The Mediterranean Sea spawns much of the Atlantic Ocean’s population of bluefin tuna, which like many species return in maturity to the waters in which they were born to spawn again. After a long period of stock decline, a combination of improved fishery management and reduced IUU fishing in the current century has allowed the gradual recovery of this overfished species, lending optimism to all those interested in this valuable resource.
However, a recent scientific study finds that the effects of worst-case climate change on temperatures of Mediterranean waters will probably harm juvenile bluefin growth and drive Atlantic population distributions and spawning grounds northward. The study, “Thermal sensitivity of field metabolic rate predicts differential futures for bluefin tuna juveniles across the Atlantic Ocean” published in the journal Nature communications, reports the results of a new scientific technique for analysing metabolic rates for bluefin in their first year of life, to find their preferred water temperature versus actual temperatures in the Mediterranean.
The findings are not favorable for bluefin starting life in those waters. The “cut-off” temperature of 28ºC—above which juvenile bluefin metabolism is damaged—will be reached in the Mediterranean Sea within a few decades at current trends (global policies to address climate change could reduce this negative impact). At and above that temperature, young bluefin experience reduced growth, and increased mortality, with consequent reductions in overall bluefin stock size.
While some individuals will adjust by spawning in northerly waters, most will return to the Mediterranean to spawn with diminishing success, shrinking the long-term fishery stock size. If populations move elsewhere, they may mix with other species such as sardines or herring, complicating the management of mixed fisheries.
The research project leader Clive Trueman, Professor of Geochemical Ecology at the University of Southampton, says that drawing on compilations of global climate model projections, the results show that most of the Mediterranean Sea will exceed that 28-degree threshold within 50 years under current predictions of climate change, making it too warm for juvenile bluefin. We would, therefore, expect the tuna to move their nursery areas, potentially into the Bay of Biscay or other cooler regions—which would place juvenile tuna within other existing fisheries.
The scientific article, with an easy-to-read summary, is available at https://www.nature.com/articles/s41467-023-41930-2 .