Circadian rhythms and individual variability of self-feeding activity in groups of rainbow trout Oncorhynchus mykiss (Walbaum)

Abstract

An experiment was conducted to examine whether circadian self-feeding rhythms exist in groups of rainbow trout Oncorhynchus mykiss (Walbaum) and to investigate individual variability of self- feeding and its influence on growth. Ten trout of 144.9 ± 12.0 g (mean ± SD) were transferred to each of eight 290-L tanks and given free access to feed via a self-feeder. Fish in four of the eight groups were individually tagged, and the individual activating the trigger was identified using video observations. The self-feeding activity of each group was recorded under a light:dark cycle of 16.5:7.5 h and under continuous light (LL) conditions. Although diel self-feeding rhythms in the groups of trout were, in general, diurnal, night-feeding activity was observed in the initial LD phase, but there was a progressive reduction and eventual cessation in most groups. Endogenous circadian self-feeding rhythms were detected in the groups of trout under LL conditions. The trout formed dominance hierarchies of self-feeding activity, with one or two individuals accounting for the majority of trigger actuations. However, the dominants did not occupy the area surrounding the trigger. Growth rates did not differ significantly between the dominants and the subordinates within the groups.