disturbance of the seabed, in terms of their contribution to climate change; and (3) Discuss how the existing approaches for assessing and prioritizing the main ecosystem stressors can be adapted to enable the assessment of fishing impacts on the carbon sequestration processes. The following lists the main findings from the workshop. 4.1 Carbon Pump There is considerable information available about the impact of the carbon pump on ocean systems, even if data availability is limited due to the high cost of collection and monitoring, and, in some cases, the infeasibility to collect data. In contrast, knowledge about shelf processes and the impact of fisheries on carbon sequestration is very limited (Zhang et al., 2023). Information about baseline carbon stores is generally in good condition for open ocean systems, but is incomplete for the seafloor with a notable knowledge gap on sediment processes (Luisetti et al., 2019). The impact of this is especially pronounced for shelf systems. The average residence time of a molecule of carbon in the ocean is of the order of hundreds of years. For a climate crisis, this may suffice, but fluxes are not synonymous with sequestration as only (an unknown) part of such fluxes will be sequestered. Processes that transport carbon deeper into the ocean are positive as it keeps carbon out of the atmosphere from a climate/carbon sequestration point of view. Environmental conditions impact the carbon pump including for seabed processes. How these processes vary with environmental conditions is unclear, but the workshop produced a risk and confidence assessment of environmental factors governing seabed carbon remineralization and storage (Table 10). Qualitatively, fish are important for carbon storage and flow, but quantitative values come with large uncertainty, and a recent review (Saba et al. 2021) found that only five published studies have provided either direct measurements or estimation of passive carbon flux of fish fecal pellets, and fewer than 10 studies have estimated active transport via DVM fishes. Fish communities are presumed to contribute to 16% of ocean carbon flux (Saba et al. 2021), and the majority of the carbon sequestered in the ocean has passed through a marine metazoan (Pinti et al. 2022). The impact of fish communities on carbon sequestration is much more than represented by biomass storage – organisms potential of sequestration is related to their metabolic activities, which is orders of magnitude greater than their biomass (Liu et al. 2022). It is thus the fish biomass plus all the ‘carbon products’ that they produce that counts. In fish, all flux parameters are a function of biomass and type of organism. Thus, abundance and distribution are important variables along with estimates of the proportion of fluxes (notably DOM and POM by type of organism) that is sequestered. It is unclear how fisheries change the stocks and pools of carbon. Fisheries will, during their development, change their gear from predominately targeting large, long-lived, slow-turnover individuals to targeting smaller, short- lived faster-turnover individuals (Christensen 1996). The impact of this on carbon sequestration is not clear. There are trade-offs between carbon, economics, yield, and cultural value (local ecological knowledge), calling for an evaluation of fish populations’ value as a provisioning service (e.g., food security) as well as a climate regulatory service (e.g., carbon sequestration) (Luisetti et al. 2019). WKFISHCARBON will recommend not to further develop mesopelagic fisheries, globally, due to their likely importance for carbon sequestration (Table 11). 29 Understanding the Role of the Fisheries and Aquaculture in Carbon Sequestration Table 10: Summary of key variables of biological carbon pump modelling, and why they matter Key variables key variables/ What are they / Why should we care processes being modelled Biomass distribution by specie or All fish undergo different metabolic processes that transform carbon, resulting in the groups of species production of POC, DOC, DIC, and carbonates at varying rates depending on the species. These productions form the foundation of fish's contribution to carbon sequestration. Consequently, a thorough characterization of fish carbon biomasses is crucial for any fish carbon model. Particulate Organic Carbon – POC Fish consume food and convert it into carbon-rich fecal pellets, which sink rapidly through the water column until they partially reaching the ocean floor. This process significantly contributes to fish carbon sequestration. Fish also consume POC. Therefore, a comprehensive understanding of POC production, sinking, and remineralization processes is essential in any modeling endeavor. Dissolved organic carbon – DOC While alive, fish produce DOC, which becomes integrated into the water column. Biological processes in the water column subsequently transform DOC into DIC, while physical processes determine its storage time in the ocean. The amount of DOC produced by marine fisheries remains poorly characterized. Dissolved inorganic carbon – DIC Through respiration, fish generate DIC, which mixes with water and is transported throughout the ocean. The duration of its storage in the ocean is determined by water flows and mixing processes. Carbonated Production Fish excrete fine-grained carbonates as a by-product of osmoregulation. These carbonates may appear as mucus-coated pellets or within feces. Their impact on carbon sequestration depends heavily on their solubility. Carcass production Mortality determines the number of marine organisms that die and naturally sink to the ocean floor, where carbon may remain sequestered for many years. Some experts suggest that the global impact of natural senescence in fish on carbon sequestration is presumed to be small, although a clear consensus has not yet been reached. Key processes What are they / Why should we care Biochemical transformations and A complex set of marine biochemical processes governs the transformation of POC and DOC remineralization into DIC (dissolved inorganic carbon), thereby influencing fish carbon sequestration. These processes are influenced by environmental variables such as oxygen levels, temperature, and others, resulting in variations depending on both the depth and region. Active vertical migration Some fish engage in vertical migrations on a daily or seasonal basis. This process enables the injected POC, DOC, and DIC to bypass certain active biochemical remineralization processes occurring in the upper layers of the ocean. These injections into lower marine layers can contribute significantly to the estimated global total biological pump sequestration Water mass flows The retention time of biologically ‘produced’ and biochemically transformed fish carbon is determined by water fluxes and advective-diffusive transport processes. Consequently, modeling fish carbon requires coupling with current-diffusive models. Fisheries-carbon sediment Subtidal marine sediments contain the ocean's largest organic carbon stock. However, interactions bottom trawling and dredging for fish and shellfish are significant disturbances in many areas. Currently, there are important scientific discussions taking place to better understand how these interrelations affect the carbon stored in the sediments and the degree of relevance of this interaction in terms of carbon sequestration. Macro benthic carbon There is increasing attention on the potential opportunities for carbon sequestration sequestration through macroalgae. Specifically, the conservation of kelp forests and certain macroalgae ecosystems may provide win-win opportunities for biodiversity and carbon sequestration. 30 Understanding the Role of the Fisheries and Aquaculture in Carbon Sequestration Table 11: Risk assessment and confidence assessment of environmental factors governing seabed carbon remineraliza- tion and storage. Draft table from the forthcoming 2023 WKFISHCARBON report, subject to change. Environmental Factor Vulnerability Severity of Confidence Impact Sea Temperatures Rising near seabed will increase sediment community oxygen Medium High consumption, driving higher carbon remineralization rates. Bottom water dissolved Bottom water oxygen concentrations are critical control on the High High Oxygen remineralization of organic matter. Decreases in bottom water oxygen will restriction the potential for aerobic metabolism and restrict remineralization rates at the seabed. Organic matter inputs Changes to the input of organic matter from the overlying water Low Low column, or lateral transport from rivers estuaries will directly affect seabed carbon sequestration potential. At present, there is a lack of consensus regarding definitions of organic matter lability, which make assessment of the potential residence times of freshly deposited carbon at the seabed challenging. Sedimentation rates There is a good understanding of the physical processes Low High and sediment transport that govern sedimentation and sediment transport through processes. fundamental processes that are predicted by the hydrodynamic regime, particle size and volume of suspended particles. Grain Size and Sediment Sediment grain size ? ? Type Sediment Organic Carbon High uncertainty about the spatial and temporal variability of High Low Content and stocks. seabed carbon stocks in shelf seas. Sediment Carbon Lability At present there is no consistent definition of sediment organic High Low matter quality. Method development required to provide operational tools for rapid assessment of organic matter quality (low-cost, rapid assessment of lability). Sediment Oxygen High High Penetration Organo-mineral Med Low associations Sediment Community High High Structure and Ecology Bioturbation and Under seabed ‘Good Environmental Status’, increased bioturbation High High Bioirrigation and bioirrigation will support the subduction of organic matter deeper into the sediment for storage. Physical Disturbance from Potential for fishing pressure to impact seabed carbon storage High Low Fishing is dependent on fishing metiér. There is likely to be a gradient of effects from static gear, demersal seine netting, otter trawl, beam trawl and dredging. There is lack of data on how sediment disturbance from fishing affects organic matter remineralization rates. Physical natural There is lack of data on how sediment disturbance from fishing High Low disturbance affects organic matter remineralization rates. 31 Understanding the Role of the Fisheries and Aquaculture in Carbon Sequestration 4.2 Fishing fleet emissions While evaluation of emissions from fishing fleets is outside the scope of this review, discussions at the WKFISHCARBON workshop are summarized here for context. At the high level, current knowledge about approximate fuel use and fuel efficiency by fishing vessels is reasonably well developed. There are global baselines for fuel use, but country-specific baselines and vessel-level information is lacking. The biggest knowledge gaps are for fuel consumption by inshore fisheries and bait fisheries. There is no clear framework for how to implement improvements, and there is a general lack of incentive for the fishing industry to implement fuel efficiency changes. There is a need to develop a framework/roadmap for how to implement improvements in terms of fuel efficiency (and thus carbon emissions), especially at the vessel level, and data collection and reporting on fuel use needs to be more clearly standardized. 4.3 Fishing impacts In general, there is good understanding of the processes that control organic matter remineralization and seabed carbon storage, and WKFISHCARBON participants generally agreed that physical seabed disturbance from fishing will drive sediment transport and has potential to facilitate remineralization. The impact level depends on substrate, fishing techniques, and fishing gear components, as not all fishing gear have an impact on the sediment, and not all fishing overlaps with areas of carbon accumulation. It is possible to identify areas that are likely to be at risk of seabed carbon loss based upon fundamental sediment geochemistry and benthic ecology. Bioturbation and macrobenthos can have important impacts on sediment carbon remineralization, but these impacts are context-dependent and are governed by community structure, temperature, dissolved oxygen levels, and disturbance regime. Last, the recent publication by Sala et al. (2021) was based on data-lite models for carbon remineralization from sediments, assuming that all carbon is available for remineralization. There was consensus at the workshop that the estimates for fishing impacts in the publication were upwards biased, which aligns to the literature review (section 3.7). 4.4 Further work At the WKFISHCARBON workshop, this World Bank related activity was presented as a foundation for collaboration to gather the information needed to evaluate ecosystem-level impacts of fisheries on carbon sequestration. While there was broad interest and while the needed information is central to the work of WKFISHCARBON, it was clear that knowledge gaps at present make it impossible to quantify the impact of fish and fisheries on carbon sequestration. At present, no-one has quantified fish contribution to carbon sequestration at the shelf ecosystem level – at least not using defensible, beyond back-of-the-envelope calculations. The status is that with the current knowledge, if cannot even be evaluated if a system with high biomass of long-lived species sequesters more than a system with lower biomass of long-lived and higher biomass of smaller species; it cannot be ascertained that current exploited systems sequester more than systems of times passed. Similar knowledge gaps exist for the actual impacts of bottom trawling fishing gear, with plenty of ongoing research but with conflicting or inconclusive outcomes. Fortunately, there is much such research ongoing, and the WKFISHCARBON working group will meet again in 2024 to continue the central and interdisciplinary discussions. The 2023 WKFISHCARBON workshop report, when published, will summarize the workshop findings and identified knowledge gaps in detail. 32 Understanding the Role of the Fisheries and Aquaculture in Carbon Sequestration 5. RELEVANT POLICY INITIATIVES While ‘coastal blue carbon’ policy initiatives are gaining momentum in the policy arena and are being supported by a network of multi-stakeholder initiatives such as the Blue Carbon Initiative coordinated by Conservation International (CI), the International Union for Conservation of Nature (IUCN), and the Intergovernmental Oceanographic Commission of the United Nations Educational, Scientific, and Cultural Organization (IOC-UNESCO, https://www. thebluecarboninitiative.org/); the Blue Forests project (https://gefblueforests.org/) - an initiative of the United Nations Environment Programme (UNEP) - managed by GRID-Arendal and supported by multiple private and public actors; and several specific project oriented actions in the context of international funding, such as the World Bank-ProBLUE program, https://www.worldbank.org/en/programs/problue). However, in general, the implementation of ‘ocean-fish blue carbon’ policy initiatives are still in their early stages. Critical for the development of actionable projects, many coastal blue carbon management projects that involve mangrove, tidal marsh and seagrass ecosystems potentially fall within different categories recognized by the Intergovernmental Panel on Climate Change (IPCC), either as wetlands or forest in the case of mangroves), enabling their inclusion within greenhouse gas accounting guidance of the IPCC. This potentially allows them to be beneficiaries of both voluntary and mandatory international carbon market schemes, increasing access to funding (Herr and Landis, 2016; Lovelock and Duarte, 2019). Other coastal projects, such as those involving kelp forests or unvegetated tidal flats, are not included in IPCC evaluation, but may be included in some voluntary schemes that might be eligible for carbon market finance at the mid-term (Vanderklift et al., 2022). However, this is not yet the case for fish carbon projects. Nevertheless, there are some first steps towards concrete policy initiatives that may pave the way for more holistic future approaches, some of which are described below. 5.1 United Kingdom The United Kingdom intends to designate the first three Highly Protected Marine Areas (HPMAs) in English waters by 6 July 2023 (UK Government, 2023). The decision on the designated areas was made through a participatory process from June 2022 through February 2023. Among the benefits of HPMAs, it is explicitly pointed out that they can help to increase carbon storage and sequestration. In practical terms, this idea has been implemented through the evaluation of the presence of habitats considered to be of importance to the long-term storage of carbon. The identified blue carbon habitats information was available in the technical reports and publicly available information prepared for the participation processes, and includes intertidal sand, muddy sand, subtidal sand, seagrass, saltmarsh, coastal sand dunes, kelp forest and rich muddy habitats. 5.2 European Union The European Commission has recently presented the action plan Protecting and Restoring Marine Ecosystems for Sustainable and Resilient Fisheries (European Commission, 2023). In this new action plan, it establishes that: • By the end of 2023, under EU environmental and fisheries law, the EU will define objectives and specific data needs for each sea basin to monitor the impact of fishing on ecosystems and carbon sequestration, involving authorities at regional level as appropriate, and then allocating sufficient funds for these activities. • During the 2021-2027 budget period the Commission will promote the use of funding to quantify the EU’s seabed carbon sequestration capacity and the potential impacts of bottom fishing. 5.3 International Council for the Exploration of the Seas ICES in a 2022 resolution asked WKFISHCARBON to identify how the knowledge on the role of fishing (by fish removals, seabed abrasion and emissions) could be translated to advice to inform ecosystem-based (fisheries) management (EBFM/EBM), and to develop a roadmap for what needs to be done next and whether further workshops would useful. 33 Understanding the Role of the Fisheries and Aquaculture in Carbon Sequestration 6. CONCLUSIONS This report summarizes the current state of scientific understanding of fish and fisheries contributions to carbon sequestration, based on a literature review and leading expert consultation via the March 2023 “Fish, Fisheries, and Carbon international workshop” of the Ocean Carbon & Biogeochemistry (OCB) Program, and the April 2023 ICES WKFISHCARBON workshop. The research conducted here identified that there is a clear consensus in the scientific literature and in scientific community that the marine biological pump plays a major role in the regulation of climate change, with a sequestration capacity of around 1300 PgC (Nowicki 2022), and a broad understanding of the main pathways by which the carbon is exported from the upper oceanic layer to the deeper ocean, with an export rate around 10 PgCyr−1 (section 5). There is also an agreement that qualitatively, fish are important for carbon storage and flow, but quantitative values come with large uncertainty (section 6 and 7). In these areas there are still significant knowledge gaps, especially for coastal and shelf areas, and the scientific community is actively striving to establish a comprehensive consensus on essential elements required for accurately evaluating the effects of fishing activities and policies on marine carbon sequestration. These gaps include the need to better characterize how different fish contribute to the various carbon flows. Carbon production depends on the biomass and type of organism. Thus, abundance and carbon distribution are important variables to be studied together with a good characterization of of POM and DOC production by type of organism Up to now this characterization is poorly understood (section 6.4 and 7). There are also relevant uncertainties on the biochemical transformations of the various carbon flows identified in the marine realm - that depend in complex ways on the environmental variables such as oxygen and temperature, and the POM characteristics that conditionate remineralization processes and sinking speeds. These processes are better understood in open ocean, while in coastal and shelf areas their characterization is more complex and require better data availability (section 6.5, section 7). While some improvements have been done in the last years there is a need to better integrate the role of active vertical migration of some marine species -especially in the context of the open ocean (section 6.6.), and the need to better characterize the interrelation between trawling activities and sedimentary ecosystems where there is still a high degree of uncertainty (sections 6.7 and 6.8). Specific difficulties have been identified arising from the fact that most fishing activities take place on the coast and marine shelf, where finer modelling resolutions are needed and key complex physicochemical and biological processes take place (Charette et al., 2016), leading to a high level of complexity in evaluating carbon sequestration times and as a consequence the role of fisheries in carbon sequestration (section 7). There is considerable research starting aimed at quantifying fish sequestration and fisheries interactions as part of policy initiatives to ensure that the impact of the marine biological pump is considered in the decision-making processes (section 8). While with the current state of knowledge it does not seems feasible in the short term to integrate new marine-fishing biological pump processes in the international carbon market schemes, the presence of habitats considered to be of importance to the long-term storage of carbon are starting to be taken into consideration both fishing and conservation policies. These policies will benefit from the scientific focus that is emerging, especially in improving the understanding of carbon sequestration times of the different carbon pathways. Acknowledging the existing uncertainties based on the current state of knowledge and based in the previous work, we can only provide some general conclusions regarding fisheries management and climate policies: 34 Understanding the Role of the Fisheries and Aquaculture in Carbon Sequestration Conclusion 1: The maintenance of healthy marine ecosystems contributes positively to climate change regulation. From a scientific perspective, it is clear that fish and fisheries through multiple complex pathways play a noticeable role in the biological marine carbon pump. The maintenance of healthy marine ecosystems contributes positively to climate change regulation. Therefore, the study of the role of the fish carbon pump supports the adoption of an ecosystem-based approach to fisheries and a precautionary approach in fisheries decision-making processes. This approach contributes to the good environmental status of marine ecosystem and supports global carbon sequestration processes. Conclusion 2: The coupling between marine oceanic modeling and biological interactions suggests that future management measures should pay special attention to areas, species, and processes (such as vertical migration) that contribute to both high exports and extended sequestration times. These areas may or may not coincide with current protected areas. Various potential management measures, including non-take areas, technical measures, and fishing fleet regulations should, among others, be evaluated based on the specific management target, employing a case-by-case approach. It is important for any fishing measure to adopt an integrative approach, and consider the interactions within the marine food web and the ecosystem. Conclusion 3: While, it is possible to identify areas that are likely to be at higher risk of seabed carbon loss based upon fundamental sediment geochemistry and benthic ecology, there is still a high level of uncertainty when it comes to quantify the benefits of specific fisheries management measures in these areas. The fishing impact level depends on factors such as substrate, fishing technique, fishing gear components, as not all fishing gear have the same impacts on the sediment, and other environmental and ecological conditions. Therefore, the implementation of specific measures will often depend in many cases on the precautionary level that is desired and the relation of these measures with other potential win-win benefits. Conclusion 4: The uncertainties associated with the current status of knowledge at present recommend against including any marine-fishing carbon pump process in carbon market schemes that may condition fisheries practices. However, due to the key role that mesopelagic fish species play in the marine ecosystem related to carbon sequestration, it is suggested not to develop new fisheries to target this unexploited resource. Conclusion 5: Further research is needed before co-benefit assessment can be provided for fisheries projects given the current modeling capacities are unable to accurately quantify the causal relations between fisheries policies and carbon sequestration benefits with enough precision. Conclusion 6: There is sufficient scientific evidence that the conservation of seaweeds ecosystems supports carbon sequestration processes, although their quantification remains uncertain. Nevertheless, the double dividend in terms of win-win biodiversity and climate change benefits recommends their conservation. This white paper concludes with a summary of how existing models could be enhanced to overcome some of the identified knowledge limitations. To accomplish this, the development of coupled food-web models with carbon sequestration models is proposed, aiming to attain a more realistic understanding of the implications of fishing activities for carbon sequestration. In the final section, this vision is elaborated on with recommendations and next steps. 35 Understanding the Role of the Fisheries and Aquaculture in Carbon Sequestration 7. RECOMMENDATIONS AND NEXT STEPS As identified in this document, research will be needed into how specific species in different environments contribute to carbon flows, into carbon sinking and benthic carbon sequestration, and in terms of fisheries, into better understanding of the role of fishing via fish removals, seabed abrasion and emissions, and into how management measures can con- tribute to stock and benthic community rebuilding and subsequent carbon sequestration. The resulting understanding of these dynamics will be fundamental to develop economic and social narratives on the larger development context of the carbon sequestration agenda. Science could be translated into advice to inform ecosystem-based (fisheries) management (EBFM/EBM) and is essential for identifying the relevant investments on fisheries and aquaculture, with associated job opportunities. The scientific gaps will need filling before any detailed roadmap for what needs to be done next can be developed with sufficient certainty. The World Bank Group is a provider of financial products and technical assistance to development projects around the globe, and with its mandate to facilitate the sharing and applying innovative knowledge, the World Bank is in a prime position to advocate for the global development of scientific understanding of fish and fisheries carbon sequestration at the global level, in particular with funding scientific capacity and knowledge building through the establishment of standard methodologies and databases. 7.1 Digital Resarch Platform To address the current knowledge gaps in fish and fisheries carbon sequestration, the scientific community needs a digital research platform that combines expert models interlinking hydrology, biochemistry, biology, fishing, and fishing interactions with the benthos to explore carbon flow dynamics. Such an platform could aid recently started research projects such as European Union Horizon 2020 projects Ocean-ICU (https://cordis.europa.eu/project/id/101083922/) and NECCTON (https://neccton.eu/) to improve carbon understanding, from individual species to entire ecosystems, for biomes at local and regional scales, from coastal areas to deep sea regions, and over time and space. Development of such a platform should initially center on a model that encompasses food web dynamics and fisheries to deliver the energy flows of off which carbon flows can be calculated. Given its capabilities, this model could be Eco- path with Ecosim (EwE; Christensen and Walters 2004, Heymans et al. 2016). EwE accounts for energy flows over time and space throughout marine food webs under influence of climate change and variability (Christensen et al., 2014), and incorporates human activities such as fishing and fisheries management (Walters et al., 1999). EwE already explicitly incorporates the energy flows and fishing dynamics from which fish and fisheries’ contributions to the various carbon flows can be computed. The open-source EwE approach is explicitly designed for model interoperability and extensi- bility (Steenbeek et al., 2016), which provides the means to interact with expert models that represent other aspects of the oceanic carbon cycle. The EwE approach is increasingly integrated with models that capture physical oceanography and biochemistry to proper understand the interplay between species, habitats, natural phenomena and anthropogenic stressors (Steenbeek et al. 2021). This makes the EwE approach a logical choice as a foundation for a fish and fisheries carbon sequestration digital research platform. It could be foreseen to extend the EwE approach with a carbon sequestration digital research platform that can be used to explore plausible quantifications of the fish carbon pump over time (via the EwE module Ecosim; Walters et al., 1997) and over time and space (via EwE module Ecospace; Walters et al., 1999). Based on the workings of EwE and the knowl- edge gaps identified, such a test environment should offer a number of specific capabilities. 7.. 1. 1 Quantify fish and fisheries carbon flows The EwE model requires specifying the biotic (animals and plants), abiotic (detritus) and human (fisheries) components and their interactions in a food web, accounting for predation, metabolic production processes such as growth, respi- ration, excretion, and decay, but also the impacts of fishing such as extraction and discarding (Christensen and Walters, 2004). Because the EwE approach explicitly accounts for the involved energy flows throughout the entire food web, it is well positioned to express these flows as contributions to the different carbon flows (POC, DOC, DIC, etc.), and how organisms consume from these flows. 36 Understanding the Role of the Fisheries and Aquaculture in Carbon Sequestration Researchers will need to quantify how different species contribute to the different carbon flows, and given the gaps in scientific understanding and uncertainty in the quantification of such flows, such quantification needs to be accompa- nied with plausible value ranges and/or uncertainty estimates to perform systematic and comprehensive uncertainty and validation assessments (Steenbeek et al., 2021). 7. 1. 2 Macroalgal dynamics In EwE, microalgae and other benthic primary producers are living components that produce, die, are consumed, and disperse where environmental conditions permit (Christensen and Walters 2004). Additionally, EwE is designed to incor- porate how living organisms utilize benthic macroalgae as a habitat in terms of feeding, recruitment, and shelter (Har- vey 2014). If distributions and quantities of microalgae biomasses change, EwE will calculate how these changes affect individual species that rely on benthic primary producers, with possible cascading effects throughout the food web. EwE can model how how fishing effort with specific benthic gears may affect macroalgae densities through a process called mediation. The model parameterization will, however, call on extensive data gathering and evaluation for param- etrization for different types of fisheries. 7. 1. 3 Interacting with the environment The EwE approach is designed to incorporate environmental static or changing environmental conditions. Similarly, the digital research platform should be able to collaborate with expert models that account for, among others, (global) cir- culation, benthic sequestration and resuspension rates, habitat disturbance to bring in the environmental context that affects the fish and fisheries carbon pump. As these models may in various stages of development, the digital research platform should provide basic assumptions about externally driving processes that can be bypassed once the required expert models have matured sufficiently for integration with the digital research platform. Refining and testing scientific hypotheses The digital research platform should first and foremost serve as a means to test hypotheses in support of developing the necessary science. As such, it could be of great benefit if the digital research platform were to be designed, imple- mented, tested and refined in close collaboration with carbon sequestration projects that use EwE. 7. 1. 4 Development timeline and wider impact The timeline for the development of the digital research platform will depend on the research projects that use and contribute to the platform. Once sufficiently mature, the carbon sequestration test environment can be deployed to as- sess potential carbon sequestration benefits of proposed fisheries management schemes, can be integrated into global modelling initiatives such as FishMIP (https://www.isimip.org/about/marine-ecosystems-fisheries/) that in turn feed into the IPCC reports, can be integrated into initiatives such as the European Digital Twin of the Oceans (Voossen, 2020), but should also be made freely available as an open source tool kit for further development by the global ecosystem modelling community where the carbon sequestration test environment can become a source of inspiration to other marine ecosystem modelling approaches. 7.2 Standardizing the dialogue Another valuable role for the World Bank Group could be in using its convening power to bring in key actors together to identify a process as lead coordination entity central dialogue to align oceanic carbon storage research around the world. Standardization of scientific approaches across research projects should be coordinated in order to fast-track the development and uptake of emergent science. Emerging data should be made publicly available in consolidated ecosys- tem based or region-specific databases such as FishBase and SeaLifeBase (regarding species Carbon flows), The Marine Stewardship Council (benthic interactions of fishing gear), Our World In Data or Copernicus (fishing greenhouse gas emissions), Global Fishing Watch (fishing intensities), etc.   37 Understanding the Role of the Fisheries and Aquaculture in Carbon Sequestration REFERENCES Anderson, T.R., Tang, K.W., 2010. Carbon cycling and POC turnover in the mesopelagic zone of the ocean: Insights from a simple model. Deep Sea Research Part II: Topical Studies in Oceanography 57, 1581–1592. https://doi.org/10.1016/J.DSR2.2010.02.024 Arjona-Camas, M., Puig, P., Palanques, A., Durán, R., White, M., Paradis, S., Emelianov, M., 2021. Natural vs. trawling-induced water turbidity and suspended sediment transport variability within the Palamós Canyon (NW Mediterranean). Marine Geophysical Research 42, 1–22. https://doi.org/10.1007/S11001-021-09457-7/TABLES/2 Atwood, T.B., Connolly, R.M., Ritchie, E.G., Lovelock, C.E., Heithaus, M.R., Hays, G.C., Fourqurean, J.W., Macreadie, P.I., 2015. Predators help protect carbon stocks in blue carbon ecosystems. Nature Climate Change 2015 5:12 5, 1038–1045. https://doi. org/10.1038/nclimate2763 Atwood, T.B., Witt, A., Mayorga, J., Hammill, E., Sala, E., 2020. Global Patterns in Marine Sediment Carbon Stocks. Front Mar Sci 7, 165. https://doi.org/10.3389/FMARS.2020.00165/BIBTEX Auster, P.J., 1998. A Conceptual Model of the Impacts of Fishing Gear on the Integrity of Fish Habitats. Conservation Biology 12, 1198–1203. https://doi.org/10.1046/j.1523-1739.1998.0120061198.x Baumas, C.M.J., Bizic, M., Baumas, C., 2023. Did you say marine snow? Zooming into different types of organic matter particles and their importance in the open ocean carbon cycle. https://doi.org/10.31223/X5RM1T Belcher, A., Henson, S.A., Manno, C., Hill, S.L., Atkinson, A., Thorpe, S.E., Fretwell, P., Ireland, L., Tarling, G.A., 2019. Krill faecal pellets drive hidden pulses of particulate organic carbon in the marginal ice zone. Nature Communications 2019 10:1 10, 1–8. https:// doi.org/10.1038/s41467-019-08847-1 Bejarano, S., Pardede, S., Campbell, S.J., Hoey, A.S., Ferse, S.C.A., 2019. Herbivorous fish rise as a destructive fishing practice falls in an Indonesian marine national park. Ecological Applications 29, e01981. https://doi.org/10.1002/eap.1981 Bianchi, D., Carozza, D.A., Galbraith, E.D., Guiet, J., DeVries, T., 2021. Estimating global biomass and biogeochemical cycling of marine fish with and without fishing. Sci Adv 7. https://doi.org/10.1126/SCIADV.ABD7554/SUPPL_FILE/SCIADV.ABD7554_DATA_ FILE_S1.ZIP Boyd, P.W., Claustre, H., Levy, M., Siegel, D.A., Weber, T., 2019. Multi-faceted particle pumps drive carbon sequestration in the ocean. Nature 2019 568:7752 568, 327–335. https://doi.org/10.1038/s41586-019-1098-2 Bray, R.N., Miller, A.C., Geesey, G.G., 1981. The Fish Connection: A Trophic Link Between Planktonic and Rocky Reef Communities? Science (1979) 214, 204–205. https://doi.org/10.1126/SCIENCE.214.4517.204 Buesseler, K.O., Boyd, P.W., 2009. Shedding light on processes that control particle export and flux attenuation in the twilight zone of the open ocean. Limnol Oceanogr 54, 1210–1232. https://doi.org/10.4319/LO.2009.54.4.1210 Butenschön, M., Clark, J., Aldridge, J.N., Allen, J.I., Artioli, Y., Blackford, J., Bruggeman, J., Cazenave, P., Ciavatta, S., Kay, S., 2016. ERSEM 15.06: a generic model for marine biogeochemistry and the ecosystem dynamics of the lower trophic levels. Geoscientific Model Development 9, 1293–1339. Carozza, D.A., Bianchi, D., Galbraith, E.D., 2017. Formulation, General Features and Global Calibration of a Bioenergetically-Constrained Fishery Model. PLOS ONE 12, e0169763. https://doi.org/10.1371/journal. pone.0169763 Butterfield, N.J., 2018. Oxygen, animals and aquatic bioturbation: An updated account. Geobiology 16, 3–16. https://doi.org/10.1111/ GBI.12267 Carter, B.R., Feely, R.A., Lauvset, S.K., Olsen, A., DeVries, T., Sonnerup, R., 2021. Preformed Properties for Marine Organic Matter and Carbonate Mineral Cycling Quantification. Global Biogeochem Cycles 35, e2020GB006623. https://doi. org/10.1029/2020GB006623 Carozza, D.A., Bianchi ¤b, D., Galbraith, E.D., 2017. Formulation, General Features and Global Calibration of a Bioenergetically- Constrained Fishery Model. PLoS One 12, e0169763. https://doi.org/10.1371/JOURNAL.PONE.0169763 Cavan, E.L., Belcher, A., Atkinson, A., Hill, S.L., Kawaguchi, S., McCormack, S., Meyer, B., Nicol, S., Ratnarajah, L., Schmidt, K., Steinberg, D.K., Tarling, G.A., Boyd, P.W., 2019. The importance of Antarctic krill in biogeochemical cycles. Nature Communications 2019 10:1 10, 1–13. https://doi.org/10.1038/s41467-019-12668-7 Cavan, E.L., Hill, S.L., 2022. Commercial fishery disturbance of the global ocean biological carbon sink. Glob Chang Biol 28, 1212–1221. https://doi.org/10.1111/GCB.16019 38 Understanding the Role of the Fisheries and Aquaculture in Carbon Sequestration Charette, M.A., Lam, P.J., Lohan, M.C., Kwon, E.Y., Hatje, V., Jeandel, C., Shiller, A.M., Cutter, G.A., Thomas, A., Boyd, P.W., Homoky, W.B., Milne, A., Thomas, H., Andersson, P.S., Porcelli, D., Tanaka, T., Geibert, W., Dehairs, F., Garcia-Orellana, J., 2016. Coastal ocean and shelf-sea biogeochemical cycling of trace elements and isotopes: lessons learned from GEOTRACES. Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 374. https://doi.org/10.1098/ RSTA.2016.0076 Christensen, V. 1996. Managing fisheries involving top predator and prey species components. Reviews in Fish Biology and Fisheries. 6:417-442. Christensen, V., Coll, M., Buszowski, J., Cheung, W.W.L., Frölicher, T., Steenbeek, J., Stock, C.A., Watson, R.A., Walters, C.J., 2015. The global ocean is an ecosystem: simulating marine life and fisheries. Global Ecology and Biogeography 24, 507–517. https://doi. org/10.1111/GEB.12281 Christensen, V., Coll, M., Piroddi, C., Steenbeek, J., Buszowski, J., Pauly, D., 2014. A century of fish biomass decline in the ocean. Mar Ecol Prog Ser 512, 155–166. https://doi.org/10.3354/MEPS10946 Christensen, V., Coll, M., Steenbeek, J., Buszowski, J., Chagaris, D., Walters, C.J., 2014. Representing Variable Habitat Quality in a Spatial Food Web Model. Ecosystems 17, 1397–1412. https://doi.org/10.1007/s10021-014-9803-3 Christensen, V., Walters, C.J., 2011. Progress in the use of ecosystem modeling for fisheries management. Ecosystem Approaches to Fisheries 189–206. https://doi.org/10.1017/CBO9780511920943.014 Ciais, P., C. Sabine, G. Bala, L. Bopp, V. Brovkin, J. Canadell, A. Chhabra, R. DeFries, J. Galloway, M. Heimann, C. Jones, C. Le Quéré, R.B. Myneni, S. Piao, P. Thornto, 2013. Carbon and Other Biogeochemical Cycles. In: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. Cabridge and New York. Clarke, A., 2019. Energy Flow in Growth and Production. Trends Ecol Evol 34, 502–509. https://doi.org/10.1016/J.TREE.2019.02.003 Countryman, C.E., Steinberg, D.K., Burd, A.B., 2022. Modelling the effects of copepod diel vertical migration and community structure on ocean carbon flux using an agent-based model. Ecol Modell 470, 110003. https://doi.org/10.1016/J. ECOLMODEL.2022.110003 Cram, J.A., Weber, T., Leung, S.W., McDonnell, A.M.P., Liang, J.H., Deutsch, C., 2018. The Role of Particle Size, Ballast, Temperature, and Oxygen in the Sinking Flux to the Deep Sea. Global Biogeochem Cycles 32, 858–876. https://doi.org/10.1029/2017GB005710 Dahlgren, C.P., Kellison, G.T., Adams, A.J., Gillanders, B.M., Kendall, M.S., Layman, C.A., Ley, J.A., Nagelkerken, I., Serafy, J.E., 2006. Marine nurseries and effective juvenile habitats: concepts and applications. Marine Ecology Progress Series 312, 291–295. https://doi.org/10.3354/meps312291 DeAngelo, J., Saenz, B.T., Arzeno-Soltero, I.B., Frieder, C.A., Long, M.C., Hamman, J., Davis, K.A., Davis, S.J., 2022. Economic and biophysical limits to seaweed farming for climate change mitigation. Nature Plants 2022 9:1 9, 45–57. https://doi.org/10.1038/ s41477-022-01305-9 DeVries, T., Weber, T., 2017. The export and fate of organic matter in the ocean: New constraints from combining satellite and oceanographic tracer observations. Global Biogeochem Cycles 31, 535–555. https://doi.org/10.1002/2016GB005551 Devries, T., 2014. The oceanic anthropogenic CO2 sink: Storage, air-sea fluxes, and transports over the industrial era. Global Biogeochem Cycles 28, 631–647. https://doi.org/10.1002/2013GB004739 DeVries, T., Primeau, F., 2011. Dynamically and Observationally Constrained Estimates of Water-Mass Distributions and Ages in the Global Ocean. J Phys Oceanogr 41, 2381–2401. https://doi.org/10.1175/JPO-D-10-05011.1 Dinauer, A., Laufkötter, C., Doney, S.C., Joos, F., 2022. What Controls the Large-Scale Efficiency of Carbon Transfer Through the Ocean’s Mesopelagic Zone? Insights From a New, Mechanistic Model (MSPACMAM). Global Biogeochem Cycles 36, e2021GB007131. https://doi.org/10.1029/2021GB007131 Duarte, C.M., Marbà, N., Gacia, E., Fourqurean, J.W., Beggins, J., Barrón, C., Apostolaki, E.T., 2010. Seagrass community metabolism: Assessing the carbon sink capacity of seagrass meadows. Global Biogeochem Cycles 24. https://doi.org/10.1029/2010GB003793 Eduardo Menschel, A., González, H.E., 2019. Carbon and Calcium Carbonate Export Driven by Appendicularian Faecal Pellets in the Humboldt Current System off Chile. Scientific Reports 2019 9:1 9, 1–12. https://doi.org/10.1038/s41598-019-52469-y Epstein, G., Middelburg, J.J., Hawkins, J.P., Norris, C.R., Roberts, C.M., 2022. The impact of mobile demersal fishing on carbon storage in seabed sediments. Glob Chang Biol 28, 2875–2894. https://doi.org/10.1111/GCB.16105 39 Understanding the Role of the Fisheries and Aquaculture in Carbon Sequestration Eger, A.M., Marzinelli, E.M., Beas-Luna, R., Blain, C.O., Blamey, L.K., Byrnes, J.E.K., Carnell, P.E., Choi, C.G., Hessing-Lewis, M., Kim, K.Y., Kumagai, N.H., Lorda, J., Moore, P., Nakamura, Y., Pérez-Matus, A., Pontier, O., Smale, D., Steinberg, P.D., Vergés, A., 2023. The value of ecosystem services in global marine kelp forests. Nature Communications 2023 14:1 14, 1–13. https://doi. org/10.1038/s41467-023-37385-0 Estes, E.R., Pockalny, R., D’Hondt, S., Inagaki, F., Morono, Y., Murray, R.W., Nordlund, D., Spivack, A.J., Wankel, S.D., Xiao, N., Hansel, C.M., 2019. Persistent organic matter in oxic subseafloor sediment. Nature Geoscience 2019 12:2 12, 126–131. https://doi.org/10.1038/s41561-018-0291-5 European Commission, 2023. Action plan: Protecting and restoring marine ecosystems for sustainable and resilient fisheries. European Commission, Brussels. Filbee-Dexter, K., Wernberg, T., 2020. Substantial blue carbon in overlooked Australian kelp forests. Scientific Reports 2020 10:1 10, 1–6. https://doi.org/10.1038/s41598-020-69258-7 Fujita, R., Collins, J., Kleisner, K., Rader, D., Mejaes, A., Augyte, S., Brittingham, P., 2022. Carbon sequestration by seaweed: Background paper for the Bezos Earth Fund EDF workshop on seaweed carbon sequestration. New York.Galí, M., Falls, M., Claustre, H., Aumont, O., Bernardello, R., 2022. Bridging the gaps between particulate backscattering measurements and modeled particulate organic carbon in the ocean. Biogeosciences 19, 1245–1275. https://doi.org/10.5194/BG-19-1245-2022 Gallagher, J.B., Shelamoff, V., Layton, C., 2022. Seaweed ecosystems may not mitigate CO2 emissions. ICES Journal of Marine Science 79, 585–592. https://doi.org/10.1093/ICESJMS/FSAC011 Giraldo-Ospina, A., Kendrick, G.A., Hovey, R.K., 2020. Depth moderates loss of marine foundation species after an extreme marine heatwave: could deep temperate reefs act as a refuge? Proceedings of the Royal Society B: Biological Sciences 287, 20200709. https://doi.org/10.1098/rspb.2020.0709 Ghilardi, M., Salter, M.A., Parravicini, V., Ferse, S.C.A., Rixen, T., Wild, C., Birkicht, M., Perry, C.T., Berry, A., Wilson, R.W., Mouillot, D., Bejarano, S., 2023. Temperature, species identity and morphological traits predict carbonate excretion and mineralogy in tropical reef fishes. Nature Communications 2023 14:1 14, 1–14. https://doi.org/10.1038/s41467-023- 36617-7 Gilman, E., Perez Roda, A., Huntington, T., Kennelly, S.J., Suuronen, P., Chaloupka, M., Medley, P.A.H., 2020. Benchmarking global fisheries discards. Scientific Reports 2020 10:1 10, 1–8. https://doi.org/10.1038/s41598-020-71021-x Graves, C.A., Benson, L., Aldridge, J., Austin, W.E.N., Dal Molin, F., Fonseca, V.G., Hicks, N., Hynes, C., Kröger, S., Lamb, P.D., Mason, C., Powell, C., Smeaton, C., Wexler, S.K., Woulds, C., Parker, R., 2022. Sedimentary carbon on the continental shelf: Emerging capabilities and research priorities for Blue Carbon. Front Mar Sci 9, 1642. https://doi.org/10.3389/ FMARS.2022.926215/BIBTEX Harrold, C., Light, K., Lisin, S., 1998. Organic enrichment of submarine-canyon and continental-shelf benthic communities by macroalgal drift imported from nearshore kelp forests. Limnol Oceanogr 43, 669–678. https://doi. org/10.4319/LO.1998.43.4.0669 Harvey, C.J., 2014. Mediation functions in Ecopath with Ecosim: handle with care. Can. J. Fisheries Aquatic Sciences 71, 1020–1029. https://doi.org/10.1139/cjfas-2013-0594 Herr, D., Landis, E., 2016. Coastal blue carbon ecosystems. Opportunities for Nationally Determined Contributions. Gland, Switzerland: IUCN and Washington, DC, USA: TNC. Heymans, J.J., Coll, M., Link, J.S., Mackinson, S., Steenbeek, J., Walters, C., Christensen, V., 2016. Best practice in Ecopath with Ecosim food-web models for ecosystem-based management. Ecol Modell 331, 173–184. https://doi.org/10.1016/J. ECOLMODEL.2015.12.007 Hiddink, J.G., van de Velde, S.J., McConnaughey, R.A., De Borger, E., O’Neill, B., Tiano, J., Kaiser, M.J., Sweetman, A.K., Sciberras, M., 2021. Quantifying the carbon benefits of ending bottom trawling. https://doi.org/10.6084/ M9.FIGSHARE.16722808.V1 40 Understanding the Role of the Fisheries and Aquaculture in Carbon Sequestration Hiddink, J.G., van de Velde, S.J., McConnaughey, R.A., De Borger, E., Tiano, J., Kaiser, M.J., Sweetman, A.K., Sciberras, M., 2023. Quantifying the carbon benefits of ending bottom trawling. Nature 617, E1–E2. https://doi.org/10.1038/s41586- 023-06014-7 Hilborn, R., Kaiser, M.J., 2022. A path forward for analysing the impacts of marine protected areas. Nature 2022 607:7917 607, E1–E2. https://doi.org/10.1038/s41586-022-04775-1 Ikeda, T., 2016. Routine metabolic rates of pelagic marine fishes and cephalopods as a function of body mass, habitat temperature and habitat depth. J Exp Mar Biol Ecol 480, 74–86. https://doi.org/10.1016/J.JEMBE.2016.03.012 Ikeda, T., 2014. Respiration and ammonia excretion by marine metazooplankton taxa: synthesis toward a global- bathymetric model. Mar Biol 161, 2753–2766. https://doi.org/10.1007/S00227-014-2540-5/FIGURES/2 IPCC, 2021. Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge, United Kingdom and New York. https://doi. org/10.1017/9781009157896 Ivlev V.S., 1955. Experimental ecology of fish nutrition. Pishchepromizdat, Moscow. Klevjer, T.A., Irigoien, X., Røstad, A., Fraile-Nuez, E., Benítez-Barrios, V.M., Kaartvedt., S., 2016. Large scale patterns in vertical distribution and behaviour of mesopelagic scattering layers. Scientific Reports 2016 6:1 6, 1–11. https://doi. org/10.1038/srep19873 Krabbe, N., Langlet, D., Belgrano, A., Villasante, S., 2022. Reforming International Fisheries Law Can Increase Blue Carbon Sequestration. Frontiers in Marine Science 9. https://doi.org/10.3389/fmars.2022.800972 Krause-Jensen, D., Duarte, C.M., 2016. Substantial role of macroalgae in marine carbon sequestration. Nature Geoscience 2016 9:10 9, 737–742. https://doi.org/10.1038/ngeo2790 Krause-Jensen, D., Lavery, P., Serrano, O., Marba, N., Masque, P., Duarte, C.M., 2018. Sequestration of macroalgal carbon: the elephant in the Blue Carbon room. Biol Lett 14, 23955–6900. https://doi.org/10.1098/RSBL.2018.0236 Kwon, E.Y., Primeau, F., Sarmiento, J.L., 2009. The impact of remineralization depth on the air–sea carbon balance. Nature Geoscience 2009 2:9 2, 630–635. https://doi.org/10.1038/ngeo612 Lambert, G.I., Jennings, S., Kaiser, M.J., Davies, T.W., Hiddink, J.G., 2014. Quantifying recovery rates and resilience of seabed habitats impacted by bottom fishing. Journal of Applied Ecology 51, 1326–1336. https://doi.org/10.1111/1365- 2664.12277 Laufkötter, C., John, J.G., Stock, C.A., Dunne, J.P., 2017. Temperature and oxygen dependence of the remineralization of organic matter. Global Biogeochem Cycles 31, 1038–1050. https://doi.org/10.1002/2017GB005643 Lebling, K., Northrop, E., McCormick, C., Bridgwater, E., 2022. Toward Responsible and Informed Ocean-Based Carbon Dioxide Removal: Research and Governance Priorities, World Resources Institute. World Resources Institute. https://doi.org/10.46830/WRIRPT.21.00090 Lee, T.R., Wood, W.T., Phrampus, B.J., 2019. A Machine Learning (kNN) Approach to Predicting Global Seafloor Total Organic Carbon. Global Biogeochem Cycles 33, 37–46. https://doi.org/10.1029/2018GB005992 Legge, O., Johnson, M., Hicks, N., Jickells, T., Diesing, M., Aldridge, J., Andrews, J., Artioli, Y., Bakker, D.C.E., Burrows, M.T., Carr, N., Cripps, G., Felgate, S.L., Fernand, L., Greenwood, N., Hartman, S., Kröger, S., Lessin, G., Mahaffey, C., Mayor, D.J., Parker, R., Queirós, A.M., Shutler, J.D., Silva, T., Stahl, H., Tinker, J., Underwood, G.J.C., Van Der Molen, J., Wakelin, S., Weston, K., Williamson, P., 2020. Carbon on the Northwest European Shelf: Contemporary Budget and Future Influences. Front Mar Sci 7, 143. https://doi.org/10.3389/FMARS.2020.00143/BIBTEX Leifeld, J., 2023. Carbon farming: Climate change mitigation via non-permanent carbon sinks. Journal of Environmental Management 339, 117893. https://doi.org/10.1016/j.jenvman.2023.117893 Lima, I.D., Lam, P.J., Doney, S.C., 2014. Dynamics of particulate organic carbon flux in a global ocean model. Biogeosciences 11, 1177–1198. https://doi.org/10.5194/BG-11-1177-2014 41 Understanding the Role of the Fisheries and Aquaculture in Carbon Sequestration Liu, Q., Zhou, L., Wu, Y., Huang, H., He, X., Gao, N., Zhang, L., 2022. Quantification of the carbon released by a marine fish using a carbon release model and radiocarbon. Mar Pollut Bull 181, 113908. https://doi.org/10.1016/J. MARPOLBUL.2022.113908 Lovelock, C.E., Duarte, C.M., 2019. Dimensions of Blue Carbon and emerging perspectives. Biol Lett 15, 23955–6900. https://doi.org/10.1098/RSBL.2018.0781 Luisetti T, Turner, R.K., Andrews, J.E., Jickells, T.D., Krager, S., Diesing, M., Paltriguera, L., Johnson, M. T., Parker, E. R. and Bakker, D. 2019. Quantifying and valuing carbon flows and stores in coastal and shelf ecosystems in the UK. Ecosystem Services 35(C): 67-76. https://doi.org/10.1016/j.ecoser.2018.10.013 Lutz, S.J., Martin, A.H., 2014. Fish carbon: exploring marine vertebrate carbon services. Arendal, Norway: GRID-Arendal. Macreadie, P.I., Baird, M.E., Trevathan-Tackett, S.M., Larkum, A.W.D., Ralph, P.J., 2014. Quantifying and modelling the carbon sequestration capacity of seagrass meadows – A critical assessment. Mar Pollut Bull 83, 430–439. https:// doi.org/10.1016/J.MARPOLBUL.2013.07.038 Mariani, G., Cheung, W.W.L., Lyet, A., Sala, E., Mayorga, J., Velez, L., Gaines, S.D., Dejean, T., Troussellier, M., Mouillot, D., 2020. Let more big fish sink: Fisheries prevent blue carbon sequestration-half in unprofitable areas. Sci Adv 6. https://doi.org/10.1126/SCIADV.ABB4848/SUPPL_FILE/ABB4848_SM.PDF Martin, J.H., Knauer, G.A., Karl, D.M., Broenkow, W.W., 1987. VERTEX: carbon cycling in the northeast Pacific. Deep Sea Research Part A. Oceanographic Research Papers 34, 267–285. https://doi.org/10.1016/0198-0149(87)90086-0 Mengual, B., Le Hir, P., Cayocca, F., Garlan, T., 2019. Bottom trawling contribution to the spatio-temporal variability of sediment fluxes on the continental shelf of the Bay of Biscay (France). Mar Geol 414, 77–91. https://doi.org/10.1016/J. MARGEO.2019.05.009 Miyajima, T., Hamaguchi, M., 2019. Carbon Sequestration in Sediment as an Ecosystem Function of Seagrass Meadows. Blue Carbon in Shallow Coastal Ecosystems 33–71. https://doi.org/10.1007/978-981-13-1295-3_2 Moura, A., Os, Q., Stephens, Nicholas, Widdicombe, Stephen, Tait, Karen, Mccoy, Sophie J, Ingels, Jeroen, Uhl, S.R.€, Airs, Ruth, Beesley, Amanda, Carnovale, Giorgia, Cazenave, Pierre, Dashfield, Sarah, Hua, E.R., Jones, Mark, Lindeque, P., Mcneill, Caroline L, Nunes, Joana, Parry, Helen, Pascoe, Christine, Widdicombe, Claire, Smyth, Tim, Atkinson, Angus, Krause-Jensen, Dorte, Somerfield, Paul J, Queir, M., Stephens, N, Widdicombe, S, Tait, K, Mccoy, S J, Ingels, J, R€ Uhl, S., Airs, R, Beesley, A, Carnovale, G, Cazenave, P, Dashfield, S, Hua, E., Jones, M, Linde-Que, P., Mcneill, C L, Nunes, J, Parry, H, Pascoe, C, Widdicombe, C, Smyth, T, Atkinson, A, Krause-Jensen, D, Somerfield, P J, 2019. Connected macroalgal-sediment systems: blue carbon and food webs in the deep coastal ocean. Ecol Monogr 89, e01366. https://doi.org/10.1002/ECM.1366 Nowicki, M., DeVries, T., Siegel, D.A., 2022. Quantifying the Carbon Export and Sequestration Pathways of the Ocean’s Biological Carbon Pump. Global Biogeochem Cycles 36, e2021GB007083. https://doi.org/10.1029/2021GB007083 Ocean Carbon and Biogeochemistry program, 2023. Fish, fisheries and carbon: Ocean Carbon & Biogeochemistry [WWW Document]. URL https://www.us-ocb.org/fish-fisheries-and-carbon/ (accessed 3.13.23). Omand, M.M., Govindarajan, R., He, J., Mahadevan, A., 2020. Sinking flux of particulate organic matter in the oceans: Sensitivity to particle characteristics. Scientific Reports 2020 10:1 10, 1–16. https://doi.org/10.1038/s41598-020-60424-5 Paradis, S., Goñi, M., Masqué, P., Durán, R., Arjona-Camas, M., Palanques, A., Puig, P., 2021. Persistence of Biogeochemical Alterations of Deep-Sea Sediments by Bottom Trawling. Geophys Res Lett 48, e2020GL091279. https://doi.org/https:// doi.org/10.1029/2020GL091279 Paradis, S., Puig, P., Masqué, P., Juan-Diáz, X., Martín, J., Palanques, A., 2017. Bottom-trawling along submarine canyons impacts deep sedimentary regimes. Scientific Reports 2017 7:1 7, 1–12. https://doi.org/10.1038/srep43332 Pauli, N.C., Flintrop, C.M., Konrad, C., Pakhomov, E.A., Swoboda, S., Koch, F., Wang, X.L., Zhang, J.C., Brierley, A.S., Bernasconi, M., Meyer, B., Iversen, M.H., 2021. Krill and salp faecal pellets contribute equally to the carbon flux at the Antarctic Peninsula. Nature Communications 2021 12:1 12, 1–12. https://doi.org/10.1038/s41467-021-27436-9 42 Understanding the Role of the Fisheries and Aquaculture in Carbon Sequestration Pauly, D., Christensen, V., 1995. Primary production required to sustain global fisheries. Nature 1995 374:6519 374, 255– 257. https://doi.org/10.1038/374255a0 Pearson, H., 2021. Assessment of Oceanic Blue Carbon in the UAE: Biomass Carbon Mechanism with a Focus on Abu Dhabi Emirates. Arendal, Norway. Pearson, H.C., Savoca, M.S., Costa, D.P., Lomas, M.W., Molina, R., Pershing, A.J., Smith, C.R., Villaseñor-Derbez, J.C., Wing, S.R., Roman, J., 2023. Whales in the carbon cycle: can recovery remove carbon dioxide? Trends Ecol Evol 38, 238–249. https://doi.org/10.1016/J.TREE.2022.10.012Pedersen, M.F., Filbee-Dexter, K., Frisk, N.L., Sárossy, Z., Wernberg, T., 2021. Carbon sequestration potential increased by incomplete anaerobic decomposition of kelp detritus. Mar Ecol Prog Ser 660, 53–67. https://doi.org/10.3354/MEPS13613 Pershing, A.J., Christensen, L.B., Record, N.R., Sherwood, G.D., Stetson, P.B., 2010. The Impact of Whaling on the Ocean Carbon Cycle: Why Bigger Was Better. PLoS One 5, e12444. https://doi.org/10.1371/JOURNAL.PONE.0012444 Petrik, C.M., Stock, C.A., Andersen, K.H., van Denderen, P.D., Watson, J.R., 2019. Bottom-up drivers of global patterns of demersal, forage, and pelagic fishes. Progress in Oceanography 176, 102124. https://doi.org/10.1016/j. pocean.2019.102124 Pinnegar, J.K., Polunin, N.V.C., Videler, J.J., de Wiljes, J.J., 2007. Daily carbon, nitrogen and phosphorus budgets for the Mediterranean planktivorous damselfish Chromis chromis. J Exp Mar Biol Ecol 352, 378–391. https://doi.org/10.1016/J. JEMBE.2007.08.016 Pinti, J., DeVries, T., Norin, T., Serra-Pompei, C., Proud, R., Siegel, D.A., Kiorboe, T., Petrik, C., Andersen, K.H., Brierley, A.S., Visser, A.W., 2022. Model estimates of metazoans’ contributions to the biological carbon pump. Preprint egusphere-2022-1227. Pinti, J., DeVries, T., Norin, T., Serra-Pompei, C., Proud, R., Siegel, D.A., Kiørboe, T., Petrik, C.M., Andersen, K.H., Brierley, A.S., Visser, A.W., 2023. Model estimates of metazoans’ contributions to the biological carbon pump. Biogeosciences 20, 997–1009. https://doi.org/10.5194/BG-20-997-2023 Primeau, F., 2006a. On the variability of the exponent in the power law depth dependence of POC flux estimated from sediment traps. Deep Sea Research Part I: Oceanographic Research Papers 53, 1335–1343. https://doi.org/10.1016/J. DSR.2006.06.003 Primeau, F., 2006b. On the variability of the exponent in the power law depth dependence of POC flux estimated from sediment traps. Deep Sea Research Part I: Oceanographic Research Papers 53, 1335–1343. https://doi.org/10.1016/J. DSR.2006.06.003 Proud, R., Handegard, N.O., Kloser, R.J., Cox, M.J., Brierley, A.S., Demer, D., 2019. From siphonophores to deep scattering layers: uncertainty ranges for the estimation of global mesopelagic fish biomass. ICES Journal of Marine Science 76, 718–733. https://doi.org/10.1093/ICESJMS/FSY037 Puig, P., Canals, M., Company, J.B., Martín, J., Amblas, D., Lastras, G., Palanques, A., Calafat, A.M., 2012. Ploughing the deep sea floor. Nature 2012 489:7415 489, 286–289. https://doi.org/10.1038/nature11410 Robison, B.H., Bailey, T.G., 1981. Sinking rates and dissolution of midwater fish fecal matter. Mar Biol 65, 135–142. https:// doi.org/10.1007/BF00397077/METRICS Ross, F., Tarbuck, P., Macreadie, P.I., 2022. Seaweed afforestation at large-scales exclusively for carbon sequestration: Critical assessment of risks, viability and the state of knowledge. Front Mar Sci 9, 2269. https://doi.org/10.3389/ FMARS.2022.1015612/BIBTEX Saba, G.K., Burd, A.B., Dunne, J.P., Hernández-León, S., Martin, A.H., Rose, K.A., Salisbury, J., Steinberg, D.K., Trueman, C.N., Wilson, R.W., Wilson, S.E., 2021. Toward a better understanding of fish-based contribution to ocean carbon flux. Limnol Oceanogr 66, 1639–1664. https://doi.org/10.1002/LNO.11709 Saba, G.K., Steinberg, D.K., 2012. Abundance, Composition and Sinking Rates of Fish Fecal Pellets in the Santa Barbara Channel. Scientific Reports 2012 2:1 2, 1–6. https://doi.org/10.1038/srep00716 43 Understanding the Role of the Fisheries and Aquaculture in Carbon Sequestration Sala, E., Mayorga, J., Bradley, D., Cabral, R.B., Atwood, T.B., Auber, A., Cheung, W., Costello, C., Ferretti, F., Friedlander, A.M., Gaines, S.D., Garilao, C., Goodell, W., Halpern, B.S., Hinson, A., Kaschner, K., Kesner-Reyes, K., Leprieur, F., McGowan, J., Morgan, L.E., Mouillot, D., Palacios-Abrantes, J., Possingham, H.P., Rechberger, K.D., Worm, B., Lubchenco, J., 2021. Protecting the global ocean for biodiversity, food and climate. Nature 2021 592:7854 592, 397– 402. https://doi.org/10.1038/s41586-021-03371-z Salter, M.A., Perry, C.T., Smith, A.M., 2019. Calcium carbonate production by fish in temperate marine environments. Limnol Oceanogr 64, 2755–2770. https://doi.org/10.1002/LNO.11339 Serra-Pompei, C., Ward, B.A., Pinti, J., Visser, A.W., Kiørboe, T., Andersen, K.H., 2022. Linking Plankton Size Spectra and Community Composition to Carbon Export and Its Efficiency. Global Biogeochem Cycles 36, e2021GB007275. https:// doi.org/10.1029/2021GB007275 Siegel, D.A., DeVries, T., Cetinić, I., Bisson, K.M., 2023. Quantifying the Ocean’s Biological Pump and Its Carbon Cycle Impacts on Global Scales. Ann Rev Mar Sci 15, 329–356. https://doi.org/10.1146/annurev-marine-040722-115226 Siegel, D.A., Devries, T., Doney, S.C., Bell, T., 2021. Assessing the sequestration time scales of some ocean-based carbon dioxide reduction strategies. Environmental Research Letters 16. https://doi.org/10.1088/1748-9326/ac0be0 Smeaton, C., Austin, W.E.N., 2022. Quality Not Quantity: Prioritizing the Management of Sedimentary Organic Matter Across Continental Shelf Seas. Geophys Res Lett 49. https://doi.org/10.1029/2021GL097481 Smith, C.R., Glover, A.G., Treude, T., Higgs, N.D., Amon, D.J., 2015. Whale-Fall Ecosystems: Recent Insights into Ecology, Paleoecology, and Evolution. https://doi.org/10.1146/annurev-marine-010213-135144 7, 571–596. https://doi. org/10.1146/ANNUREV-MARINE-010213-135144 Spiers, E.K.A., Stafford, R., Ramirez, M., Vera Izurieta, D.F., Cornejo, M., Chavarria, J., 2016. Potential role of predators on carbon dynamics of marine ecosystems as assessed by a Bayesian belief network. Ecol Inform 36, 77–83. https://doi. org/10.1016/J.ECOINF.2016.10.003 Stafford, R., Boakes, Z., Hall, A.E., Jones, G.C.A., 2021. The Role of Predator Removal by Fishing on Ocean Carbon Dynamics. Anthropocene Science 2021 1:1 1, 204–210. https://doi.org/10.1007/S44177-021-00005-X Staresinic, N., Farrington, J., Gagosian, R.B., Clifford, C.H., Hulburt, E.M., 1983. DOWNWARD TRANSPORT OF PARTICULATE MATTER IN THE PERU COASTAL UPWELLING: ROLE OF THE ANCHOVETA, ENGRAULTS RINGENS. NATO Conference Series, (Series) 4: Marine Sciences 10 A, 225–240. https://doi.org/10.1007/978-1-4615-6651-9_12/ COVER Steenbeek, J., Buszowski, J., Christensen, V., Akoglu, E., Aydin, K., Ellis, N., Felinto, D., Guitton, J., Lucey, S., Kearney, K., Mackinson, S., Pan, M., Platts, M., Walters, C., 2016. Ecopath with Ecosim as a model-building toolbox: Source code capabilities, extensions, and variations. Ecological Modelling 319, 178–189. https://doi.org/10.1016/j. ecolmodel.2015.06.031 Steenbeek, J., Buszowski, J., Chagaris, D., Christensen, V., Coll, M., Fulton, E.A., Katsanevakis, S., Lewis, K.A., Mazaris, A.D., Macias, D., de Mutsert, K., Oldford, G., Pennino, M.G., Piroddi, C., Romagnoni, G., Serpetti, N., Shin, Y.-J., Spence, M.A., Stelzenmüller, V., 2021. Making spatial-temporal marine ecosystem modelling better – A perspective. Environmental Modelling & Software 145, 105209. https://doi.org/10.1016/j.envsoft.2021.105209 Steinberg, D.K., Landry, M.R., 2017. Zooplankton and the Ocean Carbon Cycle. https://doi.org/10.1146/annurev- marine-010814-015924 9, 413–444. https://doi.org/10.1146/ANNUREV-MARINE-010814-015924 Stock, C.A., Dunne, J.P., John, J.G., 2014. Global-scale carbon and energy flows through the marine planktonic food web: An analysis with a coupled physical–biological model. Progress in Oceanography 120, 1–28. https://doi.org/10.1016/j. pocean.2013.07.001 Stock, C.A., John, J.G., Rykaczewski, R.R., Asch, R.G., Cheung, W.W.L., Dunne, J.P., Friedland, K.D., Lam, V.W.Y., Sarmiento, J.L., Watson, R.A., 2017. Reconciling fisheries catch and ocean productivity. Proc Natl Acad Sci U S A 114, E1441–E1449. https://doi.org/10.1073/PNAS.1610238114/SUPPL_FILE/PNAS.1610238114.SM01.MOV 44 Understanding the Role of the Fisheries and Aquaculture in Carbon Sequestration Stukel, M.R., Décima, M., Landry, M.R., 2022. Quantifying biological carbon pump pathways with a data-constrained mechanistic model ensemble approach. Biogeosciences 19, 3595–3624. https://doi.org/10.5194/BG-19-3595-2022 Suka, R., Huntington, B., Morioka, J., O’Brien, K., Acoba, T., 2020. Successful application of a novel technique to quantify negative impacts of derelict fishing nets on Northwestern Hawaiian Island reefs. Marine Pollution Bulletin 157, 111312. https://doi.org/10.1016/j.marpolbul.2020.111312 Sulpis, O., Jeansson, E., Dinauer, A., Lauvset, S.K., Middelburg, J.J., 2021. Calcium carbonate dissolution patterns in the ocean. Nature Geoscience 2021 14:6 14, 423–428. https://doi.org/10.1038/s41561-021-00743-y UK Government, 2023. Highly Protected Marine Areas (HPMAs) - GOV.UK [WWW Document]. URL https://www.gov.uk/ government/publications/highly-protected-marine-areas/highly-protected-marine-areas-hpmas (accessed 3.12.23). Vanderklift, M.A., Herr, D., Lovelock, C.E., Murdiyarso, D., Raw, J.L., Steven, A.D.L., 2022. A Guide to International Climate Mitigation Policy and Finance Frameworks Relevant to the Protection and Restoration of Blue Carbon Ecosystems. Front Mar Sci 9. https://doi.org/10.3389/fmars.2022.872064 Voosen, P., 2020. Europe builds ‘digital twin’ of Earth to hone climate forecasts. Science 370, 16–17. https://doi.org/10.1126/ science.370.6512.16 Wagner, S., Schubotz, F., Kaiser, K., Hallmann, C., Waska, H., Rossel, P.E., Hansman, R., Elvert, M., Middelburg, J.J., Engel, A., Blattmann, T.M., Catalá, T.S., Lennartz, S.T., Gomez-Saez, G. V., Pantoja-Gutiérrez, S., Bao, R., Galy, V., 2020. Soothsaying DOM: A Current Perspective on the Future of Oceanic Dissolved Organic Carbon. Front Mar Sci 7, 341. https://doi.org/10.3389/FMARS.2020.00341/BIBTEX Watanabe, K., Yoshida, G., Hori, M., Umezawa, Y., Moki, H., Kuwae, T., 2020. Macroalgal metabolism and lateral carbon flows can create significant carbon sinks. Biogeosciences 17, 2425–2440. https://doi.org/10.5194/BG-17-2425-2020 Walters, C.J., Christensen, V., Pauly, D., 1997. Structuring dynamic models of exploited ecosystems from trophic mass- balance assessments. Reviews in fish biology and fisheries 7, 139–172 Walters, C.J., Pauly, D., Christensen, V., 1999. Ecospace: Prediction of Mesoscale Spatial Patterns in Trophic Relationships of Exploited Ecosystems, with Emphasis on the Impacts of Marine Protected Areas. Ecosystems 2, 539–554. https:// doi.org/10.1007/s100219900101 Wernberg, T., Filbee-Dexter, K., 2018. Grazers extend blue carbon transfer by slowing sinking speeds of kelp detritus. Scientific Reports 2018 8:1 8, 1–7. https://doi.org/10.1038/s41598-018-34721-z Wilson, J.D., Andrews, O., Katavouta, A., de Melo Viríssimo, F., Death, R.M., Adloff, M., Baker, C.A., Blackledge, B., Goldsworth, F.W., Kennedy-Asser, A.T., Liu, Q., Sieradzan, K.R., Vosper, E., Ying, R., 2022. The biological carbon pump in CMIP6 models: 21st century trends and uncertainties. Proc Natl Acad Sci U S A 119. https://doi.org/10.1073/ PNAS.2204369119 Wilson, R.W., Millero, F.J., Taylor, J.R., Walsh, P.J., Christensen, V., Jennings, S., Grosell, M., 2009. Contribution of fish to the marine inorganic carbon cycle. Science 323, 359–362. https://doi.org/10.1126/SCIENCE.1157972 Woosley, R.J., Millero, F.J., Grosell, M., 2012. The solubility of fish-produced high magnesium calcite in seawater. J Geophys Res Oceans 117, 4018. https://doi.org/10.1029/2011JC007599 Zhang, W., Porz, L., Yilmaz, R., Kuhlmann, J., Neumann, A., Liu, B., Müller, D., Spiegel, T., Holtappels, M., Ziebarth, N., Taylor, B., Wallmann, K., Kasten, S., Daewel, U., Schrum, C., 2023. Impact of bottom trawling on long-term carbon sequestration in shelf sea sediments. Authorea Preprints. https://doi.org/10.22541/ESSOAR.167578408.84551876/V1 45