The Mediterranean sea is the primary spawning ground for the eastern Atlantic bluefin tuna population. Juvenile tuna, which are unable to regulate their body temperature during their first year of life, remain in their nursery grounds in the Mediterranean until they are ready to head out into the Atlantic ocean, later returning to the same area to spawn.
However, a new study published in Nature Communications shows that with marine temperatures in the Mediterranean sea expected to rise beyond 28ºC due to climate change, tuna will be driven out of their traditional spawning and nursery grounds within 50 years in search of better growth conditions in cooler waters, for example, in the Bay of Biscay.
This poses a significant risk to tuna populations, the researchers warn. The tuna may be accidentally caught in existing sardine and anchovy fisheries in the region – requiring fishery managers to adapt their methods to allow tuna nurseries to establish in their new locations.
"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."
Prof. Clive Trueman, University of Southampton
The research project led by Clive Trueman, Professor of Geochemical Ecology at the University of Southampton, found that temperatures exceeding 28ºC will negatively impact the bluefin tuna's growth, forcing the fish to seek new locations.
"Drawing on compilations of global climate model projections, we 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," Trueman said in a press release.
"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 existing fisheries for species like anchovy and sardine.”
To discover the cut-off temperature for bluefins to thrive, the international research team – comprising scientists from eight countries, including the British Antarctic Survey and the AZTI science and technology centre in Spain – used a pioneering method of analysing a tissue called the otolith.
"This can tell us when waters are getting too hot for the fish to handle - it’s like a natural fitness tracker.”
Prof. Clive Trueman, University of Southampton
The otolith tissue is found behind the brains of bony fish, can be used by scientists to determine the age of individual fish, and also provides information about population dynamics, growth rates and overall health of fish populations.
In this latest research project, the team used the chemistry of tuna otoliths to measure the "pace of life" of bluefin tuna, Trueman explained.
“All fish have a stony tissue in their ear called an otolith, or ‘ear stone’, which enables them to perceive noise and to balance," said Trueman, who pioneered the otolith decoding method.
"Our new work uses variations in the levels of natural stable isotopes of carbon and oxygen in the otolith to reveal each tuna’s individual metabolic rate. This can tell us when waters are getting too hot for the fish to handle - it’s like a natural fitness tracker.”
