Submitted
Modelling density-dependent fish shoal distributions
in the laboratory and field
Hensor, E.M.A. (1*), Couzin, I.D. (2), James, J (3) & Krause,
J (1)
1 School of Biology, University of Leeds, Leeds, LS2 9JT, UK
2 Department of Ecology and Evolutionary Biology, Princeton University,
Princeton, NJ 08544, USA
3 Department of Physics, University of Bath, Bath BA2 7AY, UK.
*Author for correspondence.
Density-dependent variables have long been established
as an important area of ecological research, but the effects of the
local density of conspecifics on behaviour are less well-studied. We
compared the influence of the density of conspecifics on the shoal size
distribution of killifish, Fundulus diaphanus, in the laboratory and
the field. In both environments we observed an increase in shoal size
and shoal number with the density of individuals present. The increase
in shoal size was markedly more steep in the field than in the laboratory,
but direct comparison of the two was complicated by the fact that the
absolute numbers of fish present at the field site were considerably
higher than those used in the laboratory trials. We developed an individual-based
model that was first used as a null model of shoal formation (defined
by proximity to others) in fish with no shoaling tendency over the same
range of densities used in the laboratory. Group size increased much
more rapidly with increasing density in the laboratory than predicted
by the null model. When we incorporated shoaling behaviour into our
model, the laboratory results could be reproduced with high accuracy.
However, when extrapolated to match conditions in the field, the model
predicted smaller, more numerous shoals than were actually observed.
We suggest this is due to heterogeneity of the field environment because
fish were found to be highly aggregated in certain areas of our field
site. The predictive power of laboratory studies for the field is discussed
with regards to using individual-based modelling as tool for deriving
such predictions.
