Protecting and Restoring Aquatic Ecosystems in Multiple Stressor Environments

Aquatic ecosystems around the globe are under threat from pollution, invasive species, over-exploitation, and other stressors. Given synergistic effects, policy measures to address particular stressors should be developed in tandem with policy measures to address others. We present a bio-economic model that addresses the optimal management of an aquatic ecosystem subject to multiple stressors. Specifically, we consider optimal management of a native fishery in a lake ecosystem subject to risks from pollution and an invasive species. Optimal plans exist for various cases defined by whether, one, both, or neither of the stressor events has occurred. Optimal fishery stocks vary between these cases, and depend on the order in which the stressor event occur if realized. The optimal native stock is the highest in the absence of either stressor. However, the combined influence of the multiple risks can rapidly reduce the probability of maintaining an un-invaded and un-polluted state for long. The synergistic effects of the risks interconnect optimal policies in interesting ways. We find that optimal pollution abatement in absence of both these stressors may turn out to be lower than when either or both stressors are present. The effectiveness of native fish stock in mitigating the risk of alien fish invasion can have a bearing on whether optimal native fish stock and abatement effort are used as substitutes or as complements. Pollution abatement levels that are chosen without consideration of alien invasion risk can lead indirectly to increased societal costs for invasion risk mitigation.