Provenance, shoal size and the sociobiology of predator-evasion behaviour in minnow shoals

Abstract

An investigation of the predator-evasion behaviour of minnow ( Phoxinus phoxinus ) shoals confronted with a pike ( Esox lucius ) showed that individual minnows generally chose the behaviour that minimized their chance of being eaten by the predator. As soon as the pike had been detected, minnows switched from dispersed small shoals to a single compact school. They then commenced inspection behaviour, during which individuals or groups approached the predator. This inspection served to confirm recognition of the pike and provide information on its behaviour. Avoidance and skittering behaviour took place when the pike began stalking. It was only when the predator escalated its attack and struck at the shoal that the minnows performed their most costly predator evasion tactics, such as flash expansion and fountain. After such tactics individuals often became separated from the shoal and as such were most vulnerable to capture. As a last resort, individual minnows hid among stones. Minnows from provenances with and without pike exhibited a similar repertoire of antipredator behaviour patterns, but those sympatric with the predator integrated their tactics more effectively and regained pre-exposure behaviour sooner after each encounter. Shoal size had an important effect on the execution of tactics. Minnows in shoals of 10 were more likely than minnows in shoals of 20 or 50 to abandon schooling behaviour and seek cover as individuals.