Long-Term Changes in the Winter Diet of Common Dolphins Reflects Ecological Shifts and Bycatch Dynamics in the Bay of Biscay.

Bycatch (e.g., the accidental capture of non-targeted species by fisheries) is a leading cause of human-induced mortality, contributing to significant population declines worldwide. Often stemming from the overlap between food resources and fishery target species, dietary analysis is key to understanding bycatch patterns. In the Bay of Biscay, common dolphin () bycatch has strongly increased since 2016. Addressing the potential trophic relationship between dolphins and fisheries is essential for developing effective conservation strategies to ensure the sustainability of both dolphin populations and fisheries. Using stomach content analysis, we investigated temporal changes in the occurrence, abundance, and importance by mass of prey between 1999 and 2019. We found no difference in overall diet over time, still composed of pelagic energy-rich prey (pilchards ; horse mackerel spp. and anchovy ). However, we observed a significant decrease in the importance by mass of horse mackerel ( = 2.8365,  < 0.01) and an increase in anchovy ( = -4.2636,  < 0.01), as well as a decrease in the average size of major species, mainly related to environmental variations in abundance and size distribution of the small pelagic fish. We also identified a shift in minor species from upper slope habitats (e.g., blue whiting ) to species inhabiting coastal waters (e.g., sprat ), reflecting changes in the distribution of common dolphins within the Bay of Biscay. Finally, we highlighted the consistency over time in the prevalence of fresh pilchards and anchovies in the dolphin stomachs, suggesting they are more likely to feed specifically on these species when bycatch occurs. The risk of bycatch may therefore be increased by dolphins targeting pilchards and anchovies in more coastal waters.