L.A. McEvoy
Abstract:
Marine fish larvae are generally unable to synthesise sufficient amounts of essential polyunsaturated fatty acid (EFAs) from dietary precursors and require them to be supplied preformed in their diet. Since the most convenient and commonly used live-prey species, rotifers and Artemia nauplii, are deficient in these EFAs, it is necessary to increase ("enrich") polyunsaturated fatty acid (PUFA) levels, particularly docosahexaenoic acid, prior to feeding the prey to fish larvae. Traditionally, this has been carried out by allowing the prey species to filter-feed on microalgae, modified yeast, or micellar emulsions of PUFA-rich marine oils, but recently the use of other preparations such as liposomes, spray-dried algae/fungi, microcapsules and silages have been reported. In addition to total PUFA-enrichment, recent emphasis is being placed on attempting to increase the polar lipid content of live-prey or, at last, to increasing the content of PUFAs in polar lipid form, because copepods, the natural prey of marine larvae, tend to be rich in polar lipids and there is evidence that these lipids may be more easily digestible. Both PUFA and polar lipid enrichment are made more difficult to achieve by the following problems inherent in the enrichment process: (a) autoxidation and instability of enrichment diets, (b) dietary lipid class conversions by live-prey, (c) retroconversion of bioencapsulated docosahexaenoic acid (DHA, 22:6n-3) to eicosapentaenoic acid (EPA, 20:5n-3), and (d) post-enrichment loss of PUFAs from Artemia nauplii prior to their ingestion by larval fish. The relative merits of the different techniques of live-prey enrichment will be discussed, together with protocol modifications such as duration and frequency of enrichment which may help to ameliorate some of the problems listed above.
(Unit of Aquatic Biochemistry, University of Stirling, Stirling, Scotland, FK9 4LA, UK.)
***************