Lipids and their constituent fatty acids are, along with proteins, the major organic constituents of fish, and they play major roles as sources of metabolic energy for growth including reproduction, and movement including migration. Furthermore, the fatty acids of fish lipids are rich in ω3 long chain, highly unsaturated fatty acids (n-3 HUFA) that have particularly important roles in animal nutrition, including fish and human nutrition, reflecting their roles in critical physiological processes. Indeed, fish are the most important food source of these vital nutrients for man Thus, the long standing interest in fish lipids stems from their abundance and their uniqueness. This review attempts to summarise our present state of knowledge of various aspects of the basic biochemistry, metabolism and functions of fatty acids, and the lipids they constitute part of, in fish, seeking where possible to relate that understanding as much to fish in their natural environment as to farmed fish. In doing so, it highlights the areas that require to be investigated in greater depth and also the increasing application of molecular technologies in fish lipid metabolism which will fascilitate further advances through molecular biological and genetic techniques including genomics and proteomics.

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Abstract.Because of competitive interactions in the metabolism of polyunsaturated fatty acids, tissue and bodily requirements for each of the three dietary essential fatty acids in marine fish, 22:6n-3, 20:5n-3 and 20:4n-6, cannot be meaningfully considered in isolation. Rather, it is necessary to consider requirements in relative as well as absolute amounts, i.e. in terms of the ratio of 22:6n-3 : 20:5n-3 : 20:4n-6. This is illustrated by recent research in our laboratories which has suggested that the optimal dietary ratio of 22:6n-3 : 20:5n-3 in sea bass larvae is circa 2:1 with the optimal dietary ratio of 20:5n-3 : 20:4n-6 being circa 1:1. The optimal dietary ratio of 22:6n-3 : 20:5n-3 in turbot and halibut larvae is similarly circa 2:1 but the optimal dietary ratio of 20:5n-3 :20:4n-6 in these species is 10:1 or greater. In addition, studies with salmon parr point to dietary 18:3n-3 and 18:2n-6 being important in determining the optimal tissue ratio of 20:5n-3 : 20:4n-6 for successful parr -smolt transition. We deduce that differences in essential fatty acid requirements for different species of fish reflect different dietary and metabolic adaptations to different habitats, and consider how such knowledge can be exploited to develop improved diets for fish, especially in their early stages of development.

Essential fatty acid (EFA) requirements vary qualitatively and quantitatively with both species and during ontogeny of fish, with early developmental stages and broodstock being critical periods. Environment and/or trophic level are major factors, with freshwater/diadromous species generally requiring C18 polyunsaturated fatty acids (PUFA) whereas marine fish have a strict requirement for long chain PUFA, eicosapentaenoic, docosahexaenoic and arachidonic acids. Other than marine fish larvae, defining precise quantitative or semi-quantitative EFA requirements in fish has received less attention in recent years. However, the changes to feed formulations being forced upon the aquaculture industry by the pressing need for sustainable development, namely the replacement of marine fish meal and oils with plant–derived products, has reintroduced EFA into the research agenda. It is particularly important to note that the physiological requirements of the fish to prevent deficiency pathologies and produce optimal growth may not parallel the requirements for maintaining nutritional quality. For instance, salmonids can be successfully cultured on vegetable oils devoid of long-chain n-3 PUFA but not without potentially compromising their health benefits to the human consumer. Solving this problem will require detailed knowledge of the biochemical and molecular basis of EFA requirements and metabolism