Risky business the combined effects of fishing and changes in primary productivity on fish communities
There is an increasing need to understand ecosystem responses to multiple stressors in that such complex responses depend not only on species level responses, but also on species interactions and ecosystem structure. In this study, we used a multi model ecosystem simulation approach to explore the combined effects of fishing and primary productivity on different components of the food web across a suite of ecosystems and a range of model types. Simulations were carried out under different levels of primary productivity and various fishing scenarios. In addition to exploring synergistic, additive or antagonistic combined effects of multiple stressors, we included a fourth category "dampened" which refers to less negative or less positive impacts compared to additive ones, and in contrast to previous studies, we explicitly considered the direction (positive or negative) of the combined effects. We focused on two specific combined effects (negative synergism and positive dampened) associated with the risk of resultant lower fish biomass than expected under additive effects. Through a meta analysis of the multi models' simulation results, we found that (i) the risk of negative synergism was generally higher for low trophic level (LTL) taxa, implying that following an increase of fishing pressure on a given LTL stock, the subsequent decrease of biomass under low primary productivity would be larger than expected under additive effects and (ii) the risk of positive dampened effects was generally higher for high trophic level (HTL) taxa, implying that given a management measure aimed at reducing the impact of fishing on HTL stocks, the subsequent rebuilding of these stocks would be slower than expected. Our approach to categorizing and exploring cumulative effects can be applied to evaluate other community properties, and provide guidance for fisheries management.