Body length assortative shoaling in the European minnow, Phoxinus phoxinus

We investigated the functional aspects of active shoal choice behaviour and its role in generating body length assortative fish shoals. We presented single European minnows with a choice between two conspecific shoals of equal number, one of which consisted of fish of similar body length and the other of fish that were smaller or larger than the focal fish. The preference of the focal fish for the matched shoal increased with increasing difference in body length between itself and the unmatched shoal. To test a potential function of this, we used feeding trials to investigate the competitive ability of a focal fish in a shoal of smaller or larger conspecifics. As body length differences between the focal fish and its shoalmates increased, the focal fish took fewer prey items and took longer to take the first one. Collection of free-ranging fish shoals at the study site showed a within-shoal variation of body length consistent with the preference behaviour observed in the laboratory. We discuss the importance of this work in the context of body length distributions in free-ranging shoals. (C) 2001 The Association for the Study of Animal Behaviour.