Optimising lipid nutrition in first-feeding flatfish larvae

Although global production of flatfish has increased in recent years, both in terms of numbers of fish and diversification into new species, problems still remain with low survival rates and difficulties with metamorphosis. This short review highlights some advances made in optimising lipid nutrition in an attempt to overcome some of these problems. Copepod nauplii are the best live prey for first-feeding flatfish larvae. Rotifers can provide a useful method of essential fatty acid delivery but Artemia are poorer in this regard, especially at first feeding. Copepods are nutritionally beneficial due to their naturally high levels of the essential highly unsaturated fatty acids (HUFA), 20:5n-3 (eicosapentaenoic acid; EPA) and 22:6n-3 (docosahexaenoic acid; DHA), which are predominantly in the form of phospholipids. Rotifers can be enriched with fish oil emulsions to provide compositions similar to copepods, while enriched Artemia are difficult to enrich with high levels of DHA and the HUFA tend to be located in triglycerides rather than phospholipids. There is considerable evidence that the superior efficacy of copepods and rotifers is largely due to the digestibility and availability of HUFA supplied as pre-formed phospholipids. In addition to the essentiality of EPA and DHA, the requirement for 20:4n-6 (arachidonic acid; ARA) should also be considered. Improvements in dorsal pigmentation in turbot and halibut can be achieved by providing ratios of DHA/EPA of >2:1 but, perhaps more importantly, an EPA/ARA ratio of >5:1. This suggests that eicosanoids are involved in the control of pigmentation and this is further supported by the use of enrichments containing 18:3n-6, the elongation product of which (20:3n-6), is a potent inhibitor of ARA-derived eicosanoid formation, as is EPA.