The nutritional value of seaweed (macroalgae) has been recognized in several Asian and South American countries for centuries, and a growing global appreciation for utilizing seaweed as food has emerged since the turn of the millennium (Cavallo et al., 2021; Naylor et al., 2021). Seaweed are low-fat food sources containing heterogeneous levels of nutrients (Leandro et al., 2020; Slegers et al., 2021). Although the lipid levels in seaweed are generally low, unlike some terrestrial plants, they contain the highly unsaturated omega-3 fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) (Cavallo et al., 2021).

An evaluation of under-exploited edible seaweed (Ganesan et al., 2019) found substantial intra- and interspecies variation in dried samples of key nutrients including iron (14.8–72 mg/100 g) and zinc (1.5–5.2 μg/100 g). Origin, geographic position, seasonality, species, and processing need to be considered when investigating bioavailability, metabolic processes, and optimal use of seaweed products (Cavallo et al., 2021; Naylor et al., 2021). Due to the high levels of iodine in seaweed (Leandro et al., 2020) a diet including seaweed may lead to excessive iodine exposure. Therefore, to enhance awareness and ensure consumer safety, iodine concentrations in seaweed should be disclosed. Additionally, nutrient and contaminant levels in some seaweeds require further study to evaluate their potential as healthy and sustainable foods (Grebe et al., 2019).

There is inadequate policy and poor management of nascent seaweed farms, frequently attributed to a lack of connection of research efforts between the Global North and South (Krumhansl et al., 2017). In a study from Latin America only three out of twenty countries mention seaweed in their fisheries policy (Alemañ et al., 2019). Chile has pioneered seaweed policy and developed regulations to encourage diversification of the sector through large- and small-scale farming (Henriquez-Antipa and Carcamo, 2019), including multi-criteria decision analyses to assess perspectives of stakeholders and identifying current policy implementation gaps (Henriquez-Antipa and Carcamo, 2019). Furthermore, the compliance between the ecosystem approach to aquaculture developed by FAO and seaweed cultivation represents an opportunity to promote both social and ecological sustainability (Grebe et al., 2019).

While up to 85% of global seaweed production is used for human consumption in Asia (van den Burg et al., 2021), diverging utilization of seaweed in the Global North and South is reflected in market prices. Kelp produced in the US and Europe is retailed for over five times the price (US$ 944 t⁻¹ wet weight) compared to kelp produced in Korea (US$ 177 t⁻¹) (Grebe et al., 2019). A question arises as to how expansion of seaweed production in Western countries could fit in and compete in existing global markets. The high volume and low prices of commercialized seaweed in the Global South further entails its perception as a lower value source of income for small-scale producers and harvesters (Henriquez-Antipa and Carcamo, 2019). Activities such as small-scale kelp farming require little startup capital, but the missing standardization of cultivation techniques required to establish profitable businesses limit its expansion (Grebe et al., 2019; Farmery et al., 2020). Despite that most small-scale seaweed farmers in the Global South are women, their significant contributions to the fisheries remain undervalued, and their voices are not resonated in fisheries policy (Mirera et al., 2020; Thomas A. et al., 2021). The role of seaweed farming for achieving the SDGs and for stimulating the blue economic growth in Bangladesh has been assessed (Thomas J. B. E. et al., 2021), presenting key linkages between small-scale seaweed farming and 26 targets of 8 the SDGs. This included increased resource access for marginalized population groups and stimulation of the local economy, underpinned by the untapped opportunity of seaweed aquaculture expansion in tropical climates (Hossain et al., 2021). A Kenyan case study demonstrated that income generated from introduction of small-scale seaweed farming in rural villages improved food security and nutrition, and stimulated infrastructural development (Mirera et al., 2020). Dried seaweed for human consumption was the main study output, but it also included manufacturing of seaweed products including soaps, salad, and fish feed (Mirera et al., 2020).

The absence of feed and excreta from seaweed production, accompanied by a rapid increase in biomass, make it one of the least invasive and most productive aquatic food systems (Grebe et al., 2019; Hossain et al., 2021). Seaweed provides ecosystem services, like reducing local eutrophication via nutrient extraction and re-oxygenation, and could be used for amelioration near fish farms (van den Burg et al., 2021). For instance, large-scale cultivation of red algae Gracilaria lemaneiformis and Pyropia yezoensis has effectively mitigated the incidence of harmful algal blooms in China (Grebe et al., 2019). However, this ability may prove exhaustive in already nutrient-poor areas, and severe nutrient depletion has been documented in areas with intensive seaweed farming (Grebe et al., 2019).

Increasing evidence indicates that seaweed farming stimulates local biodiversity by providing valuable habitat structures and contributing as a direct food subsidy for herbivorous fish and invertebrates (Stentiford et al., 2020; Theuerkauf et al., 2021). Preliminary studies from Southeast Asia and East Africa also suggest that higher species richness and abundance are found in large scale seaweed farms compared with small-scale farms or wild kelp beds (Grebe et al., 2019; Theuerkauf et al., 2021). Structural additions from seaweed aquaculture may prevent seashore erosion (Hossain et al., 2021), while increased activity in this zone may compromise the foraging habitat of seabirds and other wildlife (Theuerkauf et al., 2021). A case study from Canada also points to the different ecosystem-level consequences of production yield and process, finding that small-scale harvest poses minimal impact on recovery rates, survival, and biomass dynamics of kelp (Krumhansl et al., 2017).

Notwithstanding its vast potential, disease outbreaks encouraged by seaweed farming are one of the leading causes of decreasing native seaweed stocks and loss of genetic diversity (van den Burg et al., 2021). Increased ship traffic has accelerated the spread of seaweed species such as wakame (Undaria pinnatifida), earning it a spot among the world's top invasive species (Grebe et al., 2019). A related challenge is that hypoxic effects on meiofaunal populations are caused by eutrophication and associated filamentous algae drifts (Thomas J. B. E. et al., 2021). Furthermore, shading from large-scale kelp farms and use of equipment such as moorings for securing longlines can have detrimental effects on benthic ecosystems and cause entanglement of marine mammals (Grebe et al., 2019). Poorly located seaweed farms may also decrease the heterogeneity of macrofaunal communities, for example through shading and trampling of subtidal seagrass habitats (Theuerkauf et al., 2021).

The synergistic effects of climate change and anthropogenic activity are affecting seaweed farming to a growing extent. Beas-Luna et al. (2020) and Krumhansl et al. (2017) found that kelp ecosystems appear vulnerable to changes in ocean temperature, and that both episodic heat waves and long-term warming induce pronounced effects on kelp formation. This trend is particularly challenging given the role of seaweed as a primary producer, provider of habitat structure, and as a carbon sink. Increased anthropogenic nutrient pollution may cause eutrophication and harmful algal blooms, which are recurring more frequently (Kumar and Sharma, 2020), and elevate contaminant levels in seaweed (Cavallo et al., 2021).

New frontiers in seaweed production may enhance sustainability through multisectoral use (van den Burg et al., 2021). The environmental benefits of seaweed may also be mediated via the protein transition, representing the shift from human diets rich in meat and dairy to utilizing more plant-based protein sources. In an evaluation of the environmental performance of different burgers, in terms of global warming potential and land use, a burger containing up to 60% sugar kelp (Saccharina latissima) scored better, compared to soy- or wheat-based vegetarian burgers (Slegers et al., 2021).

Globally, seaweed and their by-products are used in food products, animal feed, pharmaceuticals, cosmetics, agriculture and biofuels (Grebe et al., 2019). Little evidence supports the role of seaweed as a dietary substitute for macronutrients from terrestrial crops and animals, but its use as fertilizer and animal feed is increasingly bridging the marine-land loop in nutrient circularity (Naylor et al., 2021). As an added benefit, specific species of seaweed may reduce enteric fermentation in ruminants, reducing the climate footprint of beef or mutton production (van den Burg et al., 2021). Algal biomass has also been suggested as a cost-effective replacement for parts of the fish meal in aquaculture, both in terms of environmental output and to offset food security issues attached to the estimated 15.6 million tons of wild fish used in fish meal and oil production annually (Stentiford et al., 2020). However, the wide application of seaweed entails multiple processing routes, many of which are in their infancy. Naylor et al. (2021) proposes that adopting a “biorefinery” approach to seaweed processing, similar to that of agriculture, could add value and minimize waste. The environmental output of processing must also be considered in a circular model. The following steps were suggested: (i) Applying the full mass of seaweed in products and prevent waste. (ii) Taking fractionation steps in an order that yields the most valuable compounds and maintains their functionality. (iii) Investing energy and chemical resources only if it increases the product value. Combined, this may reduce the environmental impact of seaweed processing (Grebe et al., 2019).

To mitigate climate change, it has been estimated that the global seaweed population could sequester the equivalent amount of carbon as mangroves, salt marshes and seagrasses combined (Grebe et al., 2019). Still, to assess whether the absorption of carbon and nutrients during seaweed cultivation contributes to an overall positive environmental effect, full carbon and nutrient cycles need to be modeled (van den Burg et al., 2021). In a cradle-to-gate life cycle assessment, a study of the nutrient return from kelp aquaculture in Sweden (Lemasson et al., 2019) reported that kelp could capture finite nutrients to be recirculated, supporting the circular blue-green bioeconomy. Challenges of kelp aquaculture were also pointed to, identifying land-based seaweed hatcheries as energy intensive compared with cultivation of other low-trophic species such as bivalves (Thomas J. B. E. et al., 2021). Another opportunity is the inclusion of seaweed in integrated multi-trophic aquaculture. Studies with seaweed co-cultured with shrimp, mollusks or salmon have proven successful in terms of commercial yields, providing up to twice the profit compared with mono-cultured seaweed, while absorbing a part of the climatic impact of production (Alemañ et al., 2019).

Implementing a circular approach is also important in a market context. For instance, current strategies in European seaweed production follow a linear pattern, with an emphasis on quantitative yields and reduced-price levels (van den Burg et al., 2021). On the contrary, qualitative aspects are suggested as key drivers toward more circular food systems, and that the focus should be shifted to producing the optimal quantity of seaweed while balancing the carrying capacity of the oceans and keeping the preferences of end-users in mind (Grebe et al., 2019).

Endowed with key micronutrients including vitamin B₁₂, iron, zinc, calcium, selenium, and omega-3 fatty acids, bivalves have a favorable nutritional profile for human health (Lemasson et al., 2019; Farmery et al., 2020). A substantial range in nutrient content has been recorded between species; for example, oysters (Crassostrea gigas) are known as a good source of dietary calcium while other species such as sici-shells (Polinicies aemingiana) have lower concentrations of calcium (Farmery et al., 2020). This variation extends to the marine omega-3 fatty acids where higher average concentrations of EPA and DHA have been determined in clams and oysters than in scallops (Tan et al., 2020). As formerly reviewed by Tan et al. (2021), marine habitats at high latitudes have been positively linked with increasing long-chain marine omega-3 content. The global supply of marine long-chain omega-3 for human consumption is under pressure, and currently meets 30% of global demand (Willer et al., 2021). On average, concentrations of EPA and DHA in bivalves range between 9.50 and 35.28 mg/g, which illustrates the capability of increased bivalve production to satiate this demand (Tan et al., 2020; Willer et al., 2021). This potential remains untapped in many of the world's fastest growing populations, including India and Kenya, which have some of the highest potential for bivalve aquaculture expansion (Gentry et al., 2017).

Possible changes in the nutritional properties of two commercially valuable oysters, Crassostrea gigas and Ostrea edulis, were modeled under climate-change scenarios. Mid- to end-of-century predictions suggest that increased ocean acidification and warming are likely to impair the nutritive composition of both species in terms of reduced concentration of essential minerals and lipids (Lemasson et al., 2019). Furthermore, Oliva et al. (2019) reported that ocean acidification may reduce consumer appeal to bivalve species due to altered color and texture. So far, this has been opposed in a sensory study by Lemasson et al. (2017), indicating that increasing levels of ocean acidification and warming do not significantly alter the sensory properties of the oyster C. gigas. Adapting practices toward safeguarding climate resilient species less prone to quality deterioration is a key to maximizing future nutrient yields from bivalve food systems (Lemasson et al., 2019).

Stentiford et al. (2020) reported that the rapid growth of the bivalve aquaculture industry has taken place under an absence of robust legal frameworks. This misreckoning of potential pitfalls and hazards has had severe impacts along the value chain, and currently limits exports for many countries in the Global South. Ninety percent of aquaculture output is currently not directed toward export, whereby certification has been proposed to increase both the market value and transparency of aquaculture products from low-income countries (Naylor et al., 2021). In the Global North, certification may also provide added value to bivalve products by ensuring the certainty of origin or the exclusivity of traditional production methods (Avdelas et al., 2021). Still, the low compliance between global certification schemes and small-scale actors has been scrutinized, listing insufficient finances, illiteracy, inadequate management skills, and low product demand as persistent barriers (Naylor et al., 2021). The tendency to focus on negative effects of aquaculture in certification schemes has also been critiqued, which, in the case of bivalve aquaculture, misses the mark on the potential provision of ecosystem services (Theuerkauf et al., 2021).

Low barriers of entry are facilitated by the modest need for farming infrastructure, particularly for small-scale production, which makes bivalve farming accessible to more people (Gentry et al., 2019; Tan et al., 2022). However, bivalve farming is also prone to mass mortality outbreaks, exemplified by recurrent mass mortality of juvenile oysters in France due to Ostreid herpes-virus infection. Annual economic losses due to bivalve mass mortalities have been estimated to US$270 million in Northwestern USA alone (Tan et al., 2021). While large farms may have the capacity to sustain such shocks, many small-scale actors do not have the financial flexibility to invest in disease diagnostics or quality seeds (Henriksson et al., 2021). The world's least developed counties are also predicted to have the lowest capacity for climate change adaptations in bivalve aquaculture, largely attributed to governance issues (Stewart-Sinclair et al., 2020). On the other hand, reduced adaptive capacity in bivalve aquaculture in high-income countries has been linked to limited species diversity (Stewart-Sinclair et al., 2020), highlighting the importance of developing policies and frameworks compliant with different socio-ecological settings.

Rebuilding of bivalve stocks in a context of poverty can only be achieved by addressing social challenges (Calvo-Ugarteburu et al., 2016). By including goals to support local management institutions and food security in a South African study, the mussel surface cover increased from 1 to 80% in an exploited area over a 10-year period (Calvo-Ugarteburu et al., 2016). Compared with large-scale actors such as Chile, the fragmentation in European production has led to greater bargaining power allocated to the depurating and processing sectors rather than the producers (Avdelas et al., 2021). In many West African countries, bivalve harvesting is exclusively practiced by women. A study from Guinea-Bissau highlighted that this practice is not only important for household food security, but also for increased purchasing power and female empowerment (Giselle Alvarenga et al., 2022). A similar study in Fiji supported this outcome while adding that female actors in small-scale bivalve harvest are often marginalized and need greater participation in management decisions and policies (Thomas A. et al., 2021). This current tension leads to the potential loss of traditional ecological knowledge of native bivalve species.

Non-fed bivalve farming is considered one of the most environmentally sustainable food systems (Farmery et al., 2016; Avdelas et al., 2021; Henriksson et al., 2021; Koehn et al., 2022a). Bivalves are among the food products generating the lowest greenhouse gas footprints (Koehn et al., 2022a). The environmental sustainability of non-fed bivalve food systems is attributed to the superfluousness of feed, and the provision of diverse ecosystem services (Farmery et al., 2016; Avdelas et al., 2021; Henriksson et al., 2021). By filtering phytoplankton and accumulating nitrogen and phosphorous from the ambient environment, bivalves mitigate coastal eutrophication (Naylor et al., 2021). In coastal waters, eutrophication may enhance algal blooms, which in turn may deprive water bodies of oxygen or contribute to shellfish poisoning (Estevez et al., 2019). For instance, in 2016 algal blooms in Chile caused substantial mortalities in several bivalve operations which had to close for 2 years, generating extensive economic losses (Naylor et al., 2021). Other beneficial outcomes of bivalve aquaculture include increased abundance and species richness of wild mobile macrofauna (Theuerkauf et al., 2021) and the potential of bivalve farms as net carbon sequesters (Willer et al., 2021). While political incentives to reward measures to enhance species abundance are still in their infancy, farms in Italy are presently awarded “green” certificates for the CO₂ uptake of shells during production (Avdelas et al., 2021). This suggests that monetizing ecosystem services and rewarding bivalve industry for sustainable innovations may be a key to stimulate their growth (Theuerkauf et al., 2021).

Nevertheless, the cultivation of bivalves is not exempt from environmental challenges. These include the introduction of non-native invasive species, cultivation gear which may entrap wildlife as well as the release of microplastics, and depleting effects on benthic ecosystems through increased deposition of excrement, increasing eutrophication followed by oxygen depletion (Farmery et al., 2016; Avdelas et al., 2021; Willer et al., 2021). Many of these challenges are attributed to unsustainable management of culturing systems and typically manifest in overstocked farms with inappropriate siting, as reported in case-studies from China (Naylor et al., 2021). One of the overarching challenges remains that expansion of bivalve aquaculture will require occupation of large areas of ocean space, which in many cases are in use for other forms of aquaculture, shoreside infrastructure, or as protected areas (Gentry et al., 2017), requiring shared spatial management. Lack of access to ocean space has been identified as a key barrier in many European countries, where bivalve production is currently experiencing a downward trend (Avdelas et al., 2021). Gentry et al. (2017) projected that bivalve aquaculture still has expansive potential, with areas in warm tropical regions displaying the highest growth potential.

Accounting for the detrimental effects of ocean warming and acidification on species growth (Oliva et al., 2019), end-of-century predictions estimate a global loss of 5–20% of suitable areas for bivalve aquaculture (Cubillo et al., 2021), even though bivalves as an inhabitant of naturally variable environments are considered relatively resilient to environmental change (Shalders et al., 2022). The projected reductions are not evenly dispersed between regions, nor within each exclusive economic zone (EEZ), but climate changes are expected to induce dramatic declines in production potential for major bivalve producers including China, Thailand, and Canada (50–100%) (Froehlich et al., 2018). Adaptive planning, sensitive to location, choice of species, and exploration of alternative initiatives such as offshore aquaculture (also in conjunction with wind farms), will therefore become increasingly important to alleviate the lack of suitable ocean space (Avdelas et al., 2021).

Climate changes have already resulted in increased migration of biotoxin producing algae, causing more frequent algal blooms and mass mortality outbreaks in bivalves (Estevez et al., 2019; Tan et al., 2021). To safeguard sustainability, developing bivalve seeds resistant to specific pathogens represents an opportunity to mitigate disease risk and stabilize production, particularly in large-scale farming systems (Henriksson et al., 2021; Tan et al., 2021). In Europe, economic uncertainty is expected to increase under both moderate and extreme end-of-century carbon emission scenarios, following decreasing trends in most productivity parameters across both species and regions (Cubillo et al., 2021). A study from Canada shows that the sensitivity of bivalves is species-specific, with oysters (Crassostrea virginica) predicted to generally outperform mussels (Mytilus edulis) in future ocean warming scenarios (Littman et al., 2020). They also point to how the continuous urbanization of coastal marine environments may further compromise food safety through micro-debris and human pathogen contamination. In this case, climatic shifts such as increased precipitation are also forecasted to enhance bacterial transportation offshore, thus adding to the causal nexus (Colaiuda et al., 2021).

The reliance on natural ecosystems for feed entails that including bivalve species in polyculture systems or integrated multitrophic aquaculture may allow for greater nutrient-circularity (Farmery et al., 2016). Co-culturing bivalves and fed monoculture species such as salmon, may be mutually beneficial, where by-products from one species act as a fertilizer or food for another species and vice versa (Avdelas et al., 2021). As a net result, both the use of feed and the environmental impact may be reduced while producing more aquatic foods to support healthy diets (Farmery et al., 2016). One of the challenges in bivalve cultivation is the low edible yield. Constituting as little as 10% for some species, aggregation of shell waste continues to be a barrier to sustainable growth (Henriksson et al., 2021). Shell valorization routes are facing multiple hinders, including cleaning and preparation, long distance transportation of shells to points of utilization, and an overall dearth of waste-management regulations (Morris et al., 2021). Regardless, utilization of shells is well-established in a number of products used in food production systems, including fertilizers and supplements for poultry and livestock (Naylor et al., 2021) as well as water treatment systems and biofilters (Summa et al., 2022).

Phosphorous is one of the cornerstones of global food security, and is essential for optimal growth of plants, aquatic foods and mammals. The current extraction rate of phosphorous is unsustainable and is expected to be depleted within the next 50–100 years, giving rise to a major sustainability challenge (Morris et al., 2021). Upscaling bivalve production and harvest shows great potential toward achieving a more circular phosphorous-cycle, by capturing excessive phosphorous in marine environments for use on land (Thomas J. B. E. et al., 2021). Calcium carbonate is commonly used as a liming agent and is currently being mined in large quantities. An added incongruity therefore exists in the circular valorization of bivalve shells, which are mainly composed of calcium carbonate, but in many cases are considered waste (Morris et al., 2021). The loss of shells as structural components, such as reef building oysters, have also been identified as detrimental to many ecosystems. In terms of environmental circularity, simply cleaning and returning shells to their marine environment may therefore be the most cost-effective solution to conserve vulnerable ecosystems (Morris et al., 2021).

Tuna, with its large stocks in the Indian and Pacific Oceans, represents a cornerstone in current aquatic food systems (Bell et al., 2015; Lucena-Frédou et al., 2021). Renowned for its high protein content (25/100 g), tuna is also a source of vitamin A, vitamin B₁₂ and iron (Farmery et al., 2020). Tuna is an essential protein source for food security in the Pacific Island Countries and Territories (PICTs) (Syddall et al., 2022b,a) where 50–90% of animal protein stems from marine resources, mainly caught in small-scale fisheries (SSF) (Pilling et al., 2015; Bell et al., 2018). Population growth and climate-change induced shifts in tuna abundance are modeled and observed to challenge this resource base, and in the future other sources will need to complement tuna to maintain current protein consumption levels (Bell et al., 2015; Pilling et al., 2015; Erauskin-Extramiana et al., 2019; Andriamahefazafy et al., 2020; Nicol et al., 2022; Tran et al., 2022).

Balancing revenues from foreign fishing fleets while ensuring local availability is one of the dilemmas in managing tuna fisheries in PICTs (James et al., 2018), highlighting a common challenge in the Global South where food insecurity is increasing because local aquatic resources are exported. Increased access to processed foods has spurred the Westernization of Pacific Islanders' diets, leading to the world's highest prevalence of obesity, and associated non-communicable diseases, co-occurring alongside micronutrient deficiencies (Farmery et al., 2020).

In Tuvalu (James et al., 2018), it was estimated that 266 Mt of edible fish are lost from the SFF due to foreign industrial fishing. Bycatch from global high-seas fisheries targeting tuna and tuna-like species amount to ~270,000 Mt annually, comprising small non-targeted tunas and more than 50 other fish species (Sardenne et al., 2020). Transshipping some of this bycatch to local ports has been presented as an opportunity to stimulate food availability (James et al., 2018). In analyses of 20 by-caught species, most were good sources of omega-3 fatty acids and protein, reinforcing their potential as a complementary nutrient supply (Sardenne et al., 2020). This approach has shown promise in Nauru and Tuvalu, where non-target species could provide an additional 20 kg protein per capita per annum (Pilling et al., 2015). Yet, the levels and stability of non-target catches are difficult to quantify and need further study. Further negative effects of transshipment in Tuvalu included lost employment days, reduced catches and potential losses in income due to reduced tuna availability (James et al., 2018). Potential benefits, such as spending in local businesses and by-catch offloads, were likely offset by a Pareto loss whereby benefits do not befall those who experience losses.

Canning tuna extends shelf life, boosting availability and affordability for low-income consumers (Bell et al., 2015). Newly established fish processing plants in Papua New Guinea are increasing the availability of tuna to the population and providing thousands of jobs (Pilling et al., 2015). However, it is difficult to compete with the cost-effective mass production in countries such as Thailand and Indonesia (Weng et al., 2015). The growing appreciation of sustainable seafood presents market potential through certification programs. Yet, a case study on Indonesian SFF of tuna reveals a disconnect between Northern consumer preferences and the infrastructure required in the Global South to obtain certifications (e.g., Fair Trade, MSC) (Duggan and Kochen, 2016).

The world's tuna fishery is facing many challenges as it seeks to conform to equitable and sustainable management (Weng et al., 2015). Tuna is the most important group of fishes by catch volume in the SSF of PICTs, and other countries such as Indonesia (Willis and Bailey, 2020). Almost half of households in PICTs acquire major parts of their income from SSF, from fishers and traders to processors and distributors (Weng et al., 2015). The exploitation of tuna also delivers important economic benefits to PICTs through the sale of fishing access rights to industrial distant water fleets. Effective management policy for tuna must therefore consider the socioeconomic importance of both small-scale and industrial sectors.

The trade-off between catching low-value juvenile tuna for maximum food availability vs. high-value adult tuna for export was identified as a major management tradeoff in the Western Central Pacific (Willis and Bailey, 2020). A disproportionate allocation of capacity-enhancing subsidies to the industrial sector was also pointed to, in addition to the skewness of workforce involved in industrial fisheries vs. SSF and the social benefits that follow suit (Willis and Bailey, 2020). A 13-year tagging experiment of skipjack and yellowfin tuna off eastern Papua New Guinea showed how industrial fishing activity may limit the availability and access of tuna to SSF (Leroy et al., 2016). Area closures for industrial fisheries may therefore improve access and increase catches for SSF.

Bell et al. (2015) emphasized the neglect of food security in tuna fisheries policy and suggested prioritizing the protection of economic benefits derived from selling licenses to distant water fishing nations in PICTs to improve local food security. By allowing distant water fishing in their EEZs, achieving sustainable fisheries in PICTs increasingly depends on decisions made by distant water fishing nations. Foregoing domestic fisheries for access payments from distant water fleets may result in negative trade-offs, where fishery resources are not equitably compensated through access fees and retention of local fleets (Nichols et al., 2015). A case study in Ghana, where tuna also represents the biggest seafood export, uncovered structural changes in tuna production from traditional SSF to Asia owned industrial purse-seine fishing (O'Neill et al., 2018).

In a review of tuna fisheries management documents in PICTs, Karcher et al. (2020) found that tuna fisheries have an established hard policy arena, with most legal instruments focused on offshore fisheries and illegal fishing. In contrast, food security, sustainability, and climate change tended to be covered in non-binding strategies. To maintain the contribution of SSF to food security in PICTs, policies should facilitate easier access to the nearshore tuna resources. This includes extending the use of Fish Attraction Devices (FAD), as a part of national infrastructure to safeguard food sovereignty. However, as Pilling et al. (2015) suggest, policy decisions in tuna fisheries should be tailored at the national level to account for local differences in tuna availability and access, cost efficiency, and social significance.

Climate change exacerbates the economic disruptions in tuna fisheries, making sustainable management of the sector crucial for achieving key development goals in PICTs and other dependent nations amidst growing uncertainties (Evans et al., 2015). Current SSF tuna catches are poorly documented and are managed at national and local levels in PICTs (Weng et al., 2015). Combined with the hesitancy of some distant water fishing nations to convey precise harvest numbers, increasing fishery monitoring, research, and capacity building through intersectoral collaboration and mutual transparency is essential to move forward. Evans et al. (2015) identified a wide range of research priorities to progress tuna fisheries' management under climate change scenarios, including refining climate models to more accurately forecast tuna distribution and abundance, in addition to assessing optimal access rights and the influence different scenarios of tuna distribution and abundance have on decision power.

Modeling long term economic consequences of supply shocks from climate change in four fish exporting PICTs, Dey et al. (2016b) reported that without policy advancements, several PICTs risk becoming net importers of fish by 2050. In a similar case study from India, responses in mackerel tuna (Euthynnus affinis) production under long-term climate change (4°C increase) showed a projected decrease in catch and food security, even under a sustainable management scenario (Das et al., 2020). Dueri et al. (2016) also investigated the interactions between climate change and socioeconomic development under different skipjack tuna (Katsuwonus pelamis) fishery management strategies. The study found that a maximum sustainable yield strategy may become increasingly profitable due to increased fishing efficiency from technological advancements, while the low catches and high prices resulting from a maximum economic yield strategy comes up short in terms of future food security. These results call for increased adaptive capacity in tuna fisheries, and to prioritize strengthening the resilience of coastal populations. However, future reduced tuna catches may still be sufficient to support local food security if policy is designed to reallocate tuna resources toward local consumption (Erauskin-Extramiana et al., 2019).

Harvesting tuna from the top of the food web releases lower tier groups from predation, which may impact the whole ecosystem through top-down and bottom-up processes (Weng et al., 2015). Tuna distributions are largely conditioned by ocean temperature, and their widespread populations may therefore be good indicators of the effects of climate change (Weng et al., 2015). Erauskin-Extramiana et al. (2019) estimated end-of-century changes in distribution and abundance of six tuna species under a high-greenhouse gas concentration scenario. They found that temperate tunas like albacore (Thunnus alalunga), Atlantic bluefin (Thunnus thynnus), southern bluefin (Thunnus maccoyii), and tropical bigeye tuna (Thunnus obesus) were expected to shift poleward and decline in the tropics. In contrast, skipjack and yellowfin (Thunnus albacares) tunas were projected to become more abundant in the tropics and EEZs of most PICTs (Erauskin-Extramiana et al., 2019). In a study by Dueri et al. (2016), projections for the global skipjack biomass showed an increase in scenarios for 2050 and 2095, with major changes in distribution patterns. Concurrent with Erauskin-Extramiana et al. (2019) and Dueri et al. (2016) found that the Pacific Ocean is likely to continue dominating the skipjack tuna fisheries, and that distribution in this area may shift eastward. This regional distribution shift is likely to impact the economy and food security of several PICTs, with a projected increase in tuna abundance in Fiji and Vanuatu, stable supply in the Solomon Islands and a decrease westward (Dey et al., 2016b). For mackerel tuna (Euthynnus affinis), end-of-century changes in ocean temperature and primary production may negatively impact the productivity of Indian tuna fisheries (Das et al., 2020). According to an ecosystem model, ocean warming will have the most pronounced effects leading to an eastward shift in the population by 2050 (Nicol et al., 2022). These projected changes will likely impact all types of fishing activities in the Pacific, suggesting marked losses (75,000–149,000 Mt) in the combined catches from Papua New Guinea, Solomon Islands, Nauru, Federated states of Micronesia, and the Republic of the Marshall Islands. For the SSF in the region, the catch reduction was estimated to 29,000 Mt. On a global scale, tuna fisheries are most vulnerable to climate change in the Global South (Erauskin-Extramiana et al., 2019).

Increasing the number of FADs has been proposed as a sustainable way of boosting tuna catches in SSF (Dey et al., 2016a; Bell et al., 2018). However, a related challenge is the associated reduction in tuna prices expected from increased use, estimated to increase toward the mid-century (Dey et al., 2016a). Tuna ranching represents another adaptive strategy, where wild bluefin juveniles are captured, fed, and raised to maturity. However, a systematic analysis of Atlantic bluefin tuna governance found that tuna ranching appears to promote unsustainable harvest (Epstein et al., 2014). Another challenge is attached to the high environmental footprint of niche species aimed for high-income consumers, including Pacific bluefin tuna (Thunnus orientalis), which has not benefited from efficiency improvements brought by the blue economy (Henriksson et al., 2021). In this context, a study by Andriamahefazafy et al. (2020) spotlights the paradox of the blue economy, where sustainability and economic growth are presented as jointly achievable. This is spurred by the geopolitical web of tuna access, combined with the stock dependency of local populations, which makes sustainable management measures difficult to implement (Andriamahefazafy et al., 2020).

Tuna fisheries generate food waste from by-catch, on-board waste, home waste, and during processing, which is limiting its circular stance (Sardenne et al., 2020). To increase production value in PICTs, tuna bycatch and discards are required by law to be offloaded at port (James et al., 2018). Retention of bycatch in a single port in Tuvalu amounted to 10–30 Mt in 1 year. While being labeled as an untapped resource, local access, unstable supply, and local food preferences are obstacles for the utilization of bycatch (Pilling et al., 2015). However, retaining non-targeted tunas that are considered too small for canning remains a potential way of reducing food waste in the tuna fishery. Also, tuna trimmings are used for fish meal in countries such as Thailand, which has both lowered the ratio of wild fish inputs in aquafeed and reduced waste (Naylor et al., 2021).