Behavioural and heart rate responses to food limitation and predation risk: an experimental study on rainbow trout

Food-restricted rainbow trout Oncorhynchus mykiss maintained a lower basal heart rate than satiated fish, probably as a result of reduced metabolic ra te. Food-restricted fish were also more active during feeding and were more willing to take risks than satiated fish. Both satiated and food-restricte d fish were positioned lower in the tank after the predator attack. Heart r ate increased more during feeding in the food-restricted fish compared to t he satiated, but energy status had no general effect on the relation betwee n heart rate and behaviour. Hence, the increase in heart rate was mainly a response to the more active foraging behaviour in the food-restricted fish. Moreover, behavioural activity in the food-restricted fish was associated with a higher heart rate after the predator attack than when the fish was u ndisturbed, which may reflect physiological preparation for flight. These f indings suggest that behavioural and cardiac responses are coadapted to mee t variation in food availability and predation risk in the wild. (C) 1999 T he Fisheries Society of the British Isles.