The effect of 'Schreckstoff' on the shoaling behaviour of the minnow: a test of Hamilton's selfish herd theory

Abstract

Abstract. According to Hamilton's selfish herd theory, per capita predation risk is lower in the centre than on the periphery of a group. If this is so, it should influence the positioning behaviour of individuals in groups. Most cyprinid fishes live in shoals and respond to Schreckstoff, a fright substance located in the skin, by forming tight shoals. Fourteen dace, Leuciscus leuciscus , habituated to Schreckstoff and a single naive minnow, Phoxinus phoxinus, were combined in order to study how a single frightened fish positioned itself in a shoal. Before Schreckstoff was added to the water there were no differences in the positioning of the minnow and the dace in the shoal. However, after the introduction of Schreckstoff, the minnows moved closer to other fish and positioned themselves so that they tended to be surrounded by near neighbours on all sides. These results suggest that peripheral positions in fish shoals suffer higher predation risks and support Hamilton's selfish herd theory.