Genetic potential for simultaneous selection of growth and body composition in rainbow trout (Oncorhynchus mykiss) depends on the dietary protein and lipid content: Phenotypic and genetic correlations on two diets

In farmed salmonids, there is an unfavourable genetic relationship between high body weight and increased body lipid percent at a fixed age. This forces breeders to control lipid deposition in order to maintain appropriate end-product quality. Here we tested the hypothesis that this unfavourable genetic relationship can be diminished when fish are reared on a relatively low lipid and high protein diet. To test the hypothesis, a total of 2931 rainbow trout from 210 families were reared using two diets in a split-family design to weight of 2.5-2.7 kg. The diets were a normal protein, high lipid diet (NP) representing modem-type feed, and an experimental high protein, low lipid diet (HP). As hypothesised, phenotypic and genetic correlations of muscle and body lipid percent with body weight were more favourable on HP than NP diet. The correlations ranged from strongly negative to close-to-zero on HP diet but from strongly positive to close-to-zero on NP diet. These results indicate that alternative high protein, low lipid diet partly uncouples lipid deposition from growth, providing more favourable genetic architecture for the simultaneous genetic improvement of growth, body composition and end-product quality. The results for viscera percent from body weight, which is an indirect estimate of visceral lipid, differed from those of muscle and body lipid percent. Phenotypic and genetic correlations between viscera percent and body weights were negative or weakly positive without differences between diets. The correlations differed between diets for muscle and body lipid but not for visceral lipid because these traits are genetically different. (C) 2007 Elsevier B.V. All rights reserved.