Evidence for visual constraints in large marine fish stocks

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 76-85% 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.