Species segregation during predator evasion in cyprinid fish shoals

SUMMARY. 1. When a predator attacks a mixed‐species group, individual prey arc faced with a conflict between retaining the advantages of large group size and the disadvantages of remaining among aliens. Experiments were designed to show how this conflict is resolved in fish shoals. 2. Simulated attacks with a model predator were made upon mixed and single species shoals of three cyprinid species under semi‐natural conditions in a fluvarium. 3. Measurements of elective group size, neighbour identity and distance, and encounters within and between species revealed behaviour which sorted fish into species at the onset of predator attack. 4. A control experiment demonstrated the same effect for fish of matched size. 5. The value to the individual of segregative behaviour in response to predator threat is discussed.