Growth and food utilisation of rainbow trout (Oncorhynchus mykiss) fed low-fat herring and dry diets enriched with fish oil

Two-year-old rainbow trout (initial weight 695 g) were fed for 15 weeks low-fat Baltic herring diets either with or without lipid enrichment. For comparison, similar groups of trout were fed dry pellets with and without top-dressing with fish oil. Inclusion of lipid increased growth rate in a similar manner for both types of diets. For the herring-based diet, increased lipid resulted in increased dry matter consumption but for the dry diet, feed efficiency (dry weight growth per dry weight food consumption) was improved by channelling more nutrients to lipid deposition. If compared within a stated lipid level, fish fed the herring-based diets grew at a slower rate than fish fed the dry diets. This could have been because of high dietary water content and also due to changes in herring protein quality during the frozen storage since the protein retention efficiencies were lower for the herring diets.