Abstract and Summary
The aim of this work was to investigate the relationship between shoal size and vigilance. The behaviour of minnows (Phoxinus phoxinus) foraging on an artificial food patch during the simulated stalking approach of a model predator (pike: Esox lucius) was recorded for shoals of 20, 12, 6 and 3 fish. Minnows in large shoals reduced their foraging sooner but remained feeding on the patch longer than in small shoals. The relatively late reaction of small shoals to the model and the rapid cessation of feeding once the predator was detected, indicates that small shoals were less vigilant than large shoals. The gradual reduction of foraging in large shoals was accompanied by an increasing number of investigative approaches in which individuals monitored the model's approach. This enabled minnows in larger shoals to balance more efficiently the conflicting demands of feeding and watching for predators.