Background

Aquaculture and fisheries contribute equally to the world seafood supply. Both industries are crucial in terms of their contribution to food security, nutrition, and the livelihoods of billions of people globally. However, capture fisheries supply plateaued in the 1990s, and nearly all the increase in global seafood demand has been met by aquaculture in the last three decades. Freshwater farming, such as carps and tilapia accounted for three-quarters of the global edible aquaculture production (excluding aquatic plants), followed up by mariculture, including salmon and brackish water aquaculture, including shrimp. Some think that aquaculture will replace fisheries in the long run, but we believe that wild stocks will continue to make a critical and significant contribution to the global food basket, with fish (from fisheries and aquaculture) being produced from a continuum, rather than a dichotomy. These systems are and will remain interlinked, in terms of production sites and nutrient flows. 

Some farmed species, in particular the more carnivorous Atlantic salmon and shrimp, consume most of the 20% of annual global marine fisheries catch, mainly pelagic fish rendered into fishmeal and fish oil. Some people argue that highly nutritious pelagic fish species should be directed to human consumption, while others argue that there is limited market potential for these species. Nevertheless, farmed Atlantic salmon is an excellent source of nutrition, and is one of the best converters of feed of any farmed animal. While farmed Atlantic salmon is considered a net neutral consumer and producer of fish biomass, it uses more of certain key nutrients than it produces for human consumption. Therefore, it is crucial to develop a better understanding of the interlinkages between aquaculture and capture fisheries and how we can optimise nutrient availability for the global human population. This requires different data sources (aquaculture and fisheries production, feeds, consumption, diets etc.) and expertise in seafood and aquaculture production systems, food system efficiency, sustainability and Life Cycle Analysis (LCA), diets and health, provided by the interdisciplinary team affiliated to the University of Cambridge, University of Lancaster, Institute of Aquaculture (University of Stirling) and the Rowett Institute (University of Aberdeen).

Results

Atlantic salmon farming produces as much fish biomass as is consumed in feed. However, for the industry to grow, retention of nutrients from marine resources needs to improve. We focused on nine nutrients that are important in human diets and concentrated in seafood, and on forage fish species including anchovies, herring, mackerel, sprat, and blue whiting that are consumed by people and used in fish feed. We found that feeding forage fish species to salmon, leads to a decrease in six out of nine essential nutrients in the salmon fillet – calcium, iodine, iron, omega-3, vitamin B12 and vitamin A (Figure 1), relative to directly consuming the forage fish. However, zinc and selenium were found to be higher in salmon than the forage feed species, reflecting the high levels of these nutrients in the plant-based ingredients used in salmon feeds – an important and often unrecognised example of how salmon farming can turn plant nutrients into a palatable and bioavailable form in salmon fillet.

Whilst still enjoying eating salmon and supporting sustainable growth in the sector, people should consider eating a greater and wider variety of wild fish species like sardines, mackerel, and anchovies, to get more essential nutrients straight to their plate. In the UK, 71% of adults have insufficient vitamin D in winter, and teenage girls and women are more likely to have deficiencies of iodine, selenium, and iron. Yet, dietary surveys have suggested that whilst 24% of adults ate a weekly portion of salmon, only 5.4% ate mackerel, 1% ate anchovies and just 0.4% consumed herring. Making a few small changes to our diet around the type of fish that we eat can go a long way to changing some of these deficiencies and increasing the health of both our population and planet. 

Our simulations show that relatively small increases in direct consumption of ‘feed’ fish, currently used in farmed salmon diets, can lead to substantial increases in nutrient retention (Figure 2a). More specifically, we find that re-allocating one-third of mackerel currently used in the feed of Norwegian-reared salmon for direct consumption would support a 66% increase in direct annual mackerel consumption in the UK. Considering by-product utilisation from the processing of mackerel, such a strategy would only require a tiny proportion of fish oil replacement in salmon feeds to maintain 2020 salmon production levels (Figure 2b). In contrast, while anchovies accounted for most of the fishmeal and fish oil production, removing it from feeds would result in most fish for direct consumption, but it would require a large amount of fish oil replacement in salmon feeds.

The direct consumption of forage fish species, such as mackerel, is associated with a higher density of most essential nutrients compared to salmon fillets. However, consumer demand for forage fish species is low and increased consumption should be supported by product innovation focussing on convenience, visual appearance, taste, and flavour, to enhance consumer appeal. 

Further research

This paper has laid the foundation for the development of a robust and standardised metric, based upon edible nutrient retention, empowering seafood producers and policy makers to assess, compare and improve nutrient efficiencies between aquaculture and capture fisheries. However, it is important that such a metric considers the complexities of fishmeal and fish oil production and the dynamics of the broader food system, such as accounting for the nutrient yields in spare fishmeal used elsewhere, and the utilisation of fish by-products in other locations (e.g., salmon heads to Asian markets), and industries (e.g., livestock feeds). This also requires better (nutritional) data on feeds, marine ingredients, seafood consumption, and by-product utilisation pathways. Additionally, the methodology should be designed so it can be aligned with other impact categories in LCA software for a more comprehensive assessment. Such a strategy would enable decision makers to make better informed choices and to optimise global nutrient availability from capture fisheries and aquaculture combined.

Contributions:
All authors contributed to this blog.