Interannual trophic behaviour of a pelagic fish predator in the western Mediterranean Sea

The exploitation of forage fish species can modify the functioning of marine ecosystems potentially impacting the population status of predators. This may be the case for the western Mediterranean Sea, where a reduction in the biomass of two key pelagic forage fish (European anchovy Engraulis encrasicolus and European sardine Sardina pilchardus) could produce a change in the diet composition of their main predators, which would consume alternative preys or change the size of the prey consumed. Here, we aimed to investigate the potential effect of biomass reduction of sardine and anchovy in the western Mediterranean Sea on the trophic preferences of the little tunny (Euthynnus alletteratus), a medium-sized predator that present a high consumption of these forage fish. We compared its interannual trophic ecology by combining the analysis of stomach contents and stable isotopes. Specifically, we examined if the diet of little tunny changed in its main trophic habits (diet composition, prey size, and trophic niche) during a 6-year period. We found that small pelagic fish, especially clupeiformes, were the most important prey group for the little tunny during the study period. However, we found changes in the body size of anchovy and the relative importance of sardine in recent years, probably reflecting the reported reduction in the biomass and body size of these two forage fish in the study area. In addition to these changes, we found an increase in some demersal and benthopelagic species in the diet of little tunny, which could act as an alternative diet resource.