Indications of a negative genetic association between growth and digestibility in juvenile Atlantic salmon (Salmo salar)

Abstract Digestibility is a part of the feed efficiency complex. Improved understanding of the genetics of digestibility and its relation to the most important traits in a breeding program, such as growth rate, is important for enhanced knowledge on the biological aspects of selective breeding for growth. The aim of the study was to estimate the genetic variance and heritability of nitrogen and carbon digestibility parameters and their genetic and phenotypic correlations with growth using individually recorded phenotypes and genotypes. The fish were kept in a common tank from start-feeding until the end of the experiment. All fish were individually tagged and genotyped (56 K SNP-chip) at ~5–10 g body weight. A total of 129 fish from 14families (average initial body weight of 194g) were included in an individual digestibility trial carried out over 30 days. Individual digestibility was measured as apparent digestibility of nitrogen and carbon in fecal samples, using a fishmeal-based diet with yttrium oxide as an inert marker. To obtain enough feces per fish for digestibility analysis, the fish were stripped four times, i.e., once a week. Results showed significant differences between families with respect to digestibility. Heritabilities were 0.39 ± 0.17 and 0.51 ± 0.18 for digestibility of nitrogen and carbon, respectively. Digestibility showed adverse genetic correlations to the growth rate (−0.77 ± 0.24 to −0.85 ± 0.16). A possible explanation may be that a high growth rate is related to higher feed intake, increasing the passage rate in the gastrointestinal tract and thereby reducing the digestibility of the nutrients. This, however, does not imply that there is an adverse genetic relationship between growth and feed efficiency, as the latter is determined by a number of other factors in addition to digestibility.