Climate change will render size-selective harvest of cold-water fish species unsustainable in Mediterranean freshwaters.
Climate change is impacting the composition and functioning of virtually every ecosystem on Earth, and disrupting the productivity of exploited ones. Species are rapidly adjusting to their changing environments through evolutionary and/or plastic phenotypic changes in behavioural, physiological, phenological and life-history traits. Size-selective harvest produces severe demographic impacts on exploited populations and induces individual phenotypic changes in many of the same fitness-related traits as climate change and thus can impair local adaptation and acclimation. We addressed in the context of inland recreational fisheries two interrelated questions: (1) Will fisheries-induced phenotypic changes operate at different rates and direction than those induced by climate change, and thus hinder local adaptation and acclimation, threatening population persistence?; (2) which harvest regulations most likely lead to overexploitation of populations under the new environmental conditions? We used an eco-genetic individual-based model to simulate the consequences of size-selective fishing for a cold-water fish species brown trout Salmo trutta across a range of regulatory (defined by exploitation rate and size-based limits) and environmental scenarios (warming vs. concurrent warming and streamflow reduction) in a Mediterranean system. We ran 1,620 combinations of fishing and environmental scenarios and analysed results using artificial neural networks. In our simulations, (a) climate change and size-selective fishing both led to a reduced, truncated population, with increased juvenile but decreased adult growth and earlier maturation at smaller size, but fisheries-induced changes were stronger than those produced by climate change; (b) their effects were additive or dampened but rarely synergistic and (c) phenotypic changes in fitness-related traits resulted from both evolutionary and plastic processes. Synthesis and applications. Our model-based analyses highlight that any size-selective fishing regime would lead to the overexploitation of cold-water freshwater fish populations if climate warming is accompanied by streamflow reduction - as projected in Mediterranean fisheries. Even if we assumed no future streamflow regime changes, only a limited range of size-based harvest regulations may provide an acceptable balance between conservation and fishery objectives. Thus, recreational fisheries of cold-water fish in Mediterranean climates might be more sustainably managed under climate change if conservation-oriented strategies based on harvest bans (e.g. catch-and-release fishing) were implemented.