Minnows and the selfish herd: effects of predation risk on shoaling behaviour are dependent on habitat complexity

Hamilton's theory of the selfish herd suggests that gregarious behaviour is a strategy by which individuals seek cover behind other group members to reduce the risk of predation. Therefore, it can be predicted that individuals will have less reason to seek cover within a group if the habitat itself provides physical structure, and therefore cover. We tested this prediction with European minnows, Phoxinus phoxinus, in a large, seminatural stream channel using a fully factorial design in which habitat complexity and predation risk were manipulated independently. We recorded shoal size, rates of movement and association preferences for familiar fish. Consistent with predictions, European minnows responded to the presence of predatory pike, Esox lucius, by forming larger shoals, but this effect was observed only in structurally simple habitats. Fish also responded to increased predation risk by reducing their rates of movement, and this was particularly pronounced in complex habitats. Association preferences for familiar fish were unaffected by predation risk in simple habitats. Together, these results suggest that when at risk from predation, European minnows shoal to obtain shelter, but only when there is insufficient physical structure available. Moreover, the results show the extent to which physical habitat complexity can affect the expression of gregarious behaviour.