Bycatch (e.g., the accidental capture of non-targeted species by fisheries) is a leading cause of human-induced mortality, contributing to significant populations declines worldwide. Often stemming from the overlap between food resources and fishery target species, dietary analysis is key to understand bycatch patterns. In the Bay of Biscay, common dolphin (Delphinus delphis) bycatch has strongly increased since 2016. Addressing the potential trophic relationship between dolphins and fisheries is essential for developing effective conservation strategies and 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 preys between 1999 and 2019. We found no difference in overall diet over time, still composed of pelagic energy-rich prey (pilchards Sardina pilchardus; horse mackerel Trachurus spp. and anchovy Engraulis encrasicolus). However, we observed significant decrease in the importance by mass of horse mackerel (t = 2.8365, p = 0.0052) and increase in anchovy (t = -4.2636, p < 0.005), as well as a decrease in the average size of major species, including pilchards, horse mackerel and anchovy; 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 Micromesistius poutassou) to species inhabiting coastal waters (e.g., sprat Sprattus sprattus), 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, anchovies and horse mackerels 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 modulated by dolphins’ target species, with a higher risk being associated with dolphins feeding on pilchards, anchovies or horse mackerel rather than other prey species