As the Mediterranean Sea continues to warm, it may become more favorable for some tropical tuna species. However, warmer waters do not necessarily provide all the environmental conditions these highly migratory fish need.

A new study shows that improved temperature conditions do not necessarily make the Mediterranean more environmentally accessible to several tuna species. The region's relatively high salinity, changes in upper-ocean mixing, and other environmental differences may continue to limit their potential use of the basin.

A research team led by Prof. ZHANG Hui from the Institute of Oceanology of the Chinese Academy of Sciences (IOCAS) has investigated how five tuna species may respond to changing Mediterranean conditions. Their findings were published in ICES Journal of Marine Science on October 21.

The Mediterranean is one of the marine regions experiencing pronounced warming. It also provides important habitat and spawning grounds for commercially valuable species such as Atlantic bluefin tuna. The researchers combined global species occurrence records, fishing catch data, and marine environmental information to analyze whether the Mediterranean continues to provide environmental conditions suitable for tropical tunas in the face of rising temperatures.

They developed ensemble species distribution models to estimate habitat suitability under present-day and future climate scenarios. They also assessed how closely Mediterranean conditions correspond to the environments associated with each species' known occurrences. The assessment covered albacore, Atlantic bluefin tuna, yellowfin tuna, bigeye tuna, and skipjack tuna.

The results revealed marked differences among the five species. Under present-day conditions, the Mediterranean offers broadly suitable habitat for albacore and Atlantic bluefin tuna, while predicted suitable areas for yellowfin, bigeye, and skipjack tuna are more limited.

By the end of the century, under a high-emissions scenario, the proportion of the Mediterranean predicted to be suitable for Atlantic bluefin tuna could fall from 100.0% to 69.1%. In contrast, the corresponding proportion for yellowfin tuna could rise from 9.5% to 18.6%. Bigeye and skipjack tuna are also projected to gain suitable habitat.

However, greater temperature suitability does not necessarily translate into greater environmental accessibility. The researchers found that relatively high salinity, changes in upper-ocean mixing, and other environmental differences could continue to constrain the ability of tropical tuna species to use the Mediterranean.

Atlantic bluefin tuna showed relatively strong environmental correspondence with the Mediterranean, consistent with its established regional presence. Yellowfin and bigeye tuna, however, remained poorly matched with Mediterranean conditions across multiple environmental variables, despite projected gains in habitat suitability.

The findings suggest that climate warming will not open the Mediterranean equally to all tuna species. They also highlight the need to look beyond habitat suitability maps when assessing future shifts in marine species distributions.

"Our study offers useful insights for understanding climate-related changes in tuna habitats and informing fisheries management," said Prof. ZHANG.