Bluefin tuna (Thunnus thynnus) larvae captured during the TUNIBAL campaign.
Photo: Asvin Pérez, Centro Oceanográfico de Baleares / IEO-CSIC.
Atlantic bluefin tuna larvae in the western Mediterranean are already living relatively close to their upper thermal limit, according to new research involving scientists from the Spanish Institute of Oceanography (IEO-CSIC).
The study, published in the Journal of Thermal Biology, examined the temperature limits of Atlantic bluefin tuna (Thunnus thynnus) larvae, finding a mean upper thermal limit of 31.7°C and a lower limit of 17.9°C.
According to the researchers, the findings help explain why bluefin tuna do not reproduce during colder periods or in areas where water temperatures fall below the range tolerated by their larvae.
However, the scientists highlighted the upper limit as a particular concern as the Mediterranean continues to warm.
In the waters around the Balearic Islands, a major spawning area for Atlantic bluefin tuna in the western Mediterranean, current sea temperatures are already relatively close to the upper thermal limit identified by the researchers.
The study found that larvae currently have a thermal safety margin of around 3.6°C in these waters.
The findings add to previous research suggesting that warming could reshape where Atlantic bluefin tuna reproduce and develop. A 2023 study reported by WeAreAquaculture found that temperatures above 28°C negatively affect juvenile bluefin tuna growth, and projected that much of the Mediterranean could exceed that threshold within 50 years. Researchers said this could push nursery areas towards cooler waters, including the Bay of Biscay.
This latest study by IEO-CSIC indicates that tuna larvae are potentially vulnerable to marine heatwaves during the summer, which coincides with the bluefin tuna spawning season. During such events, water temperatures can rise rapidly towards levels that are critical for larvae.
As ocean temperatures continue to increase, marine heatwaves are also expected to become more frequent and intense, further reducing the margin between normal conditions and the larvae's thermal limits.
“We are seeing how the earliest life stages of bluefin tuna are operating very close to their thermal limits. This increases their vulnerability to extreme events such as marine heatwaves,” the researchers said.
The study also highlights the difficulties involved in investigating the early life stages of open-ocean species under controlled conditions.
Bluefin tuna larvae are particularly sensitive, making experimental research on their development challenging. The researchers said this adds to the value of establishing their thermal limits and contributes to broader scientific efforts to build datasets on temperature tolerance during the early life stages of fish.
Understanding these limits could help researchers assess how warming seas may affect the future resilience of Atlantic bluefin tuna populations.
“Atlantic bluefin tuna is much more than a fisheries resource: its role as an apex predator makes it a key part of the balance of marine ecosystems. Understanding and protecting its early life stages will be essential to ensuring its resilience in the future,” the research team concluded.
The study, Critical thermal limits of Atlantic bluefin tuna (Thunnus thynnus) larvae, is available online in the Journal of Thermal Biology. The research was partly funded through the TUNAWAVE project, the EU's Horizon 2020 programme, and the Government of the Balearic Islands and its Sustainable Tourism Tax.