Two feeding experiments were conducted to evaluate the nutritive value of rotifers enriched with different levels of docosahexaenoic acid (DHA) for larval cod Gadus macrocephalus.
Rotifers containing different levels of DHA (0-3.1%) were prepared by the direct method in which triglycerides, methyl esters, Euglena and shark's egg (dried powder), each containing different amounts of DHA and eicosapentaenoic acid (EPA), were emulsified. The larvae fed on rotifers containing more than 1.5% DHA showed high percentages of abnormal fish floating on the water surface together with high mortalities and hydrops in eyes and fins. These abnormalities and mortality were lowest in the fish fed the rotifers containing about 1% DHA. The larvae fed the rotifers containing 2% DHA also showed good growth performances. These fish contained 11μg/g vitamin E (VE) dry basis in the whole body, while other groups of fish had only a trace amount of VE. On the other hand, the DHA content in fish rapidly decreased from 0.6 to 0.1-0.2g/100g (wet basis) during 20 days after hatching in all groups of fish.
It is inferred that a suitable level of DHA in rotifers for larval cod is approximately 1% and that co-existance of VE seems to be required for effective utilization of DHA.