Selection for growth in brown trout increases feed intake capacity without affecting maintenance and growth requirements1

The correlated responses in feed intake and G:F ratio with selection for increased growth rate were evaluated by comparing selected (S) and control (C) brown trout (Salmo trutta) reared under conditions known to affect feed efficiency: feed restriction and periods of compensatory growth. Nitrogen and energy requirements for maintenance and growth were also measured. Trout were allotted at comparable BW (3.7 ± 0.06 and 3.8 ± 0.04 g, for C and S respectively) to triplicate groups per treatment. The experiment lasted a total of 198 d, during which animals were successively submitted to a 116-d feeding phase and fed 10, 30, 50, 70, 100, and 140% of their usual daily ration (UDR), a 35-d phase of food deprivation, and a 47-d refeeding phase. The G:F of C and S were comparable in all experimental conditions tested. During the feeding phase, S grew better than C only when fed 100 and 140% UDR (P< 0.001). This was explained by a higher feed intake capacity. The requirements for growth and maintenance were similar among the lines, which is in agreement with their comparable loss of weight (mean energy loss of −53 and −55 kJ/(kg•d) for C and S, respectively; P> 0.38) observed during the feed deprivation phase and the lack of differences in carcass composition (fat, P> 0.35; protein, P> 0.54). During the refeeding phase, growth performance and G:F were high in all groups. The daily growth coefficient was higher in S than in C (P< 0.001) because of a higher feed intake (P< 0.001). An increase in absolute individual variability in final BW and length was associated with the level of food restriction in both lines; however, it always remained lower in S than in C. In conclusion, fish selected for growth under ad libitum conditions will only exhibit growth superiority when fed diets close to ad libitum, and there was no evidence that selection was associated with an improvement in efficiency of maintenance nor in retention of body tissues.