Several micronutrients in the rotifer Brachionus sp. may not fulfil the nutritional requirements of marine fish larvae

The current best practice intensive culture of larval Atlantic cod includes feeding rotifers from onset of exogenous feeding until 25–30 days after hatching. These larvae grow considerably slower and develop higher frequencies of deformities than larvae reared in semi-extensive systems, using copepods as feed. The present study compares the micronutrient concentrations in rotifers with those of copepods, with the aim of identifying nutrients that may be limiting for normal growth and development of cod larvae. An additional criterion used is the nutrient requirements given for fish in general, by NRC (1993), as nutrient requirements of cod remains to be determined. Rotifers were fed on four different diets, consisting of baker's yeast with cod liver oil (3.3 : 1 dry weight (DW)/v), baker's yeast with Algamac 2000TM (3.5 : 1 DW), baker's yeast with live algae Chlorella (4.1 : 1 DW), and Culture Selco 3000TM (CS). CS was a complete commercial diet for rotifers while the other diets are considered as based on raw ingredients. Compared with copepod nutrient levels, rotifers grown on yeast-based diets supplemented with either cod liver oil, Algamac 2000 or Chlorella were apparently sufficient for covering the requirements in cod larvae for all the B-vitamins, except thiamine. Rotifers cultured on the CS diet also had sufficient amounts of thiamine. Of the minerals, only calcium and magnesium were sufficient, using this criterion while iron was on the borderline. However, with reference to the requirements given for larger fish (NRC 1993), only thiamine, vitamin A, manganese, selenium and perhaps copper, appear too low in the rotifers cultured without extra micronutrient supplementation. The other nutrients were present at levels intermediate between copepod and fish requirement levels. This study suggests that it is necessary to develop enrichment techniques to produce rotifers with sufficient amounts of all micronutrients. Such techniques will also be important tools for determining which nutrients are present at levels below the actual requirements in cod larvae.