Fish and Fisheries, 2003 4, 183-146.
When fish schools meet: outcomes for evolution
and fisheries
Darren P. Croft1, Jens Krause1 Iain D. Couzin2& Tony J.
Pitcher3
1School of Biology, University of Leeds, Leeds LS2 9JT, UK.
2 Department of Ecology and Evolutionary Biology, Princeton University,
Princeton, NJ 08544, USA.
3Fisheries Centre, 2204 Main Mall, University of British Columbia, Vancouver,
Canada, V6T 1ZA.
The mechanisms underlying the social structure of free ranging fish
shoals have received little attention in comparison to functional studies
on shoaling. Recently however, a number of investigations both in the
marine and freshwater environment have begun to address the underlying
mechanisms by concentrating on interactions between free-ranging shoals.
The rates of shoal encounters can influence the opportunities for individual
assortment by phenotype and selection of shoal size. Encounter rates
can act as a constraint on the observed patterns of shoal structure
by restricting individual choice behaviour. Encounter rates were found
to be high in several small freshwater species (where inter-shoal distances
were small) but lower in marine species where shoals were more dispersed.
Information on the outcome of shoal encounters is largely missing from
the literature. It has been suggested that the duration of encounters
may play a role in that it affects both the time available for assessment
(of the encountered shoal) as well as that for exchange of individuals.
The little empirical data that is available suggest that the outcome
of shoal encounters is strongly influenced by shoal composition but
not by shoal size. Individual behaviour may have evolved to maintain
shoal size when shoals encounter, but when fish populations are depleted
by fishing this trait can exacerbate range and stock collapse. Furthermore,
an understanding of the dynamics of shoal encounters has important consequences
for the evolution of reciprocal altruism and the transmission of information
through social learning within populations. Finally, information on
encounter rates between shoals and on the number of individuals that
are exchanged on such occasions could be important for making predictions
about the spread of disease through fish populations.
