Entire cultures have survived on diets of fish. Civilizations grew along the shores of the sea and the banks of rivers. The oceans, seas, lakes, rivers, and streams often provided an incredible bounty.

In the human diet, fish are a source of high quality protein, essential vitamins and minerals but, above all, a virtually unique, rich source of omega-3 (or n-3) long-chain polyunsaturated fatty acids (LC-PUFA) sometimes termed highly unsaturated fatty acids (HUFA). This page summarises the origins of omega-3 PUFA in fish, the increasingly important role of aquaculture in the provision of fish for human consumption, and the major issues currently facing aquaculture focusing on sustainability, driven by the urgent need to replace traditional fish oil and meal in formulated diets, and safety, driven by the requirement to reduce feed-derived contaminants.

PUFA originate in the primary producers, essentially the plants, in aquatic food chains as in all ecosystems. The marine system is characterised by high levels of eicosapentaenoic acid (EPA; 20:5n-3) and, especially docosahexaenoic acid (DHA; 22:6n-3) that originate in the microalgae (phytoplankton) and accumulate up the food chain to fish. Fish themselves vary in their ability to produce long-chain PUFA from linoleic (18:2n-6) and alpha-linoleic acids (18:3n-3). Thus, freshwater and diadromous species including salmonids such as Atlantic salmon and trout, have the enzymatic machinery to produce EPA and DHA from alpha-linolenic acid whereas marine fish are unable to perform these conversions due to deficiencies at one or more enzymatic steps in the HUFA biosynthetic pathway.

Around the globe, virtually every fishery is in decline due to over-exploitation. As a result, farmed fish represent an ever-increasing proportion of fish in the human food basket. Presently, one third of all fish marketed is farmed, and aquaculture is the fasted growing animal protein food sector, currently increasing at about 9% per annum. Paradoxically, traditional diets in aquaculture have been based on fishmeal and fish oil, themselves derived from industrial reduction fisheries. These fisheries have, at best, reached their sustainable limit making fish meal and oil finite resources. To continue to expand, aquaculture must find suitable, sustainable alternatives. Plant-based products are the obvious choice but vegetable oils are devoid of LC-PUFA and fish fed high levels of vegetable oil are characterised by increased levels of C18 fatty acids and reduced levels of the omega-3 LC-PUFA, EPA and DHA, compromising their nutritional benefit to the human consumer. Short-term strategies for addressing this issue include the use of fish oil “finishing’ diets to restore levels of EPA and DHA prior to harvest. Longer-term strategies involve finding other sources of omega-3 LC-PUFA. Alternative marine sources such as sourcing from lower trophic levels such as zooplankton pose serious technological and environmental problems. Increasing primary production appears the only lasting solution. Algal culture and microorganism fermentation technologies, although developing, are not currently capable of supplying the volume of oil required. Production of omega-3 LC-PUFA by oilseeds is the most promising solution but this is not possible by conventional breeding programmes as the necessary genes are not present in higher plants. Therefore, a transgenic approach is required and genetically-modified oilseeds with the ability to produce and accumulate EPA are already on the horizon. A much greater biotechnological challenge will be to develop transgenic oilseed plants with the ability to produce and accumulate DHA in their seeds.

Contamination by harmful pollutant

As well as accumulating the beneficial omega-3 LC-PUFA from their diet, fish also receive a number of undesirable compounds through the diet. These include toxic elements such as mercury and arsenic, as well as lipid soluble contaminants such as dioxin/furans, polychlorinated biphenyls (PCBs) and the polybrominated diphenylether flame retardants (PBDEs), collectively referred to as persistent organic pollutants (POPs). The oceans are a sink for these pollutants, and they accumulate up the food chain. Therefore, it is important to stress that wild-caught fish contain these contaminants at varying levels depending upon the physiology of the species (flesh lipid contents etc.) and geographical location (e.g. Baltic). Farmed fish fed marine products also accumulate these contaminants and this has led to damaging media scare stories. The levels of contaminants allowed in food are strictly controlled by a number of regulatory bodies, and likely to show a decreasing trend and so reduction of contaminant levels in farmed fish has a high priority. Strategies include sourcing raw materials from “clean” environments, replacement of dietary marine products (as described above) and the use of decontaminated fish meals and oils. The overarching challenge for the sustainable development of aquaculture is to continue to produce safe and nutritious fish for the burgeoning human population against the background of diminishing global marine resources.

For further related information, please find these useful articles:

Cod Liver Oil: The SuperFood

Health Benefits of Omega-3 Fish Oil

References:

1. Tocher, D. R. (2003) Metabolism and functions of lipids and fatty acids in teleost fish. Reviews in Fisheries Science 11, 107-184.
2. Bell, J.G. and Waagbø, R. (2008) Safe and nutritious aquaculture produce: benefits and risks of alternative sustainable aquafeeds. In: Holmer M, Black KD, Duarte CM, Marba N, Karakassis I, editors. Aquaculture in the Ecosystem. Berlin: Springer Verlag BV; pp. 185-225.