Feeding, growth and social interactions during smolting and seawater acclimation in Atlantic salmon, Salmo salar L.

Abstract

In the wild, parr of Atlantic salmon, Salmo salar L., normally show aggressive, territorial behaviour, whereas smolts form non-aggressive schools. In order to study changes in feed acquisition in hatchery reared Atlantic salmon smolts, individual feed intake and hypoosmoregulatory capacity were monitored during smolting and seawater acclimation, using X-radiography and 24 h seawater challenge tests. In both the seawater transferred fish and the control fish remaining in freshwater, weight-specific feed intake and specific growth rates increased during the spring. One week after seawater transfer, there was a temporary decrease in feed intake and an increase in the number of non-feeding salmon. Four weeks after transfer, the fish had compensated for the growth suppression, and there were no differences in weight, feed intake and growth rates. Feed intake was weakly correlated with hypoosmoregulatory capacity, both in freshwater and after seawater transfer. There were no changes in the coefficient of variation in feed intake during smolting and seawater acclimation, and there was no relationship between weight, feed intake or growth in the freshwater phase and the subsequent feed intake in seawater. The study indicated that the decrease in feed intake after seawater transfer may not be explained as a behavioural change in social interactions, but rather as a transient acclimation to seawater.