Genetic variability in residual feed intake in rainbow trout clones and testing of indirect selection criteria (Open Access publication).

Little is known about the genetic basis of residual feed intake (RFI) variation in fish, since this trait is highly sensitive to environmental influences, and feed intake of individuals is difficult to measure accurately. The purpose of this work was (i) to assess the genetic variability of RFI estimated by an X-ray technique and (ii) to develop predictive criteria for RFI. Two predictive criteria were tested: loss of body weight during feed deprivation and compensatory growth during re-feeding. Ten heterozygous rainbow trout clones were used. Individual intake and body weight were measured three times at three week intervals. Then, individual body weight was recorded after two cycles of a three-week feed deprivation followed by a three-week re-feeding. The ratio of the genetic variance to the phenotypic variance was found high to moderate for growth, feed intake, and RFI (VG/VP = 0.63+/-0.11, 0.29 +/-0.11, 0.29 +/-0.09, respectively). The index that integrates performances achieved during deprivation and re-feeding periods explained 59% of RFI variations. These results provide a basis for further studies on the origin of RFI differences and show that indirect criteria are good candidates for future selective breeding programs.

Group feeding behavior of brown trout is a correlated response to selection for growth shaped by the environment.

Group feeding pattern is a response correlated to selection for growth in brown trout. Previous results suggest that the feeding pattern of two lines (NL and SY), selected with the same process at two different fish farms, differ. This study addresses the following questions: (i) are the patterns differentially fixed by the selection process and if yes (ii) is this explained by the different genetic background of the lines? In a first experiment, we tested if the group feeding behavior can be inherited. We produced and reared under the same experimental conditions selected hybrids (NLS x SYS) and pure SY selected line (SYS). We compared their behavior on self-feeders when reared single or mixed (groups of 500 fish; mean initial BW = 6 g, 90-day period). Growth rates were comparable among the groups. In the lines reared single, NLS x SYS exhibited greater morning feeding activity than SYS. Feeding activity of SYS was higher later in the day. The inter-day variation of the morning peak was less variable for the hybrid than for the SYS line. In the mixed group, the pattern was intermediate between that of the hybrid and the pure line. In a second experiment we compared the feeding pattern of control hybrids (NLC x SYC) to pure control lines (SYC and NLC) reared single or mixed (groups of 500 fish; mean initial BW = 12 g, 92-day period). Feeding activity patterns were similar among the control groups. These results show that group feeding pattern is a heritable trait that can be indirectly selected with growth, which shape is less influenced by the growth performance and the genetic background than by the environmental conditions applied during the selection process.

The evaluation of energy intake adjustments and preferences in juvenile rainbow trout fed increasing amounts of lipid.

This study aimed at a better understanding of the feed intake (FI) regulation in rainbow trout by dietary digestible energy (DE). The DE contents of the three diets (20.5, 23.0 and 24.7 kJ per g dry diet) were modified by supplementing different amounts of fish oil. The crude lipid and protein levels were 13%, 26% and 34% and 64%, 54% and 48%, respectively (% dry diet). The daily FI was measured by means of self-feeders in groups of rainbow trout (32-55 g, initial BW) during two 5 to 6-week trials. Their eventual preference for one of the lipid levels was evaluated by offering the choice between a low and higher lipid diet. The results indicated that fish of a similar body mass had a similar FI without apparent energy intake compensations. It is believed that the excessive energy intakes with the higher lipid diets were not caused by a higher palatability of these diets since the trout did not express any particular preference. The trout fed the lipid rich diets had a higher level of body adiposity, but a similar protein growth. The observation that the trout did not reduce FI when fed the high lipid diet implies a low negative feedback by the ingested fat or by the increase in body adiposity. The similarities in lean carcass growth favour the idea that growing animals regulate their FI in order to meet the demand for maximal protein growth rather than to satisfy a predetermined energy requirement.