A study by IEO-CSIC and ICM-CSIC shows that juvenile tuna adapt to the scarcity of traditional prey by replacing sardines and anchovies with horse mackerel.
Research led by the Institute of Marine Sciences (ICM-CSIC) and the Spanish Institute of Oceanography (IEO-CSIC) has revealed that juvenile Atlantic bluefin tuna (Thunnus thynnus) have significantly altered their diet in the western Mediterranean over the past three decades. The study, published in Estuarine, Coastal and Shelf Science, shows that as sardine and anchovy numbers decline, young tuna have increased their consumption of horse mackerel (Trachurus spp.), demonstrating a remarkable ability to adapt to ecosystem changes.
The work, based on samples collected in the Gulf of Valencia in 1989, 2012–2014 and 2018–2019, combines traditional stomach content analysis with stable isotope techniques to reconstruct the diet and trophic role of juvenile bluefin tuna over time.
“Bluefin tuna is an opportunistic and flexible predator. Our results show that it responds to changes in prey availability, allowing it to adapt to an environment increasingly altered by overfishing and climate change,” explains Joan Giménez, researcher at the Málaga Oceanographic Centre of IEO-CSIC and lead author of the study.
The study concludes that concerns within the fishing sector about the impact of the growing tuna population on sardine and anchovy stocks are not justified, at least in the case of juvenile individuals.
“The proportion of sardines and anchovies in the current diet of juveniles is very low. There is no evidence that their recovery is affecting small pelagic populations,” adds Marta Coll, co-author of the article and researcher at ICM-CSIC.
Furthermore, the results suggest that the decline of sardines and anchovies in the western Mediterranean is more closely related to factors such as overexploitation, rising temperatures and changes in plankton communities than to tuna predation.
The study is part of the SEINE-ETP and PELWEB projects, funded by the Ocean Stewardship Fund and the Spanish Ministry of Science, Innovation and Universities, and involved researchers from ICM-CSIC, IEO-CSIC (oceanographic centres of Gijón, Málaga and Murcia), the University of Cádiz and the International Campus of Excellence of the Sea (CEI·MAR). Tuna sampling was made possible thanks to the monitoring programme carried out by the Tuna Group of the Málaga Oceanographic Centre (IEO-CSIC).