Biochemical composition of live feed used in cod larvae production - As good as it gets, or room for improvement?

Abstract

Low survival rate, slow growth and developmental abnormalities during the larval stage are challenges related to aquaculture of marine species. Several studies have shown that feeding the larvae their natural feed source, zooplankton, improves these factors significantly compared to feeding them intensive feeds like rotifers. The nutritional quality of the feed is likely to play a crucial role. Components suggested to have an impact are, for instance, the typical marine fatty acids eicosapentaenoic (EPA) and docosehexaenoic acid (DHA). A high content of free amino acids (FAA) and peptides are also believed to be of importance due to limited differentiation of the digestive system in the larvae. In addition, the content of some minerals is believed to be crucial. The main objective of this study was to examine whether there were differences in biochemical composition between zooplankton and two intensive feeds (A and B) that could, at least in part, explain the increased growth when feeding on zooplankton at Lofilab AS (Steine, Norway). Natural rotifers are deficient or low in several nutritional components required for fish and thus they are normally enriched prior to feeding. A secondary aim was to compare the biochemical composition of the rotifers prior to and after enrichment. Three commercial enrichment media were also analysed and two underutilised marine sources of nutrients (krill and calanus) were included. The results showed large differences in biochemical composition between zooplankton and the intensive feeds, in particular in components of typical marine origin. The sum of the most typical marine fatty acids, EPA+ DHA, in zooplankton was 38.5% of the total lipid fraction, corresponding to 3.1% of the diet. In the intensive feeds the EPA+DHA were 21.7% (A) and 20.5% (B) respectively, corresponding to 2.6% and 2.4% of the diets. The relative amount of FAA in zooplankton was 15.7%, while the equivalent amounts in the intensive feeds were 10.4% and 12.1% in feed A and B respectively. Zooplankton was rich in taurine, 8.4 mg/g dry weight (DW), while the two intensive feeds were almost devoid of it, reflecting the composition of the enrichment media. Except for phosphorus, zooplankton was richer in all of the minerals analysed. The main effect of the enrichment of rotifers was an increase of the amount of EPA and DHA in two out of three enrichment protocols. The existing enrichment protocols were ineffective if the target was to increase the content of the other nutritional components. The biochemical compositions of krill and calanus were more similar to zooplankton, as the content of taurine and EPA+DHA was higher than in the commercial enrichment media, thus supporting the idea of utilising these marine resources as new and improved enrichment media.