Coupling body weight and its composition:: a quantitative genetic analysis in rainbow trout

We examined whether selection for rapid growth rate in rainbow trout, Oncorhynchus mykiss, will potentially lead to correlated genetic responses in body composition. The potential for correlated responses was evaluated by estimating heritabilities and genetic correlations for body weight and traits characterising body composition (condition factor, wasted biomass, fillet weight, three measures of fat, flesh redness, percent fillet protein, ash and water) at the age of 3 years in a half/full-sib mating design. Percent abdominal fat from body weight, percent fillet protein, ash and water were the only traits to display heritabilities close to zero (h(2)=0.02-0.06). The other body composition traits and body weight displayed moderate levels of genetic variation (h(2)=0.11-0.45), making direct and correlated genetic responses to selection possible. Genetic correlations of body weight with percent fillet fat, protein, ash and water and flesh redness were low (r(A)=-0.12-0.36), indicating that the quality of fillets is not strongly changed by the selection for rapid growth rate. In contrast, the weight of abdominal fat and waste will be increased as a correlated response, because due to a part-whole relationship, the traits showed strong positive genetic correlations with the body weight (r(A)=0.63-0.98). Moreover, we found large differences between sexes especially in the traits that are closely related to body size (i.e., condition factor, weight of waste and abdominal fat) and, to a lesser extent, in the traits related to fillet quality. However, the differences between sexes in body composition could be mainly explained by the difference between sexes in body size. In conclusion, the genetic properties of body weight and body composition traits of the rainbow trout population studied do not impose strong constraints on the progress of the breeding programme. (C) 2002 Elsevier Science B.V. All rights reserved.