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lead to additional top-down control. The inclusion of passive movement into
the model had very little effect on the average size spectrum. There was,
however, an increase in variability as the size range affected by the velocity
field and the variability of the velocity field increased. These results were
unsurprising, as only the smallest size classes are significantly affected by the
velocity field and larger organisms with active movement have the capacity to
track movements in areas of high production. However, we took no account
of the capacity of larger organisms to use passive transport by choice or the
capacity of smaller organisms to selectively use residual flows by migrating
vertically in the water column. Thus, some species of migrating fishes may
use passive transport to reduce energy costs even though their swimming
speeds would allow them to overcome the velocity field ( Metcalfe et al.,
1990 ), and larval fish can use vertical migration control their exposure to
water masses that are moving at different rates ( van der Molen et al., 2007 ).
ACKNOWLEDGEMENTS
We thank Richard Law, Olivier Maury and Ken Andersen for stimulating
and helpful discussions during the early stages of this work and in the
European Science Foundation SIZEMIC Network. The study was funded
by the Centre for Environment, Fisheries and Aquaculture Science, the UK
Department of Environment, Food and Rural Affairs (Project M1001) and
the European Union (Framework 6 Projects BECAUSE and IMAGE).
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