Daily “predawn” peak of locomotion in fish

Abstract

Bluegill and largemouth bass held under alternating, 12-hour periods of light and “darkness” (very dim light) and fed daily at the onset of light show a predawn peak of locomotion. The peak appeared between the tenth and twentieth days when the fish became active daily during the last 1 to 3 hours of the “dark” period. When the onset of light and feeding were shifted 6 hours to a later time of day, the predawn peak delayed a small amount a day for 4 to 6 days to restore its original relation to the onset of light. When the onset of light and feeding time were advanced 6 hours to an earlier time of day, the predawn peak reappeared in 2 to 3 days. Six-hour shifts in the onset of “darkness” or in feeding time failed to cause corresponding shifts in the predawn peak. When the onset of light and “darkness” were both shifted 6 hours, the predawn peak shifted as though the onset of light alone had been altered. A peak of locomotion corresponding to the predawn peak persisted in constant “darkness” without feeding for 2 days, then disappeared and reappeared in 1 to 2 days when the periodicities of illumination and feeding were resumed. The conclusion is discussed that the predawn peak is regulated from within the fish by an internal rhythm which itself is co-ordinated by the daily change from “dark” to light.