Species portfolio schemes buffering the risk of overexploitation in mixed fisheries management

Mixed fisheries capture multiple species simultaneously, posing significant challenges to fisheries assessment and management, and leading to systematic risk of overfishing. While the Multi-Species Total Allowable Catch (MSTAC) approach provides a feasible tool for managing mixed fisheries, there remains a lack of clarity on how to design effective management units within MSTAC. To achieve robust and conservative management, we designed and evaluated different schemes of MSTAC based on the species portfolio effect that is crucial for multispecies productivity. We considered four ecological traits to construct the MSTAC schemes, including habitat preference, feeding mode, body size and thermophily. We simulated the population dynamics of mixed fisheries and the biotic and technical interactions within the fish community using a dynamic multispecies size-spectrum model (MSSM) developed for the North Yellow Sea, China. We assessed the effects of these schemes in terms of fishery yield, stock biomass, potential productivity, and risk of depletion. Our evaluation suggests that appropriate portfolio-based schemes could enhance sustainability better than maintaining current yield through Single-Species TAC (SSTAC), which might result in rapid productivity losses and a higher risk of depletion. All the candidate schemes exhibit necessary trade-off between fisheries yields and depletion risks of the fish community. Notably, the size-based scheme provides significant benefits in buffering against depletion risks while conserving the potential productivity of the holistic fish community, followed by the habitat-based scheme. We highlight that the emergent portfolio effects from the aggregated MSTAC can enhance sustainability in mixed fisheries management and potentially serve as "insurance" against climate variability.