Seasonal changes in feeding motivation of juvenile Atlantic salmon (Salmo salar)

Abstract

It is known that juvenile Atlantic salmon populations divide into two size categories in their first autumn. The larger fish migrate to sea the next summer, whereas the lower modal group fish remain in freshwaters for at least another year. The size difference arises from a reduced food intake by lower modal group fish in autumn. We experimentally investigated the feeding motivation of lower modal group salmon from July to September of their 1st year. Feeding motivation declined through the season, regardless of competition, food supply, or water temperature. This was evident at each stage in the sequence of capturing prey: orientation was less likely to lead to attack, and an attack was less likely to result in the food being consumed. Fish also reduced the distances over which they would travel to intercept food, and delayed their attacks until passing particles had reached their closest approach. There was thus a general shift in foraging strategy in most fish towards minimizing the costs of obtaining a maintenance ration. It would therefore appear that lower modal group juvenile salmon do not grow during the winter as a result of internal suppression of appetite, which commences in midsummer.