Overseas
The diet of young Atlantic bluefin tuna in the Mediterranean has changed profoundly over the past thirty years, revealing the remarkable adaptability of one of the most iconic predators in our sea. This is confirmed by a new study published in Estuarine, Coastal and Shelf Science and led by researchers from the Institute of Marine Sciences (ICM-CSIC) and the Spanish Institute of Oceanography (IEO-CSIC). The work offers a crucial perspective on how the trophic balance of the western Mediterranean is shifting in response to human pressures and climate change.
The study analyzes three distinct periods — 1989, 2012–2014, and 2018–2019 — using an integrated approach that combines stomach content examination with stable isotope analysis. This methodology allows researchers to “reconstruct” the diet of juvenile tuna over time with unusual precision, providing a dynamic picture of ecosystem evolution in the Gulf of Valencia.
The first clear finding concerns prey composition. As sardine and anchovy populations have declined — species historically central to the bluefin tuna diet — the trophic gap has been filled by a growing consumption of horse mackerel (Trachurus spp.). This shift reflects not a preference but a functional response to an environment that now offers different prey availability compared to the past.
According to Joan Giménez, researcher at the Malaga Oceanographic Centre (IEO-CSIC) and lead author of the study, this evolution confirms the opportunistic nature of Atlantic bluefin tuna: a “plastic” predator, quick to adjust its feeding behavior to ensure survival even under strong environmental pressures. This trait allows the species to maintain its top-predator role in the Mediterranean food web despite increasing habitat alteration.
Another key conclusion challenges a recurring concern within the fishing sector — the belief that growing bluefin tuna populations (Thunnus thynnus) may worsen the already fragile state of sardine and anchovy stocks. Instead, the analyses show that the impact of juvenile tuna on these species is minimal. Their presence in the current diet is “very low,” notes Marta Coll, researcher at ICM-CSIC and co-author of the study. There is no evidence that the recovery of bluefin tuna — a major achievement of international management efforts — is further deteriorating the condition of small pelagic species.
The decline of sardine and anchovy in the western Mediterranean, the research team suggests, is rooted in deeper and more complex drivers: overexploitation, rising temperatures, and changes in plankton structure seem to carry far more weight than bluefin tuna predation.
The study, part of the SEINE-ETP and PELWEB projects funded by the Ocean Stewardship Fund and the Spanish Ministry of Science, stands out for the breadth of scientific collaboration involved, including ICM-CSIC, IEO-CSIC, the University of Cádiz, and the International Campus of Excellence of the Sea (CEI·MAR). Tuna sampling was enabled through the monitoring program of the Tuna Group at the Malaga Oceanographic Centre.
For the Mediterranean fisheries sector, these findings offer a more mature and less reactionary interpretation: bluefin tuna is neither an adversary of small-scale fisheries nor the “culprit” behind the decline of pelagic species. Rather, it is a biological indicator of a sea in transformation — and of the ecosystem’s ability, or inability, to maintain balance within its communities. Understanding the species’ feeding behavior therefore means gaining deeper insight into the Mediterranean itself and the future pathways for managing its resources.
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