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Evidence for visual constraints in large marine fish stocks Dag L. Aksnes1

Summary: Evidence for visual constraints in large marine fish stocks
Dag L. Aksnes1
Department of Biology, University of Bergen, N-5020 Bergen, Norway
I tested the hypothesis that the abundance of visually constrained fish stocks relate inversely to light
attenuation for the Black Sea, where long time series of fish stocks and Secchi depths are available. Variation in
Secchi depth accounted for 76­85% of the variation in combined fish biomass, which is higher than any other
correlations previously reported for large fish stocks and a single environmental variable. Fish biomass scaled
linearly with Secchi depth, which is consistent with the visual constraints hypothesis. Two alternative hypotheses,
one involving a trophic cascade that affects water clarity and another assuming the effect of water clarity on fish
catches, were not consistent with the observations. Although causal factors other than visual constraints cannot
be excluded, the results clearly suggest that changes in water clarity must be considered in analyses of marine
ecosystem change and regime shifts.
Long time series spanning more than 50 yr demonstrate
that water clarity has decreased in some coastal regions
(Mankovsky et al. 1996; Sanden and Ha°kansson 1996;
Aksnes and Ohman unpubl. data). Although growth and
survival of fish stocks commonly are associated with
factors such as temperature (Ottersen and Sundby 1995),
prey abundance (Benson et al. 2002), and fishing mortality


Source: Aksnes, Dag L. - Institutt for biologi, Universitetet i Bergen


Collections: Environmental Sciences and Ecology; Biology and Medicine