We investigated the functional aspects of active shoal choice behaviour and
its role in generating body length assortative fish shoals. We presented s
ingle European minnows with a choice between two conspecific shoals of equa
l number, one of which consisted of fish of similar body length and the oth
er of fish that were smaller or larger than the focal fish. The preference
of the focal fish for the matched shoal increased with increasing differenc
e in body length between itself and the unmatched shoal. To test a potentia
l function of this, we used feeding trials to investigate the competitive a
bility of a focal fish in a shoal of smaller or larger conspecifics. As bod
y length differences between the focal fish and its shoalmates increased, t
he 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-sh
oal variation of body length consistent with the preference behaviour obser
ved in the laboratory. We discuss the importance of this work in the contex
t of body length distributions in free-ranging shoals. (C) 2001 The Associa
tion for the Study of Animal Behaviour.