Journal of Animal Ecology 2002 71, 10-14
The effects of parasitism and body length on positioning
within wild fish shoals
A. J. W. WARD*, D. J. HOARE*, I . D. COUZIN*, M. BROOM†
and J. KRAUSE*
*School of Biology, University of Leeds, Leeds; and †School
of Mathematical Sciences, University of Sussex, UK
1. The influence of body length and parasitism on the
positioning behaviour of individuals in wild fish shoals was investigated
by a novel means of capturing entire shoals of the banded killifish
(Fundulus diaphanus, Lesueur) using a grid-net that maintained
the two-dimensional positions of individuals within shoals.
2. Fish in the front section of a shoal were larger than those in the
rear.
3. Individuals parasitized by the digenean trematode (Crassiphiala
bulboglossa, Haitsma) showed a tendency to occupy the front of
shoals. Parasitized fish were also found more in peripheral positions
than central ones in a significant number of shoals.
4. Shoal geometry was affected by the overall parasite prevalence of
shoal members; shoals with high parasite prevalence displayed increasingly
phallanx-like shoal formations, whereas shoals with low prevalence were
more elliptical.
5. There was no relationship between body length and parasite abundance
or prevalence in the fish population which suggests body length and
parasite status are independent predictors of positioning behaviour.
6. Solitary individuals found outside shoals were both more likely to
be parasitized and had higher parasite abundance than individuals engaged
in shoaling.
7. Differences in the shoaling behaviour of parasitized and unparasitized
fish are discussed in the context of the adaptive manipulation hypothesis.
