Daily feeding rhythms in Atlantic salmon .2. Size-related variation in feeding patterns of post-smolts under constant environmental conditions

Previous studies of feeding rhythms in post-smelt Atlantic salmon Salmo salar have produced variable results. Therefore, the present study was performed to clarify the source of these differences and to examine size-related variation in daily feeding patterns. A small group of hatchery reared post-smelts was kept in an indoor seawater tank under conditions of constant photoperiod and light intensity, and negligible diurnal variation in water temperature. Feeding responses were monitored by video camera throughout the day. The resultant feeding pattern for the population mirrored an earlier study, with the highest levels of feeding activity in the early morning and late evening. Differences between this and other studies may be due to variation in climate. season or life-history stage. Daily feeding patterns were size-dependent. The largest fish fed al a relatively constant level through the day, medium-sized fish fed in a pattern similar to that described above for the population and small fish fed most al first and last light. These differences were due probably to the effect of competitive interactions upon fish of different sizes. (C) 1997 The Fisheries Society of the British Isles.