Selection for growth and feeding hierarchy in brown trout

A line of brown trout as been selected for longer body length up to five generations and a control line has been maintained. In order to ascertain whether this selection had caused correlated responses in feeding behaviour, we measured the group feeding hierarchy and the regularity of intake at the individual level with the X-radiographic technique on four occasions, when the lines were reared separately or mixed together and fed ad libitum. Individual intakes were higher in selected than in control fish at comparable weight. Feeding ranks were highly variable in both lines, however, a feeding hierarchy was present in all groups, and the fish eating the smallest proportion of the food were those displaying the largest day-to-day variations in feed intake. Mixing of the two lines did not affect the overall variability of intake or growth. Altogether these results show that the increase in feed intake provoked by selection for growth is not accompanied by a more unequal share of food. This study demonstrate that increased feeding competition between individuals do not arise as an inevitable consequence of growth selection.