Group size influences foraging effort independent of predation risk: an experimental study on rainbow trout

Abstract

Foraging effort, swimming activity, vertical position and flight response were recorded in focal juvenile rainbow trout Oncorhynchus mykiss at three group sizes: without company, or in visual and chemical contact with either one or five companion fish at two levels of predation risk: high (simulated aerial predator attack) or low (no attack). The predator attack induced a pronounced flight reaction as well as a reduction in vertical position, feeding and swimming activity. The foraging effort of the focal fish increased with group size independent of the level of predation risk, which suggests that the group‐mediated increase in foraging activity is caused by competition rather than by reduction in perceived risk. The flight response to the predator attack, however, was stronger when the focal fish had company, suggesting that individuals may benefit from copying the anti‐predator response of other group members.