Food acquisition and growth of juvenile Atlantic salmon ( Salmo salar) in relation to spatial distribution of food

Abstract

Triplicate groups of juvenile Atlantic salmon ( Salmo salar) were exposed to either standing or flowing water (currents corresponding to a swimming speed of approximately 1.5 Bl s −1) for 79 days (17.06–04.09). Food was relatively evenly distributed throughout the entire water body in the 1.3 m 3 tanks with circumferential water currents, but food availability was limited to only a small part of the tanks with standing water. Most of the males (85%) were maturing at the end of the experiment. There were no significant differences in the frequencies of maturing males between treatments. Fish exposed to water currents grew significantly better than those held in standing water. On average, the immature exercised fish grew 14% faster than their unexercised counterparts, whereas differences in growth rates between exercised and unexercised fish were 24% for maturing fish. Amongst immature fish there was greater inter-individual variability in food intake in groups (CV g ) held in standing water than in those exposed to water currents. Day-to-day variations in food intake of individual fish (CV i ) were also found to be significantly greater for the salmon held in standing water. Differences in CV i and CV g between treatment groups, were expected to lead to corresponding differences in the variability of growth rates (CV SGR). The observed differences in CV SGR were, however, small, but there was a negative correlation between CV i and group average SGR. This may suggest that feeding behaviour was characterised by interference and scrambling conditions, rather than clear-cut hierarchies. The trends in the variabilities of food intake and growth of the maturing fish were similar to those seen in the immature fish, but overall variability was greater due to the fact that the maturing fish displayed reduced feeding and growth towards the end of the experiment. The results of this investigation clearly show that an adequate spatial distribution of food is essential in order to ensure rapid and homogeneous growth of farmed salmonids. The employment of water currents in rearing tanks would seem to be a useful measure for ensuring an even spread of food.