Exploring trade-offs in mixed fisheries by integrating fleet dynamics into multispecies size-spectrum models.
Ecosystem-based fisheries management aims to ensure ecologically sustainable fishing while maximising socio-economic benefits. Achieving this goal for mixed fisheries requires better understanding of the effects of competing fishing fleets on shared resources and economic performance. Proposed management strategies that promote either specialisation or diversification of catches may result in unintended consequences for ecosystem-based management. Here, we ask the following questions: does increased or decreased competition among fleets lead to better ecological and socio-economic fishery outcomes? How effective are currently proposed management strategies for achieving these outcomes? We integrated fleet dynamics into a multispecies size-spectrum model and parameterised this model to represent Australian Southern and Eastern Scalefish and Shark Mixed Fishery. We compared the fishery status quo to two extreme scenarios: no competition, where each species is fished only by one fleet (specialisation); and maximal competition, where all fleets catch all species (diversification). To answer our second question, we considered three more plausible scenarios resulting from proposed management strategies: decreased competition due to reduced bycatch, and increased competition due to increased catches of under-utilised or valuable species. We used indicators to explore scenarios' outcomes. Our model reproduced observed trends in fishing effort and yield. Extreme scenarios showed that a fishery dependent on single species management structures is more likely to achieve ecosystem-based management objectives if fleets do not compete, while maximal competition can lead to socio-economic loss as management buffers the ecological impact of diversifying. The more plausible scenarios showed little improvement over the status quo, with mixed ecological and negative economic effects. Synthesis and applications: Our model can be applied to assess mixed fisheries ecosystem-based management strategies. Our results show that, under single species management approaches, greatest outcomes can be achieved when fleets are specialised, whereas managing fleets that catch similar species is unlikely to be successful. They question the effectiveness of these management approaches in providing resilience for mixed fisheries facing changes and highlight the need to account for fleet interactions in the evaluation of management strategies to avert unintended risks.