Recent Developments in the Essential Fatty Acid Nutrition of Fish

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 : 20:5n3 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.