IS DIEL DUALISM IN FEEDING-ACTIVITY INFLUENCED BY COMPETITION BETWEENINDIVIDUALS

The feeding activity of eight groups of 15 rainbow trout (Oncorhynchus mykiss) each was recorded using self-feeders combined with passive in tegrated transponder (PIT) tag registrations. The diel pattern for all individuals was established by analysing the PIT-tag registrations. M ost trout released the self-feeders during daylight, but in all groups some individuals (1-5) were active mainly during the night. The hypot hesis that dualism in diel feeding activity is influenced by competiti on for food resources between individuals was then tested. In the firs t round, the diurnally active group members were removed, leaving grou ps with only 5 mainly nocturnally active individuals. After 3 weeks, t he nocturnal individuals still released the self-feeders mainly in dar kness. In a second round, the nocturnal individuals were removed, leav ing the diurnal ones to self-feed in smaller groups. Again, no switch in activity occurred, but the diurnal trout continued to release the t riggers in daylight. When food was available at all hours, individual fish seemed to gain little by releasing the demand feeders during a sp ecific part of the light-dark cycle. The diurnal individuals were sign ificantly (p < 0.05) larger than the nocturnal ones in the first round , but there were no significant (p > 0.05) differences in specific gro wth rate between the two categories. During the second round, there wa s no difference in mass between nocturnal and diurnal individuals, whe reas the former had a significantly (p < 0.05) higher growth rate. The ecological benefits of dualism in feeding activity within groups of f ish are discussed.