Ocean Warming Effects on Catch and Revenue Composition in the Northwestern Mediterranean Sea

Climate change-induced ocean warming can have profound implications for marine ecosystems and the socioeconomic activities dependent on them, affecting the catch composition, and fisheries revenue. Our study evaluates spatio-temporal changes in the Northwestern Mediterranean marine fisheries catch and revenue composition tied to ocean warming and disentangles the different underlying processes. To do so, we analyzed the weighted mean thermal affinity of the catch (Mean Temperature of the Catch: MTC) and revenue (Mean Temperature of Revenue: MTR) across different taxonomic groups, fishing fleets, and fishing harbors, using a 23-year time series of commercial landings. Results revealed changes in catch and revenue composition, with an overall temporal increase in the MTC (0.68°C per decade) and MTR (0.58°C per decade) linked to local sea temperature. The temporal increase in both indices prevailed across fishing fleets and taxonomic groups. The processes underpinning these changes over time were tropicalization (i.e. relative increase of warm-affinity species; 41.97% for MTC and 45.20% for MTR), and deborealization (i.e. relative decrease of cold-affinity species; 46.58% for MTC and 44.99% for MTR), with variability across dimensions. Deborealization particularly influenced pelagic fisheries (i.e. purse-seiners and surface longliners) and some commercially important species (e.g. European hake, blue whiting, and Norway lobster). Even if the temporal increase in MTC and MTR was consistent across taxonomic groups and fleets, the spatial dimension showed heterogeneity and temporal declines in some cases. In summary, our study provides valuable information about temporal changes in catch composition associated with local ocean warming and reveals potential cascading effects through the social-ecological system. In particular, we presented the MTR approach for the first time, evidencing ocean warming effects on revenue composition. We suggest that the correlation between changes in catch and revenue composition reveals the adaptive capacity, or fragility of specific fishing fleets and points to management priorities.