Seasonal changes in the short-term activity and ultradian rhythms of a cyprinid fish, the lake chub, Couesius plumbeus

Abstract

The temporal pattern of the locomotor activity of a cyprinid fish, the lake chub, Couesius plumbeus, was examined under seasonally appropriate light–dark cycles that included twilights and under constant darkness. Photoperiodically entrained and free-running circadian activity from constant darkness was composed of nonrandomly distributed short-term activity components (0.5–3.5 h) with significant ultradian frequencies. In both cases there was an annual rhythm in the length of the ultradian periods, ranging from 0.7 h in winter to 2.5 h in summer. This pattern was accompanied by annual changes in the frequency distribution of short-term activity components. Ultradian periods were correlated with both daylength in the entraining light–dark cycles and the seasonal changes in the circadian period of fish free-running under constant darkness. These results suggest that the expression of ultradian rhythms and short-term locomotor bouts is controlled by a multioscillator circadian system.