EVIDENCE FAR AN ENDOGENOUS CIRCADIAN-RHYTHM OF FEEDING IN THE TROUT (ONCORHYNCHUS-MYKISS)

The possible endogenous circadian rhythm in the feeding activity of rainbow trout (Oncorhynchus mykiss) was investigated using individual fish previously trained for self-feeding. Under LD 12:12 conditions, the fish showed a diurnal behaviour, in many cases with a feeding rhythm with two main peaks of food demand at dawn and dusk, with an 8h interval of low feeding activity, and the actograms showed an expected 24 h rhythm. Fish kept under constant conditions (L : L, 15 degrees+/-0.5 degrees C), showed free-running feeding activity for about 12 days. Food demands were concentrated at dawn, with a periodogram of 25.3 hour, under continuous environmental conditions. Results showed evidence for the endogenous origin of the circadian rhythm of feeding in this species.