Four ways blue foods can help achieve food system ambitions across nations

Blue foods, sourced in aquatic environments, are important for the economies, livelihoods, nutritional security and cultures of people in many nations. They are often nutrient rich1, generate lower emissions and impacts on land and water than many terrestrial meats2, and contribute to the health3, wellbeing and livelihoods of many rural communities4. The Blue Food Assessment recently evaluated nutritional, environmental, economic and justice dimensions of blue foods globally. Here we integrate these findings and translate them into four policy objectives to help realize the contributions that blue foods can make to national food systems around the world: ensuring supplies of critical nutrients, providing healthy alternatives to terrestrial meat, reducing dietary environmental footprints and safeguarding blue food contributions to nutrition, just economies and livelihoods under a changing climate. To account for how context-specific environmental, socio-economic and cultural aspects affect this contribution, we assess the relevance of each policy objective for individual countries, and examine associated co-benefits and trade-offs at national and international scales. We find that in many African and South American nations, facilitating consumption of culturally relevant blue food, especially among nutritionally vulnerable population segments, could address vitamin B12 and omega-3 deficiencies. Meanwhile, in many global North nations, cardiovascular disease rates and large greenhouse gas footprints from ruminant meat intake could be lowered through moderate consumption of seafood with low environmental impact. The analytical framework we provide also identifies countries with high future risk, for whom climate adaptation of blue food systems will be particularly important. Overall the framework helps decision makers to assess the blue food policy objectives most relevant to their geographies, and to compare and contrast the benefits and trade-offs associated with pursuing these objectives.

Given the diverse contribution of blue foods to society, the role that they can play in the transition to healthier, more just and less environmentally harmful food systems is an important question for both public and private decision makers. Yet, blue foods have remained remarkably absent from many contemporary food system discussions and policies on both nature and nutrition-positive outcomes [5][6][7][8] . When included, their representation is often simplified and reduced to a few types of 'fish' in dietary recommendations 9 and demand projections 10 . Similarly, ocean policy often neglects blue food contributions to human nutrition and benefits to communities producing them 11,12 . Deeper appreciation and understanding of the roles blue foods can play is essential for informing policy development that can harness their unique capacity for addressing nutritional, social and environmental food system challenges, while navigating the trade-offs of pursuing these different roles, within and across countries.
Blue foods are immensely diverse. More than 2,200 wild species are caught and more than 600 are farmed 13 , with tremendous variation in associated production and processing systems and practices 2,14 .
Aquatic food consumption profiles of nations are also remarkably diverse 10 . This diversity means that blue foods vary substantially in their contributions to human health, nutrition 1 , jobs 15 and culture 16 and their environmental impacts 2 . Natural variations in blue food diversity and abundance are compounded by social structures that exacerbate inequities 17 across socio-economic and geographic contexts. Diversity can bolster the resilience of blue foods to shocks 18,19 , but such resilience is unevenly represented across countries at present 20 .
In this diversity lies the key to understanding the geographic contexts and conditions whereby blue foods can contribute to achieving food system ambitions, such as improved nutrition, equity and lowered environmental impact-as articulated by high-level processes 21,22 and the Sustainable Development Goals of the United Nations 23 .
This paper integrates the findings of an initiative to assess the multiple roles animal-sourced blue foods play in food systems around the world (the Blue Food Assessment; https://www.bluefood.earth/) and translates them into four policy objectives that could help realize the positive health, environment, resilience and equity contributions of aquatic foods worldwide. We assess the relevance of these policy objectives for individual countries, and then examine the co-benefits and trade-offs associated with policy objectives at national and international scales. In doing so, we provide a guiding framework for decision makers across public and private spheres to assess blue food policy objectives most relevant to their geographies, and compare and contrast the benefits and trade-offs associated with pursuing these objectives.

Four ways blue foods improve food systems
The Blue Food Assessment examined the roles of blue foods in current and future food systems globally. It brought together more than 100 scholars across a wide range of disciplines to investigate the nutritional contribution of blue foods 1 , current and future demand 10 , and the environmental impacts of blue food production 2 , as well as the vulnerability of this production to environmental stressors 24 (L.C., manuscript in preparation). It synthesized key dimensions characterizing the small-scale fisheries and aquaculture (SSFA) actors 14 who produce two-thirds of aquatic foods destined for human consumption 14,25 , and evaluated injustices across the blue food system to identify policy attributes that support more equitable access to blue food benefits 17 . It also assessed the climate risks posed to nutritional, social, economic and environmental outcomes of blue food systems worldwide 20 . Finally, it explored how supporting the capabilities of actors, small and large, across the supply chains can build adaptive capacity to support a wider food system transformation (S.R.B., manuscript in preparation). This multi-perspective assessment is unique, and together with the large body of previous research helps crystallize the diverse functions blue foods play at present, and how these can be leveraged to support a food system transformation. These functions include the following.

Sources of critical nutrients
Blue foods are rich in many essential nutrients 26 . Like other animalsource foods, blue foods can enhance bioavailability of nutrients in plant-based food sources, depending on how they are combined with other foods 27 . Where blue foods are accessible and consumed in adequate quantities, they can promote nutrition by reducing deficiencies of a range of nutrients, most notably vitamin B 12 and the omega-3 long-chain polyunsaturated fatty acids docosahexaenoic acid and eicosapentaenoic acid (DHA and EPA; hereafter, fatty acids), in which blue foods are generally rich. These are among the nutrients noted as important for human nutrition 21 , showing relatively high levels of deficiency globally 1 (Extended Data Fig. 1), and blue foods are projected to contribute a global average of approximately 27% and 100% of omega-3 fatty acids and vitamin B 12 , respectively, by 2030 (ref. 1 ). Addressing these deficiencies is particularly important among vulnerable demographic groups, such as young children and older people, pregnant women and women of childbearing age 28,29 . Alongside other health-critical foods, blue foods can thus make essential contributions to maintaining and improving nutritional food system outcomes 30 . Capture fisheries constitute the last large wild-food resource. Failing to sustain it will jeopardize food security in many places and it will be challenging to replace without negative environmental consequences.

Healthy alternatives to terrestrial animal-source foods
By adding to the range of food sources associated with relative reductions in many non-communicable diseases [31][32][33] , blue foods can help to circumvent the harmful nutrition transition observed in many countries at present (sensu ref. 34 ), and contribute to reducing the overall disease burden. This may be particularly relevant in countries experiencing continued high, or growing, trends of red (particularly processed) meat intake (such as China, Argentina, Brazil, the USA and Eastern Europe) [35][36][37] . Cardiovascular disease is among the most commonly cited negative health effects of red meat consumption 32,33 , and we use it here as an example of how countries can assess the relevance of blue food policies depending on their specific disease burden. In this context, the health-promoting role of blue foods rests on the assumption that they can displace some red meat consumption 1,10 and on the plausible health contributions (for example, of DHA and EPA from aquatic foods 38,39 ), for which uncertainties persist [39][40][41] . Substitutability of red meat by fish has not been well documented, yet reverse substitution has been observed 30 , as have large-scale adoptions of new proteins when innovations are supported with public funds, and scaled by the private sector under supportive state and international policy regimes 42 . Sixty years of increased consumption of poultry compared to beef 10 suggests that poultry and seafood can replace red meat. As blue foods are already part of the local food culture in many countries with a high level of meat consumption, they constitute a promising step away from routinized overconsumption of red meat.

Nutrient sources with relatively low environmental footprints
Across the diversity of blue foods, many production systems already result in relatively lower environmental pressures compared to those associated with terrestrial animal-source food production 2 . Partial replacement of particularly ruminant meat with blue foods can therefore help to lower dietary environmental footprints. Unfed aquaculture systems, such as bivalves and seaweeds, typically result in low greenhouse gas (GHG), nitrogen and phosphorus emissions and require limited freshwater and land inputs. Many fed aquaculture systems perform similarly to or better than chicken production, which is often considered the most efficient terrestrial animal-source food production system 2,43 . GHG emissions for capture fisheries vary substantially 44 , with small pelagic fish, cod and some inland fisheries resulting in low average emissions 45 and flounder and lobsters having high emissions 2 , but all capture fisheries generally have negligible N and P emissions, and freshwater and land inputs 2,46 . Blue food production can nonetheless restructure ecological food webs and cause substantial biodiversity loss 47,48 , but there is a large potential to reduce most environmental impacts associated with blue food production. Improved fisheries management, fossil-free energy and a shift to low-impact gear are key areas of interventions for capture fisheries 49 . Impacts from aquaculture could be substantially reduced by lowering feed conversion ratios (for example, through breeding programmes), shifting species focus or feed composition (for example, to deforestation-free soy, fisheries by-products, or insect meal) and improving husbandry practices 2,50,51 .

Cornerstones in cultures, diets, economies and livelihoods
In many nations, blue foods are a cornerstone of cultures, diets and economies, fulfilling critical food and nutrition security functions 4 . Blue foods are also among the most traded commodities globally, providing substantial export revenue for many nations 13 and livelihoods for 800 million people 25 , indicating their critical role for employment and subsistence. However, although blue foods support the welfare (for example, through jobs and nutrient-rich blue food) of these actors 17 , the wealth-generating benefits (for example, export revenues) of blue foods flow predominantly to industrial-scale firms that control global Article supply chains 17,52,53 . This reflects inequities inherent across many food systems 54 . Small-scale actors are therefore often undervalued and marginalized in decision making, threatening livelihoods and their capacity to cope with changing environmental conditions 17,55 . Policies focusing on environmental or economic gains must therefore be attentive to risks of inadvertently undermining human wellbeing. Several environmental stressors affect blue food production, and climate change in particular will affect all aspects of aquatic food systems, from production to consumption 20 . Overall, the climate risk to a country's aquatic food system is determined not only by the climate hazards the country faces, but also by its dependence on the nutritional, economic, social and environmental benefits of aquatic foods 56 , and the vulnerability to losing these benefits 20 . These future threats may compound existing challenges and exacerbate inequities, by increasing barriers to

Reducing blue-food-sensitive nutrient deficiencies
Leverage consumption of blue food, alongside other nutrient-sensitive foods, as a means of reducing certain blue-food-sensitive nutrient deficiencies, particularly among poor or nutritionally vulnerable population segments.Our analysis centres on vitamin B 12 and omega-3, two nutrients for which blue foods are projected to contribute substantial portions of global supplies 1 .
When nutrient insufficiency is high and blue foods are or can be made available, together with other nutritious foods.
Successfully reducing blue-food-sensitive nutrient deficiencies means production portfolios must be managed strategically-developing blue food production systems with high capacity to satisfy nutritional needs with minimal environmental impact so that both environmental and nutritional co-benefits can be realized. Nutrition-sensitive trade, processing and distribution policies are also important, to avoid a scenario in which increased aquaculture production delivers foods that are not nutrient rich, affordable or accessible to those who need it.Trade-offs between economic and nutritional goals may emerge between directing national blue food production towards domestic markets versus exporting.Trade-offs between the environmental impacts of feed production and the nutritional quality of the fish produced also need to be assessed.
Reducing disease burden associated with high consumption of red meat (for example, cardiovascular disease risk) Leverage consumption of blue food, alongside other health-promoting foods, as a means of reducing the burden of non-communicable disease related to overconsumption of red meat.Cardiovascular disease is among the most commonly cited negative health effects of overconsumption of red meat 32,33 , and used here as an example for how blue foods can be leveraged to reduce specific non-communicable disease risks.
When red meat consumption is high, the risk of cardiovascular disease is high and low-environmental-impact blue foods are or can be made available, together with other health-promoting foods.
Blue foods vary in their environmental impacts 2 . Some have similar GHG emissions to those for poultry. By carefully considering which species are produced and traded to simultaneously minimize environmental footprints, nutritional and environmental co-benefits can be achieved. Trade-offs are otherwise likely to occur as production maximizes species that offer opportunities for efficiencies and bulk production but are not the most nutrient-dense or culturally appropriate aquatic foods.

Reducing environmental footprints of food consumption and production
Alongside overall shifts towards lower-impact diets, leverage consumption of low-impact blue foods as a means to lower GHG emissions from diets.
When red (ruminant) meat consumption is high and blue foods are or can be made available.
Reducing GHG emissions of diets through consumption of blue food can generate health co-benefits if the nutritional content of blue food groups is considered in production and trade policies. Otherwise nutritional outcomes (reduced deficiencies and disease risk) may be traded off for environmental improvement.Further health-environment co-benefits can be generated if portion sizes are limited and blue food production footprints are therefore deliberately minimized 108 .

Safeguarding contributions to nutrition, just economies, livelihoods and cultures under climate change (now and in the future)
In places where blue foods play an important role for nutrition, economies and/or employment, ensure they are climate resilient.
If blue foods contribute substantially to national employment, export revenue or nutrition and are likely to be threatened by climate hazards.
Safeguarding the contribution of blue foods in different settings entails reviewing production, processing and trade portfolios, as well as practices and preferences to identify relevant climate adaptation actions.However, the diversity of blue foods in terms of nutritional density, environmental impact and vulnerability to environmental stressors means that climate adaptations may present trade-offs, such as between farming species tolerant to new climate conditions but that are less nutritious.Co-benefits of climate adaptation, sustainability, health and livelihoods can be achieved if diversity in blue food supply chains is retained or enhanced. Diversity among production modes, supply chain actors and species can provide resilience to changing climatic and trade conditions, and if small-scale actors are given voice and support, it can simultaneously benefit blue-food-dependent livelihoods and contribute to nutritional security.
Throughout the paper, sustainability refers to the need to ensure that production and consumption meet present needs without compromising those of future generations. In column three, variables used to map policy objectives to nations are in bold, and correspond to those in Supplementary Table 2. Column four provides examples of notable co-benefits and trade-offs, discussed in more detail in the main text.
inclusive production and trade, limiting access to blue foods, and thus restricting their nutritional contributions 14,20,57 . Supporting the diversity and resilience of SSFA 14 can help build national food system resilience to climate and other shocks 14,20,58 , by providing response diversity 59 . Anticipation of how and where climate hazards will be most severe is therefore essential to help private and public actors identify appropriate actions to safeguard the contribution of blue food to the health, economies, culture and livelihoods in a way that also considers justice.

From science to policy objectives
The potential contributions of blue foods to achieving food system ambitions depend on specific environmental, socio-economic and cultural contexts 60,61 , which in turn are embedded in broader economic and political spheres 15 . We translate the blue food functions reviewed above into four policy objectives. These include leveraging consumption of blue food to: reduce vitamin B 12 and omega-3 deficiencies; reduce non-communicable disease risks related to overconsumption of red meat, particularly cardiovascular disease; and reduce GHG consumption and production footprints. A fourth policy objective centres on safeguarding blue food contributions to nutrition, just economies, livelihoods and cultures under climate change. Each objective is mapped to individual country contexts on the basis of publicly available data (Table 1), to assess the broad relevance of each policy across nations. For example, using proxy variables for insufficient nutrient intake across populations (summary exposure values of vitamin B 12 and omega-3 fatty acids), alongside blue food availability (through trade or domestic production), we identify countries for which reducing vitamin B 12 or omega-3 deficiencies among nutritionally vulnerable populations is particularly relevant (see Supplementary Table 1 for details on all variables, underlying assumptions and cutoff values). Conditions for relevance were informed by key literature and expert assessment by the interdisciplinary pool of authors. This mapping is a first step towards a more context-specific articulation of the multi-dimensional policy relevance of blue foods in food systems around the world, and could be enhanced as further data at subnational level, or for small-scale operations, become available.

National relevance of policy objectives
The relevance of each blue food policy objective is mapped across nations globally (Fig. 1). The degree of relevance of each policy objective is evaluated by a set of rules and cutoff points (detailed in Supplementary Table 1), including a sensitivity analysis of cutoffs (Extended Data Figs. 2-6). An interactive website (https://gedb.shinyapps.io/ BFA_synthesis/) presents all data and allows users to adjust cutoff points to explore the impacts of this on the relevance of each policy objective.
Fewer countries (43) were estimated to have >10% of their population at risk from inadequate intake of vitamin B 12 , compared to omega-3 deficiency (89 countries), which predominantly affects African and South American nations. Many of these nations also have a high availability of blue foods, making them well positioned to address deficiencies by promoting access and facilitating consumption of culturally relevant blue food, especially among nutritionally vulnerable population segments (Fig. 1a).
Countries with red meat intake above the threshold recommended as environmentally sustainable and healthy 31 , who also have a high incidence of cardiovascular disease, are primarily located in the global North, with the exception of several small-island states. In many of these countries, blue food is currently available ( Fig. 1b and Supplementary Table 2). In such settings, moderate consumption of seafood with low environmental impact could be encouraged as a stepping stone away from high intake of red meat.
A substantial number of countries (124) also have a high intake of ruminant meat, contributing to high dietary GHG footprints (Fig. 1c).
Many of these countries have blue food available because they are big importers (for example, Belgium) or big producers (for example, Chile and Norway), or they both produce and import (for example, France and Denmark). Although they may export some of their blue food at present, our mapping identifies countries that, with a shift in policy or prioritization, could retain some of their domestic production for domestic consumption. This would trade off export revenue, and highlights the need to balance several policy goals, discussed below.
At present, blue foods play an important role for nutrition, livelihoods or national revenue, in a substantial number of countries (103), particularly in the global South and among Indigenous communities across the global North 4 . Combining such findings with analysis of climate hazards identifies countries with high future risk, for whom climate adaptation of blue food systems will be particularly important (Fig. 1d). We illustrate some adaptation options below.
It is important to note that for a sizeable portion of countries, certain blue food policy objectives are less relevant, according to our analysis. This does not mean that food systems in these countries are devoid of challenges, but blue foods are not a panacea and do not offer suitable means to improve food systems in all geographies at present.

Overlapping policy relevance
For some countries, several policy objectives are relevant. Figure 2 shows the degree of overlap between policy relevance, in terms of the number of countries for which two objectives are both relevant. Some policies show a high degree of overlap, indicating possible win-wins. For example, in most (75%) of the 89 countries for which omega-3-enhancing policies are relevant, reducing environmental footprints is also a relevant objective. Similarly, for most (82%) of the 22 countries dealing with high cardiovascular disease risk, promoting blue foods over red (particularly ruminant) meat overconsumption as part of a whole-diet approach would simultaneously address health and environmental concerns.
It is noteworthy that 91% of countries with vitamin B 12 deficiencies also show high levels of omega-3 deficiency. Vitamin B 12 deficiency seems to reflect more general undernutrition of the population, whereas omega-3 deficiency (specifically DHA and EPA) is caused by low intake of blue foods. The large overlap is thus explained by most of the 42 countries whose populations are at risk of malnourishment also lacking in consumption of blue food.
In 50% of the 103 countries for which blue foods play an important role for nutrition, livelihoods or revenue, blue foods could also represent an avenue to reduce environmental footprints of ruminant meat consumption. Furthermore, in 46% of these 103 countries reducing omega-3 deficiencies is also relevant, and is similarly reflected in the 53% overlap in relevance between countries with high omega-3 deficiency and safeguarding food system contributions. In these settings, policies that can reduce certain types of malnutrition by implementing climate adaptations that ensure access to low-environmental-impact blue foods, while also securing quality jobs (that is, welfare benefits) and removing barriers to wealth-generating benefits, could therefore have the potential to generate substantial co-benefits 17,62 . Below we explore how potential co-benefits flagged by Fig. 2 can be realized.

Harnessing diversity for co-benefits
Achieving globally agreed targets, such as zero hunger, good health, healthy aquatic and terrestrial environments, and a stable climate requires systems thinking 11,63,64 . Optimizing one policy domain often leads to both positive and negative spillover effects in several other sectors 65,66 . Addressing food system complexities that span land and sea, and/or encompass production, processing, trade and consumption, will be feasible only through systemic food policy 64,66 that Article identifies co-benefits between policy objectives and actions to achieve them 67 . A systemic food policy agenda can provide a clearer understanding of the potential of blue food diversity for navigating trade-offs and realizing synergies between blue food policy objectives. Below we explore opportunities for co-benefits across the policy objectives proposed above. For each, we discuss how ensuring diversity in blue food actors and blue food performance across the domains of health, nutrition, environmental impact and climate risk can help realize synergistic system-level outcomes. Overall, we argue that in any setting, shaping or maintaining food environments that make blue foods an attractive food choice is a prerequisite for achieving the multiple food system goals highlighted by this paper.

Human health and environmental sustainability
Enabling this synergy will depend on the ability of sustainably sourced blue food to displace currently consumed foods with high environmental impact. Some aquatic foods, such as bivalves and small fish, are nutrient dense and have low environmental footprints 1,2 offering an environmentally sustainable way to address both vitamin B 12 and omega-3 deficiencies and cardiovascular disease risk. Along with cultural preferences, smell and taste, safety concerns and eating habits 68,69 , price is key for determining household consumption 42 . Access and affordability are therefore prerequisites for blue foods to reduce nutrient deficiencies, cardiovascular disease risk and dietary environmental footprints 70 . However, blue food diversity means that income is a poor predictor of consumption when relying on aggregate data categories such as 'fish' 10,71 . At present, some blue foods are more expensive than other animal protein, particularly in developing contexts 63,72 , but in many settings they represent affordable sources of key nutrients 10,63,73,74 . Increasing or protecting affordability will require the commoditization of low-environmental-impact blue foods through policy and regulation that promote sustainable intensification and supply chain transformation 71,75 . This can include public incentives for directing research and development investment towards specific species and production systems, and/or market incentives for value chain actors to reorient trade to low-income and nutritionally vulnerable consumers, and prioritizing increased nutrition over growth in production volumes and monetary value.
Although consumption of blue food is projected to increase (about 80% in edible weight by 2050 assuming constant prices and balance between supply and demand 10 ), the resulting nutrition and environmental impacts will depend on the substitutability among blue foods and other animal-source foods in national diets. Substituting all red meats for blue foods is neither feasible nor desirable, and adding or increasing animal-source blue foods to diets of wealthy consumers, already rich in animal-source foods, would fundamentally undermine the role of blue foods in delivering healthier and less environmentally harmful dietary outcomes 2 . Fish-meat substitutability has not been widely studied, but the possibility of replacing meats with blue foods or plant-based alternatives seems to be an attractive policy option 35,72 . Strategies to achieve these goals could include combining soft policy tools such as dietary guidelines or behavioural nudging to mainstream eating and cooking blue foods 76,77 , with harder regulatory interventions and economic disincentives for high-carbon-emissions food [78][79][80][81] .

Livelihoods, economies, health and environmental sustainability
Investments in blue food innovations have the potential to yield inclusive livelihoods and systems that produce nutritious, affordable and environmentally sustainable blue foods 14,75 . Such synergies again depend on which species and production modes are pursued, their variable environmental performance and nutrient density, and what barriers to access exist 1,2,82 . Production modes also vary greatly 14 , from un-mechanized small-scale fisheries and farming to industrial-scale, highly specialized operations. These different production modes, and the power dynamics of supply chains developed for distribution, generally affect their contribution to equitable wealth and welfare distribution 14,17 . Policy levers are therefore needed that can improve equity by removing barriers to wealth-generating benefits. This can entail inclusive financing, infrastructure and governance that lends voice and rights to all actors and avoids displacement by competitive sectors, but also maintaining traditional access rights to nutritious blue foods; all as part of efforts to implement the human right to food 14,17,62,83 . Improving equity can also yield further benefits. For instance, increasing gender equity has been found to also improve nutritional outcomes for families 84 .  Climate resilience and blue food production, employment or revenue Climate change will affect all aspects of aquatic food systems, from production to consumption, and threatens to undermine their contribution to the health, economies, culture and livelihoods of billions of people 20 . The substantial contribution that blue foods already make, particularly for livelihoods and diets, in many nations 85,86 (Fig. 1d) underscores the importance of strengthening blue food system resilience as no-or low-regret adaptation options 87 . Climate-smart production, supported through finance and adaptive governance, can reduce future disruption by promoting a multitude of different blue foods, and thus also take advantage of new opportunities that come with changing species and conditions. Examples include farming several thermally tolerant species, or introducing more flexible catch guidelines to cater for geographically extended species ranges and migration patterns of fish and fishers 88 . Addressing the current unsustainability of many fisheries by regulating harvestable quantities would simultaneously enhance stock resilience through maintenance of higher genetic diversity and thus adaptive capacity. Larger stocks are also less likely to crash when exposed to periodic shocks, such as El Niño and marine heatwaves 89 . For aquaculture, relying on a diversity of species could provide response diversity across a number of critical dimensions such as temperature, salinity or oxygen. While increasing the diversity of species to reduce climate sensitivity and increase adaptive capacity, aquaculture could also reduce the focus on fed species and promote the development of non-fed production systems 90 . Additionally, by valuing the diversity of skills and knowledge encompassed by small-scale actors and enabling their capacities to innovate and adapt to changing environmental and economic conditions, nations could further invest in the resilience of their aquatic food system (S.R.B., manuscript in preparation) 14 .
Enhanced capabilities of the small-scale sector would also increase their ability to establish rights over resources, promoting more equitable forms of production and employment. Finally, disincentivizing high concentration of economic power in supply chains, characterized by the singular pursuit of efficiency gains, and mechanization at the expense of jobs 14,54 will be important to ensure that potential synergies between SSFA diversity, climate resilience and equity materialize.

Navigating trade-offs
The complex nature of food systems, including aquatic ones, means that any action to improve performance along some dimensions will trade off performance on one or several others 65 . We identify and elaborate on three bundles of substantial trade-offs that need to be considered, but which can be navigated and minimized by making strategic use of the diversity of blue food species and production systems. We visualize an example of such trade-offs using the pursuit of either economic or nutritional blue food benefits through domestic consumption or export (Fig. 3).

Environmental sustainability versus nutritional content of aquaculture products
In aquaculture, a pressing challenge has been to reduce reliance on wild fish for feed 50,91,92 by incorporating plant-based ingredients and recycled animal processing wastes in feeds 18,93 . However, such feeds may compromise the nutritional value of the fish produced 94 and divert produce that could be used for direct human consumption. Continued innovation to develop alternative feeds that combine lower environmental footprint with high nutritional quality will therefore be important, along with lowering feed conversion ratios 2,92 , but the latter will pit improved local environmental performance against higher-quality resource requirements with consequences for sustainable and ethical resource management. Policies that promote supportive structures of governance, infrastructure and financial access to new technologies and high-quality feeds for small-scale producers will contribute to mainstreaming a move away from wild fish feeds. A regulatory environment tailored to the specific needs of blue foods, as opposed to outdated and agriculturally focused rules such as bans on use of non-ruminant processed animal proteins and genetically modified organisms, can enable rather than hinder inclusion of new protein sources. It could also avoid perverse impacts and enhance regulatory coherence for improved innovation, market access and sustainability 51 .

Domestic consumption versus export revenues
Production of blue foods for export, and allocation of fishing rights to foreign fleets, offer economic opportunities for governments, individual businesses and fishers. However, these actions can undermine domestic consumption and local livelihoods 95,96 if fishers and farmers are displaced from productive fishing or aquaculture areas 97 , or suffer knock-on impacts of damaging industrial practices and overexploitation. Small-scale producers often face a tension between local needs 98 and connecting to export markets with higher profits that leave them vulnerable to global power dynamics, price fluctuations and supply chain disruptions 99 . In some cases, such as Chile, rising blue food export is associated with declining national consumption in favour Article of terrestrial meat 10 . As demand for blue foods rises among affluent population groups because of their contributions to health and reduced environmental footprint, prices will probably increase, exacerbating this tension. Implementing environmentally sustainable commodification will therefore be important, but must ensure that small-scale actors are not marginalized in the process (for example, ref. 100 ), This requires policies that encourage collaborative practices across production scales. Cooperatives and coalitions can support complementary and synergistic production and resource access across producers 101 , and inclusive jurisdictional and landscape approaches offer means to reconcile the diverse incentives and capabilities of actors in blue food production systems, while addressing ecological and geographical mismatches of current ratings and certification systems 102 . Exempting some domestic production from export is another way to secure food access but must align with local preferences, ensuring demand exists and producers are not disenfranchised 103 .

Efficiency, affordability and availability versus diversification and resilience
The global capacity to produce increased quantities of nutrient-dense, yet low-impact aquatic foods will influence the severity of trade-offs that emerge between blue food policy objectives. Commodification often offers efficiency and economies of scale, making blue foods more affordable and accessible 75 , but may compromise nutrition, squeeze out small producers and processors or outcompete them in markets if measures are not in place to safeguard their livelihoods 104 . For example, large-scale production of tilapia and pangasius offers inexpensive sources of aquatic foods, but in some markets they have replaced more nutritious indigenous fish 82 . Ultimately, efficiencies must be balanced against food sovereignty and the many contributions of blue foods, distinct from their monetary value 17 . Policies to retain or enhance the diversity of blue food production modes, actors and species are essential for the capacity of nations and regions to build resilience against shocks associated with, for example, climate change 20,105 , trade 19,57 or new diseases 106 . Examples include government support funds to provide financial relief for small businesses highly vulnerable to environmental and trade fluctuations 106 , and improved accessibility to production-related insurance 107 . The combination of species and productions systems that provide most resilience to a changing climate will be highly context specific, yet the species that offer opportunities for efficiencies and bulk production under a changing climate (such as tilapia with a high temperature tolerance range) may not be the most culturally appropriate or nutrient-dense aquatic foods 1 . Navigating this apparent trade-off could involve complementing bulk production of fewer species with environmentally sustainable cultivation and capture of a diversity of species that provide additional nutrition (for example, dried fish powder) and more inclusive supply chains 77 .

Policy ambitions need bold visions
The coronavirus disease 2019 pandemic has changed many aspects of our lives: how we work, travel and eat. For better or worse, it has shown that radical change is feasible in a short amount of time. For example, the pandemic highlighted how the small-scale blue foods sector was able to convey resilience and fill nutritional gaps left by interrupted global markets in some contexts, whereas in others it was left highly vulnerable 14,106 . Such shocks illustrate that backcasting is not the only, or the best, way to understand the future. Envisaging alternative futures (for example, through scenarios) may be instrumental for altering entrenched, unhealthy and unsustainable ways of producing and consuming food 80,91 .
We have outlined four roles that blue foods can play now and in the future and have translated these into broad policy objectives that-if actions are developed to achieve them-could contribute to achieving articulated food system ambitions (for example, United Nations Food Systems Summit 2021). Our analysis shows that the health, environmental, economic and welfare benefits that nations derive from blue foods are diverse 60,61 . We therefore provide an analytical framework and an interactive tool (https://gedb.shinyapps.io/BFA_synthesis/) for decision makers to explore how this diversity affects the relevance of specific blue food objectives in specific contexts.
However, regardless of how environmentally sustainably produced blue foods are, the global demand for blue foods to address disease and environmental impact in one set of countries (Fig. 1b,c) may reduce the availability and affordability of blue foods for achieving improved nutritional status for vulnerable populations in another (Fig. 1a). Governments can address these tensions by regulating trade and by ensuring that diets incorporating blue foods are considered Likely outcomes for each dimension are represented by coloured boxes and the strength of outcome is represented by plus and minus symbols; with positive outcomes depicted in green, and negative in pink. Sustainable commodification aligned with local preferences and demand represents an example of how a balance could be struck to optimize positive environmental, inclusive, economic and nutritional outcomes. Unknown impacts, or where policy objectives are judged to not have a strong impact, are depicted in grey. E. Wikander/Azote. alongside other means of achieving environmentally sustainable and healthy food system outcomes, such as various forms of more diverse and plant-rich diets 31,43,72,81 . Decisions regarding the role that blue foods can and should play for any nation's journey towards a more nutritious, equitable and less environmentally harmful food system therefore need to be grounded in local context and availability of aquatic foods, but also availability and affordability of a diversity of alternatives that are equally healthy and sustainable. Furthermore, the dietary shifts associated with the nutrition transition 34 are neither globally universal, nor inevitable. Despite their growing incomes, India and most countries in Asia-Pacific, much of the Middle East and some Latin American nations show low terrestrial meat preferences, with a higher share of protein coming from other sources, such as legumes and seafood 10 . Nations whose diets were previously constrained by low income are therefore now well placed to lead the way to sustainable and healthy eating.

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Assessing degree of policy relevance
To assess the degree of relevance of each policy for each country, we rely on theory and expert-guided typology building. Such an approach centres on classifying countries on the basis of a set of a priori assumptions about the conditions when blue food policies are relevant. The analysis has three steps.
Step one uses theory and expert assessments to build a data table of conditions that logically explain the relevance or non-relevance of each of the four policies. Conditions are variables that explain an outcome. In our analysis, these variables represent proxy variables that logically explain the relevance or non-relevance of the four policies in focus (Supplementary Table 1). The proxy variables correspond to national averages of publicly available (or published) datasets. Following best practice for related methods, such as qualitative comparative analysis 109 , thresholds for inclusion (that is, cutoffs for when a policy objective is considered relevant on the basis of a country's statistic) were set on the basis of theoretical knowledge where available (Supplementary Table 1 and Extended Data Fig. 1). For many variables, however, no theoretically established cutoff existed. Thresholds were then set on the basis of natural breaks in the data after deliberation with authors of relevant expertise (see Extended Data Figs. [2][3][4][5][6]. For all cutoff values, we provide a transparent justification for the selection and specify the type of disciplinary expertise leveraged to assess cutoffs for each variable (Supplementary Table 1).
A second step involves developing Boolean logic solution formulae that allow us to classify countries in relation to the outcome variable 'degree of policy relevance' (highly relevant, relevant, less relevant and missing data). We use a crisp set methodology to define which countries are relevant for a particular policy (see Supplementary Information for elaborated justification). Crisp sets assign cases (countries) a binary value for each variable. The binary value is based on whether the data for the country fall above or below the pre-determined cutoff (Supplementary Table 1). Logic solution formulae specify the combination of binary conditions that results in a given level of policy relevance (Supplementary Table 2), and these solution formulae are based on expert judgement and logic (Supplementary Table 1). The combination of AND and OR statements in the solution formulae highlight the distinction between necessary and sufficient conditions, akin to how these are conceptualized in qualitative comparative analysis 110 .
One key reason for choosing crisp set methodology was that we wanted to maximize the ease of interpretation and potential use. Crisp sets arguably retain less information richness than fuzzy sets (for which membership of cases is not binary but assigned as degrees of membership to different categories). However, although partial set membership allows for more information from the underlying data to be maintained, it is also likely to result in situations of partial relevance in the outcome variable (degree of policy relevance). In other words, one could easily end up in a situation in which a country is classified as 33% relevant. This would be exceedingly hard for readers to interpret and act on. In other words, crisp sets were chosen in order for countries to receive a clear classification of relevance (highly relevant, relevant, less relevant and missing data) in our analysis. Another reason for opting for crisp sets is the above noted lack of scientific consensus to guide the exact cutoffs for all variables assessing the conditions. Fuzzy set analysis requires several such decisions to be made as each variable is divided into a minimum of three sets (as opposed to a case simply being in the set = 1, or out = 0), and would have thus increased the uncertainty of the analysis. We recognize that even with our analysis some cutoffs could be up for discussion. We therefore invite readers to explore different threshold values and see the change in outcome in the web-based tool available at https://gedb.shinyapps.io/BFA_synthesis/. This tool also means that, as scientific evidence for a particular cutoff value becomes available or updated, this information can easily be applied to revise the classification of nations.
The third step of our analytical approach involves matching the set configurations in the data table (step 1) to the Boolean logic solution formulae designed in step 2, to assign each case (country) to the outcome variable 'degree of policy relevance' (Supplementary Table 2). This outcome variable (for each policy objective) forms the results presented in Fig. 1, and forms the basis of the overlap analysis presented in Fig. 2.

Sensitivity analysis
To assess the sensitivity of our results to variations in thresholds, we opted for a one-at-a-time sensitivity approach 111 (Extended Data Figs. 2-6) owing to the simplicity of Boolean rules used in this analysis. This was combined with a visual examination of the underlying distribution of each variable used in the analysis, in relation to our set threshold (Extended Data Fig. 1). The typology classification model was re-run changing one variable threshold at a time, leaving all else constant, and assessing the change in the number of countries in each outcome category ('highly relevant', 'relevant' and 'less relevant'). The threshold was varied across the full range of the variable. The sensitivity analysis highlights to what degree the underlying distribution of the data (Extended Data Fig. 1), as well as the Boolean logic on which the classification model is based (Supplementary Table 2), influences the sensitivity of the threshold.

Assessing overlap in policy relevance
To assess overlap in policy objective relevance among nations, we conducted pairwise comparison of policy objectives. We calculated the percentage of the set of countries to whom a specific policy was deemed relevant, and that was also deemed relevant for a second policy objective. Nations classified as 'highly relevant' or 'relevant' were combined for the purpose of this analysis, and countries with missing data for either policy were not considered.

Reporting summary
Further information on research design is available in the Nature Portfolio Reporting Summary linked to this article.

Data availability
All data generated and analysed during the study are available in the Stockholm University Library Dataverse (https://doi.org/10.7910/ DVN/ILA0XI).

Code availability
R code used for the Boolean analysis and sensitivity analysis, as well as for producing the figures and web application, is available at https:// github.com/emmywas/BFA_Policy_analysis. All coding was carried out in R (version 4.2.0).

Corresponding author(s): Beatrice Crona
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