Recovering highly exploited stocks with heterogeneous resilience capacities in mixed fisheries under uncertain illegal fishing

Abstract

The complexity of stock recovery strategies in mixed-species fisheries involving heterogeneous resilience capacities of target and incidentally harvested species is further challenged by possible levels of illegal exploitation rates. This context adds to the uncertainty for decision-makers when establishing proper management measures to recover highly overexploited stocks towards the selected target reference point (e.g., BiMSY). With a dynamic bioeconomic model, we analyzed the stock recovery performance of a mixed fishery with two overexploited species using combinations of moratoria durations (μi) and subsequent exploitation rates (FiMSY), under possible states of nature of illegal fishing (θj). Alternative stock recovery management strategies (Dj) were considered to include the effect of possible θj. Both stocks present heterogenous abundance, renewability capacities, and relative prices. The impact of possible levels of illegal fishing on the legal resource rent and the corresponding forgone legal resource rent was also assessed. A Monte Carlo analysis was undertaken to calculate the risk of falling below BiMSY considering alternative management strategies to deal with possible illegal fishing (ρi,Dj). Overexploited species with lower resilience capacity require longer moratoria timelines, extended when dealing with alternative states of nature of illegal fishing. Management strategies based on target species achieved the highest fishery legal resource rent while having the highest risk of not recovering the stock of incidental species with lower resilience capacity. In this study, a mixed-strategy management decision where a moratorium is based on target species and subsequent exploitation rates (Fi) based on lower resilience species resulted in minimum risks of falling below species MSY at high levels of illegal fishing. This same strategy resulted in maximum NPV of resource rent per vessel for alternative levels of illegal fishing. Lack of consideration of possible levels of illegal fishing may lead to failure in achieving stock recovery to target reference points in mixed fisheries.