Is diel dualism in feeding activity influenced by competition between individuals?

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Abstract
The feeding activity of eight groups of 15 rainbow trout (Oncorhynchus mykiss) each was recorded using self-feeders combined with passive integrated transponder (PIT) tag registrations. The diel pattern for all individuals was established by analysing the PIT-tag registrations. Most trout released the self-feeders during daylight, but in all groups some individuals (1–5) were active mainly during the night. The hypothesis that dualism in diel feeding activity is influenced by competition for food resources between individuals was then tested. In the first round, the diurnally active group members were removed, leaving groups with only 5 mainly nocturnally active individuals. After 3 weeks, the nocturnal individuals still released the self-feeders mainly in darkness. In a second round, the nocturnal individuals were removed, leaving the diurnal ones to self-feed in smaller groups. Again, no switch in activity occurred, but the diurnal trout continued to release the triggers in daylight. When food was available at all hours, individual fish seemed to gain little by releasing the demand feeders during a specific part of the light–dark cycle. The diurnal individuals were significantly (p < 0.05) larger than the nocturnal ones in the first round, but there were no significant (p > 0.05) differences in specific growth rate between the two categories. During the second round, there was no difference in mass between nocturnal and diurnal individuals, whereas the former had a significantly (p < 0.05) higher growth rate. The ecological benefits of dualism in feeding activity within groups of fish are discussed.