Nutrient profiles of rotifers ( Brachionus sp.) and rotifer diets from four different marine fish hatcheries

Cultured marine fish larvae are most often fed rotifers (Brachionus sp.) the first few weeks after first-feeding. However the nutritional value of rotifers is generally inferior to the natural diet of marine fish larvae which is comprised of zooplankton, mostly copepods. Commercial diets for rotifers vary considerably in nutrient composition and this variation is partly mirrored in rotifers. The present study was undertaken to investigate the variation in rotifers and rotifer diets used in commercial marine fish hatcheries and to identify possible deficiencies or excess of individual nutrients. Culture and enrichment diets and unenriched and enriched rotifers were sampled from four hatcheries for Ballan wrasse (Labrus bergylta) and analyzed for nutrients which can potentially fall outside the safe window of supplementation for marine fish larvae. It is concluded that rotifer diets generally contain appropriate levels of fatty acids and vitamins C and E, while vitamin A, iodine and selenium need more attention. For vitamins D and K and many of the micro-minerals, data on larval requirements are still lacking and these nutrients need further research. Protein and phospholipid levels are mainly determined by the rotifer's own metabolism and can be low compared to assumed requirements. This study shows that there is a large variation in the nutrient composition of commercial rotifer diets and a resulting large variation in the nutrient composition of rotifers fed to marine fish larvae at an industrial scale. In some cases the levels of single nutrients in rotifers are below the requirements given for fish by the NRC (2011). This means that the relatively low predictability of fish larval culture can partly be explained by lack of knowledge in nutrition and inferior commercial live feed diets. This may inspire to more research into how the nutrient composition of live feeds can be improved and what are the nutrient requirements of marine fish larvae, in order to standardize diets.