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responses to local resources to maintain directed movement. This directed
movement may benefit the population on long time scales, by increasing the
probability of encountering optimal conditions for feeding, growth or repro-
duction, but the overall movement of individuals may not be dominated by
local behavioural interactions (
Dingle, 1996
). Moreover, even when beha-
viour is dominated by local interactions, predators may change their foraging
behaviour to acquire 'ideal' rather than 'local' knowledge. For example,
fishes encountering areas with a few prey may start to make longer ranging
movements in search of areas where prey are more abundant. These ranging
movements in search of high prey density may override responses to more
subtle gradients in prey abundance (
Sims et al., 2008
). The implications of
such behaviour could be assessed by modifying cost functions, although in
seeking the generality needed to describe community processes, some species-
specific detail cannot be captured.
Both bottom-up and top-down effects influenced apparent growth in the
simulations. The incorporation of spatial movement produced realistic fish-
like growth curves that were influenced by the spatial origin of an individual.
Individuals of low mass, in an area of low food availability, were not able to
move towards an area of high food availability fast enough to take advantage
of the potential for increased growth rates. Consequently, their overall
growth rates were slower than those individuals that started growing in
areas of high food availability. Although other factors such as temperature
also influence growth and survival in real environments, the results broadly
support the premise that being in a productive environment at the right time
will dictate future growth and survival. This is consistent with the match/
mismatch hypothesis of
Cushing (1990)
.
The simulations suggested that prey-seeking and predator-avoiding beha-
viour would lead to systematic changes in the relative abundance of different-
sized individuals in relation to the location of the areas of highest primary
production. Such effects would be hard to investigate in a shelf sea, where the
areas of high primary production can change rapidly in space and time, but
might be investigated around areas such as equatorial or seamount and
island associated upwellings, where there is a relatively constant source of
primary production in a defined spatial location (
Vinogradov, 1981
). If both
prey seeking and predator avoiding are important, for example, our results
suggest that the smallest individuals would be relatively more abundant
closest to the areas of upwelling but that large individuals would be more
abundant close to the upwelling than medium-sized individuals. While the
distribution of primary production dominated the overall spatial distribu-
tions of consumer-size classes, the incorporation of spatial movement terms
did cause the larger size classes to impact the distributions of smaller size
classes. This means that although bottom-up control had the largest impact
on the spatial distributions of size classes, spatial movement resulted may
