Direct and indirect selection of visceral lipid weight, fillet weight and fillet percent in a rainbow trout breeding program

¹ MTT Agrifood Research Finland, Biotechnology and Food Research, Biometrical Genetics, FI-31600 Jokioinen, Finland
² Finnish Game and Fisheries Research Institute, Tervo Fisheries Research and Aquaculture, FI-72210 Tervo, Finland

^* To whom correspondence should be addressed. E-mail: Antti.Kause{at}mtt.fi.

	Abstract

We assessed whether visceral lipid weight, fillet weight, and percent fillet from body weight, three traits laborious to record, could be genetically improved by indirect selection on more easily measured traits in farmed rainbow trout. Visceral lipid is discarded during slaughter as waste, influencing production efficiency and production costs. Fillet weight and fillet percent directly influence economic returns in trout production. The study comprised three steps. First, we assessed the degree to which selection on percent visceral weight from body weight indirectly changes visceral lipid weight and the size of intestines and internal organs. The phenotypic analysis of weights of viscera, intestines, visceral lipid, liver, and gonad measured from 40 fish revealed that phenotypic selection against visceral weight is most strongly directed to visceral lipid, and to lesser degree to intestines and gonads. Because genetic relationships between these traits were not established, it is not known whether the indirect selection leads to genetic responses. Secondly, we examined whether direct selection for the fillet traits could be replaced by indirect selection on body weight, gutted weight, visceral weight, visceral percent, head volume, and head volume relative to body weight. The selection index calculations based on the quantitative genetic parameters obtained from multigenerational pedigree data showed that genetic improvement of fillet percent through direct selection (selection accuracy, r_TI = 0.54) was equally efficient compared to indirect selection on visceral percent (r_TI = 0.54). Genetic improvement of fillet weight through direct selection (r_TI = 0.56) was always more efficient than indirect selection. Yet, indirect selection for gutted weight (r_TI = 0.50) was almost as efficient as direct selection. Thirdly, the expected genetic responses to alternative selection indices showed that improved fillet percent was mainly a result of a moderate decrease in visceral weight rather than of a major increase in absolute fillet weight. Moreover, fillet percent is challenging to improve, even if it exhibits moderate heritability (h² = 0.29). This is because fillet percent displays low phenotypic variation. In conclusion, fillet weight and fillet percent can be increased by indirect selection against visceral percent and for high gutted weight.

Key Words: fillet yield, genetic correlation, heritability, lipid deposition, quantitative genetics, rainbow trout