Diel Activity Rhythms and Thermoregulatory Behavior of Bluegill in Response to Unnatural Photoperiods

Locomotor activity patterns and temperature regulation of bluegill, acclimated to 25° C and 12-hour light/12-hour dark (LD 12: 12) or LD 15:9 were examined prior to and following shifts to unnatural photoperiods. During the control phase of both experiments activity patterns of fish were typically diurnal. Activity levels varied sharply with changes in illumination intensity: activity increased during dawn and decreased following dusk. When photoperiod was switched to LD 6:6:6:6 (from LD 12:12) fish activity levels indicated a resistance to photoperiod entrainment, particularly during the first 24 hours. Following switch from LD 15:9 to constant light (LL), the striking diurnal pattern of activity observed under LD 15:9, was no longer apparent. However, a rhythm persisted throughout the entire 96 hours of LL. Results of both experiments suggest the presence of an endogenous component mediating bluegill diel activity patterns.Switches to unnatural photoperiods were not disruptive to thermoregulatory behavior of bluegill in either experiment. Performance data collected before and after photoperiod switch in each experiment were similar. Mean preferred temperatures of grouped fish under the 4 different photoperiods (LD 12:12, LD 6:6:6:6, LD 15:9 and LL) ranged from 30.9 to 31.4° C and were not significantly different.