Abstract
Locomotor activity in the rainbow trout Oncorhynchus mykiss was recorded at 10°C using an infrared photocell system. Under light-dark cycles of 12:12 h (LD 12:12), all fishexhibited diel rhythms with higher activity during the light phase. When fish were exposed to constant conditions such as constant darkness or constant dim light, fish exhibited free-running rhythms of locomotor activity with τ in a circadian range. When fish were subjected to phase shift of LD 12:12 cyclesby doubling the light phase or the dark phase, resynchronization to the new LD cycles was attained notimmediately but a few cycles of transient phase were observed. These results indicate that diel rhythms of locomotor activity in the rainbow trout are strongly influenced by a circadian clock entrained to LD cycles.
