Europe's Seas Post Hottest Summer on Record as Marine Heatwaves Reshape Fisheries

European seas recorded their warmest summer months since reliable satellite data began in 1982, with temperature records broken from the English Channel to the Mediterranean, as prolonged marine heatwaves transform underwater ecosystems and disrupt fisheries that support coastal communities.

Between June and August, European waters averaged 21.6 degrees Celsius, the highest in 45 years of available data. An offshore buoy near Mallorca logged 33.02 C on August 5, the highest reading ever recorded in the Spanish port authority's buoy network.

The Bay of Biscay, stretching from Galicia in northwestern Spain to southern Brittany in France, stood out for its deviation from the norm. Its traditionally cool waters averaged 21.4 C over the three months, more than 2.4 degrees above the 1991-2020 average. Readings from the US National Oceanic and Atmospheric Administration peaked at 23.2 C on August 13, the highest of the satellite era.

Scientists define a marine heatwave as five or more days of water temperatures well above an area's historic norm. In the 1980s, the Bay of Biscay averaged a handful of such days per year. By the end of August this year, it had logged 186, meaning more than three out of every four days qualified.

"What is surprising is the speed at which this occurs," said Jean-Pierre Gattuso, senior research scientist at France's National Centre for Scientific Research. "Records are broken almost every day in many places."

The warming is altering habitats. Jose Manuel Rico Ordas, a professor of marine ecology at the University of Oviedo, estimates that roughly 95% of the large brown kelp stands along the rugged Asturias coast have disappeared, based on sampling since 2010. Kelp requires cold water, typically below 20 C, to survive. Overgrazing by dreamfish, which thrive in warmer temperatures, has added to the decline, he said.

"We are finding no more lobster, no more crayfish, no more sea urchins and many other species that are associated with the good status of kelp forests," Rico Ordas said.

In Galicia, fishermen have joined scientists and environmental activists to restore kelp forests by planting seedlings grown in laboratories. Manuel Angel Reiriz Gonzalez, president of a local fishermen's guild, recently ferried project members out of the port of Ribeira to scatter small stones seeded with kelp into the water. His family has dived for sea urchins and harvested clams for more than a century, but recurring heatwaves are hurting shellfish stocks, he said. The area has also seen an influx of Portuguese man o' war and trigger fish from warmer southern waters.

"We had some very bad years for clam production, but we worked on seeding them, and the species gradually adapted," Gonzalez said, expressing hope that seabed restoration can do the same for kelp. "At that point, as a fishing guild, we would cease to exist," he warned if kelp disappears.

In the Mediterranean, more than 1,000 non-indigenous species have been recorded, according to a recent report by the Cyprus Academy of Sciences, Letters and Arts based on surveys by scientists in 11 countries. New arrivals from the Red Sea and beyond are entering through the Suez Canal, displacing native populations and reshaping food chains.

Some species are adapting better than others. A massive swarm of jellyfish shut down reactors at France's Gravelines nuclear power plant in August when they clogged its cooling system, the second such incident in as many years. The plant in northern France draws cooling water from a canal connected to the North Sea.

Fishermen on England's south coast are reporting a surge in common octopus landings. Around 2,600 metric tons were caught last year and upwards of 3,000 tons so far this year, according to figures compiled by the UK Marine Management Organisation. Before 2025, annual landings were less than 30 tons.

Octopus larvae are carried into British waters on currents from breeding grounds near the Channel Islands, Brittany and further south, said Bryce Stewart, senior research fellow at the Plymouth-based Marine Biological Association. Previously, many did not survive the cold. But since 2022, the English Channel has been under heatwave conditions for around six months a year on average, logging more than 900 days in all.

"Now they're actually breeding in UK water quite prolifically," Stewart said.

For Ian Perkes, owner of a fish merchants in the south Devon town of Brixham, the octopus bloom has proved a boon. There is a ready market in southern Europe for the once-rare species and no fishing quotas, he said. "There was one week where we had 200,000 kilos."