After surface temperatures dropped to around 15°C following strong mistral winds in July, Mediterranean waters are expected to approach 30°C by the end of the week due to a heatwave. Could such swings disorient marine life?
Fish and marine life struggling to adapt
In just about ten days, surface water temperatures in the Mediterranean have experienced an unprecedented variation, rising from roughly 15°C at the end of July to nearly 30°C expected later this week. Could this drastic shift confuse fish, making them feel as if they’ve suddenly returned from Brittany? While global warming in the Mediterranean has facilitated the arrival of new species on local coasts—such as barracuda, lionfish, and blue crab—and marine heatwaves cause high mortality in sessile species like corals and gorgonians, Christine Pergent, a marine biologist at the University of Corsica, explains: “We are not yet in that situation.”
Current changes: currents inversion and jellyfish blooms
“For severe impacts, these temperature anomalies would need to last for days or even weeks, though the amplitude of the phenomenon can shorten this period,” says the researcher.
Marine life does make some adjustments. “This shift from cold to warm waters usually causes current inversions and jellyfish blooms,” notes Gérard Carrodano, fisherman-diver and coastal sentinel in La Ciotat for seventy years. “For fishing, warmth generally increases fish activity, except lobsters, which seem to prefer mistral winds. It’s a natural phenomenon we are accustomed to, though it is intensifying with climate change.”
The recent temperature rise has forced him to release about 80% of the live catches he collected in recent weeks for collaborations with aquariums. “The tanks became too warm. It’s not so much the fish that suffer, but the growth of parasites favored by heat,” explains Carrodano.
Fish farms stressed, turtles benefit
Fish farms are also heavily affected. “Aquaculture cages are typically placed between 5 and 15 meters deep, making them more exposed to temperature increases than wild fish, which can move to colder waters, down to the thermocline at 35-40 meters, where temperature remains stable. But in fish farms, mortality is more caused by bacterial proliferation due to fish density than by oxygen depletion from heat,” adds Christine Pergent.
Warmer waters appear favorable for loggerhead turtles, a threatened species that found the beaches of the Var coast particularly suitable for nesting this year, with numerous first observations in Cavalaire-sur-Mer and the Gulf of Saint-Tropez. “This is good news,” says Franck Dhermain, veterinarian with the Mediterranean Cetacean Study Group.
“Regarding cetaceans, however, we know very little about the effects of temperature variations. Sperm whales seem to be expanding northward due to warming, whereas we have hypotheses about changes in prey distribution for rorquals, which feed on shrimp favoring cooler waters,” he adds.
The main concern lies with shallow, sessile species if water temperatures remain high. After a marine heatwave in 2022, up to 90% of gorgonians (a type of coral) died at 20 meters depth. “We are also concerned about Posidonia seagrass, though scientific consensus on temperature impacts is not yet clear,” continues Christine Pergent.
Persistent conditions are the real threat
“Our hope is that marine heatwaves only affect upper layers of water, and we’ve observed that organisms living deeper can recolonize damaged areas. But slow-growing corals, which grow only a few millimeters per year, cannot withstand repeated events, and that is the problem,” she explains.
Another issue is the heat accumulated by the Mediterranean over summer. “Early in summer, only the top meters warm, easily mixed by a mistral wind. But in autumn, the top 50 meters may be heated, and it then takes four to five days of strong mistral to cool the water down. In the sea as in agriculture, the problem is persistent bad conditions,” concludes Gérard Carrodano.
