Bluefin tuna Adapt Diets in Mediterranean Sea Amidst Declining Prey
A new study reveals that juvenile bluefin tuna are demonstrating a remarkable ability to adapt to changing marine ecosystems in the western Mediterranean Sea, shifting their diets as traditional prey sources dwindle. The research, published in Estuarine, Coastal and Shelf Science, offers crucial insights into the resilience of this commercially important species in the face of overfishing and climate change.
Why is this happening? Declining populations of sardines and anchovies, traditional food sources for juvenile bluefin tuna, are forcing the fish to adapt their diets. This shift is a direct response to environmental changes impacting the marine food web.
The investigation, conducted by scientists at the Institute of Marine Sciences (ICM-CSIC) and the Spanish Institute of Oceanography (IEO-CSIC), analyzed decades of data collected from the Gulf of Valencia, spanning 1989, 2012-2014, and 2018-2019.Researchers combined traditional stomach content analysis with advanced techniques utilizing stable isotopes to reconstruct the dietary habits and ecological role of young tuna over time.
Who conducted the study? Scientists from the Institute of Marine Sciences (ICM-CSIC) and the Spanish Institute of Oceanography (IEO-CSIC) collaborated on the research. The project involved multiple oceanographic centers, universities, and the Campus of International Excellence of the Sea (CEI·MAR).
“Bluefin tuna is an opportunistic and flexible predator,” explained a researcher at the Malaga Oceanographic Center of the IEO-CSIC and lead author of the study. “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.”
What changes where observed? The study found that as populations of sardines and anchovies have declined, juvenile tuna have increasingly turned to horse mackerel as a primary food source. This dietary shift highlights the species’ capacity for ecological flexibility.
Importantly, the findings challenge concerns raised by the fishing sector regarding the impact of a recovering tuna population on already stressed sardine and anchovy stocks. According to a researcher at the ICM-CSIC, “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.”
Further analysis suggests that the decline in sardines and anchovies is primarily driven by factors beyond tuna predation, including overexploitation, rising water temperatures, and alterations in plankton populations – the base of the marine food web.[A chart illustrating the decline of sardine and anchovy populations in the western Mediterranean over the past three decades would be beneficial here.]
How did it end? The study concluded that bluefin tuna are adapting to a changing food web by shifting their diets, primarily from sardines and anchovies to horse mackerel. This adaptation doesn’t appear to be negatively impacting the already stressed sardine and anchovy populations.
The research was supported by the SEINE-ETP and PELWEB projects, funded by the Ocean Stewardship Fund and the Ministry of Science, Innovation and universities. The collaborative effort involved scientists from the ICM
