Demand-feeding rhythm in rainbow trout and European catfish - Synchronisation by photoperiod and food availability

The effect of light-dark (LD) cycle and food availability was tested on the demand-feeding rhythm of single and groups of rainbow trout and European c atfish. Under LD and free food access, most trout and catfish displayed, re spectively, a diurnal and a nocturnal pattern of demand-feeding activity, w hereas a few Fish or groups of fish switched from diurnalism to nocturnalis m or vice versa. In both species held under constant lighting conditions an d a restricted feeding (R-F) cycle (PF 20:4), the demand-feeding rhythm rap idly synchronised to food availability. The demand-feeding rhythm was under endogenous control and, in rainbow trout, periodogram analysis suggested t he existence of two oscillators, one synchronised by photoperiod (LEO) and the other by food (FEO). When submitted to both LD and R-F cycles, LD was, at least in the rainbow trout, the dominant zeitgeber synchronising the dem and-feeding rhythm. In catfish, food availability rapidly synchronised dema nd-feeding rhythm. Finally, in both species, the synchronisation of single fish to LD or feed availability appeared slower than that of groups of fish , supporting the idea that social organisation affects the cireadian activi ty in fish. (C) 2001 Elsevier Science Inc. All rights reserved.