P. Dhert, P. Lavens, P. Sorgeloos
Abstract:
Since no artificial feed formulation for start feeding marine larval fish has been developed yet, feeding live preys to the first larval stages remains essential in commercial hatchery operations. Since culture rotifers and the commercial available Artemia strains being relatively poor in eicosapentaenoic acid (EPA: 20:5n-3) and docosahexaenoic acid (DHA: 22:6n-3) it is essential and common practice to enrich these live preys with emulsions of marine oils.
In rotifers the short term exposure to oil emulsions results in lipid-encapsulated rotifers with high EPA and DHA levels. However, these rotifers are prone to fast losses of their gut content when not consumed immediately. Also, they show a distortion in their protein/lipid balance. More common and more appropriate when medium to low enrichment values in live preys are needed, are formulated culture diets which ensure a better protein/lipid ratio and lower changes in nutritional value after being transferred to fish tanks.
In Artemia the most commonly applied boosting technique is a 24 h enrichment period after hatching. However, the variability of enrichment studied in one Artemia strain (Great Salt Lake, Utah, USA) by the ICES Working Group on Mass Rearing of Juvenile Fish showed a high variability in fatty acid bio-accumulation be it under laboratory or commercial conditions. In order to avoid the variation originating from differences in commercial preparations standardized ICES emulsions with different HUFA and DHA/EPA ratio have been formulated and can be obtained for research purposes. It should be emphasized, however, that the enrichment technique has limitations as Artemia is selectively catabolizing some of the nutrients such as DHA and phospholipids. Research on the kinetics of DHA catabolism in various Artemia strains has shown that DHA catabolism is strain dependent and could partially be overcome by the use of strains of different geographical origin.
Nowadays, various enrichment emulsions have been formulated differing in the fatty acid composition of their triglycerides. In this respect the traditional formulations rich in EPA are replaced by new products rich in DHA and arachidonic acid. In order to reduce the risk for oxidation of these fatty acids higher concentrations of vitamin E are incorporated into the emulsions. Also vitamin C has been incorporated in booster formulations which enable to increase the level of ascorbic acid in Artemia to 2000 ppm.
All these changes in the formulation of the enrichment diets offer more possibilities to cover the needs of different species and help to reduce problems related to diseases, stress resistance, malformation and pigmentation in numerous fish species.
(Laboratory of Aquaculture & Artemia Reference Center, Rozier 44, B-9000 Gent, Belgium)
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