Response of growth-selected brown trout (Salmo trutta) to challenging feeding conditions

Correlated responses for feed intake, feed utilization and growth were measured in brown trout from lines after five generations of selection on fork length (S) and in control (C). Groups of fish with the same initial weight (5 g) were constituted (500 S, 500 C, 250 S + 250 C) and were either fed ad libitum using self-feeders or were food restricted (75% ad libitum) for 87 days (2 feeding levels × 3 replicates for S, C, and mixed groups). After 34 days of food deprivation, all groups were re-fed ad libitum using self-feeders for 41 days. Greater growth of S compared to C was related to higher feed intake (line effect on cumulative intake, P = 0.0041), but feed efficiency was similar (1.4 on average), irrespective of feeding level. During the food deprivation phase, S lost more weight than C (P = 0.0024). When fish from the two lines were mixed they exhibited the same growth when fed ad libitum; when food was restricted, S grew faster than C (P = 0.0190), suggesting that S may consume a higher proportion of the available food than C. The daily profile of feeding activity was comparable among the lines. Within groups, weight variation (assessed as CV) was lower in S than in C at all the experimental phases, suggesting that food sharing may be more homogeneous in S than in C. S fish were more slender than C, had lower hepatosomatic and viscerosomatic indices and a higher dressing %. The robustness of the response correlated to our selection procedure is discussed.