The intense heat that brought widespread drought and wildfires to Europe this summer extended far beyond the land.
Sea surface temperatures across the continent reached their highest levels for the summer period since dependable satellite records started in 1982, an analysis of marine data shows.
Temperature records were broken from the English Channel to the Mediterranean Sea. On 5 August, an offshore buoy anchored near Mallorca registered 33.02 degrees Celsius (91.44 degrees Fahrenheit), marking the warmest temperature ever recorded by the Spanish port authority's monitoring system.
Experts caution that marine heatwaves of this nature are growing in frequency, severity, and extent as global greenhouse gas emissions elevate air temperatures worldwide. Earth's oceans take in most of this trapped thermal energy, holding onto it far longer than land surfaces or the atmosphere.
These shifting conditions are altering aquatic ecosystems with devastating impacts on certain European fisheries and coastal livelihoods, based on scientific literature alongside testimonies from over a dozen analysts, researchers, and fishing sector figures. Marine species that long supported local communities in southern England and Spain are departing, replaced by non-native species migrating into newly warmed habitats.
"What is surprising is the speed at which this occurs," Jean-Pierre Gattuso, senior research scientist at France's National Centre for Scientific Research, said of Europe's warming seas. "Records are broken almost every day in many places."
Across the continent as a whole, European marine waters averaged 21.6 C (70.9 F) from June to August, setting a peak across 45 years of collected observations.
Among the most affected regions was the Bay of Biscay, situated along the Atlantic coast shared by France and Spain.
Reaching from Galicia’s Cape Ortegal to Pointe de Penmarch in southern Brittany, the bay—where waters are typically cool—averaged 21.4 C (70.5 F) across the summer, exceeding the 1991–2020 baseline by more than 2.4 degrees (4.4 degrees). Measurements from the US National Oceanic and Atmospheric Administration reached a satellite-era high of 23.2 C (74 F) on 13 August.
Marine heatwaves are defined by researchers as periods of five or more consecutive days where water temperatures significantly exceed historical baselines. While the Bay of Biscay experienced only a few heatwave days annually during the 1980s, analysis shows it had accumulated 186 heatwave days by late August this year, representing over three-quarters of the period.
The Bay of Biscay's cool waters, long prized by Spaniards seeking relief from the heat of the Mediterranean coast, were immortalized in Ernest Hemingway's 1926 novel "The Sun Also Rises." The book's protagonist recovers after weeks of dissolution on the northern beaches of San Sebastian.
"Now if you want to swim in cold water, you have to go to the westernmost portion of Galicia," in the far northwestern corner of Spain, said Jose Manuel Rico Ordas, a professor of marine ecology at the University of Oviedo in the northern Asturias region.
Galicia benefits from strong winds that push warm surface water away from the coast during the summer months, allowing cold, deep-sea water to take its place in a process known as upwelling.
Rico Ordas has been spending summers on the bay since he was a child and now studies its transformation.
Back then, there were extensive underwater forests of kelp, which provide vital food, shelter and nurseries for dozens of species that sustain the region's small-scale fisheries. But he estimates roughly 95% of the large, brown seaweed stands have disappeared, based on sampling along the rugged Asturias coast since 2010. He attributes the decline primarily to climate change.
Kelp needs cold water – typically below 20 C (68 F) – to survive. Higher temperatures were once exceptionally rare in the bay, Rico Ordas told Reuters.
"Now, it's the opposite," he said.
Fish overgrazing – in particular by dreamfish, which thrive in warmer temperatures – has added to the kelp's decline, he said.
The impacts have been stark.
"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.
Concerned fishermen have been teaming up with scientists and environmental activists to try to restore kelp forests by planting seedlings grown in laboratories.
Manuel Angel Reiriz Gonzalez, president of a local fishermen's guild in Galicia, recently ferried members of one such project in a boat he uses for sport fishing charters out of the port of Ribeira. The group brought a box full of small stones seeded with kelp, which they tossed into the choppy waters.
Climate change threatens a way of life that has sustained Gonzalez's community for generations, he said. His family has been diving for sea urchins and harvesting clams for more than a century. But now, he said, recurring heatwaves are hurting shellfish stocks.
The area has also seen an influx of Portuguese man o' war and trigger fish from warmer climes to the south – species that prey on local fish and crustaceans.
"We had some very bad years for clam production, but we worked on seeding them, and the species gradually adapted," Gonzalez said.
He hopes the seabed restoration efforts can do the same for kelp forests.
The disappearance of kelp could lead to the decline of sea urchins and other species fishermen depend on, he said. "At that point, as a fishing guild, we would cease to exist."
In other European waters, new, invasive species from the Red Sea and beyond are entering the Mediterranean through the Suez Canal, displacing native populations, reshaping food chains and threatening fisheries, researchers say. In all, there are more than 1,000 non-indigenous species in the Mediterranean, according to a recent report by the Cyprus Academy of Sciences, Letters and Arts, based on surveys by scientists in 11 countries.
Some native species are coping with the warmer temperatures 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 – for the second time in as many years. The plant in northern France is cooled from a canal connected to the North Sea.
Fishermen on the south coast of England are reporting a surge in landings of common octopus. 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 places further south, said Bryce Stewart, senior research fellow at the Plymouth-based Marine Biological Association (MBA).
Previously, many didn't survive the frigid temperatures. But since 2022, the English Channel has been under heatwave conditions for around six months out of the year on average, logging more than 900 days in all, the Reuters analysis found.
"Now they're actually breeding in UK water quite prolifically," Stewart said.
For Ian Perkes, owner of the eponymous fish merchants in the south Devon seaside 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 (200 tons)."
But not everyone profits from the shellfish-loving octopuses, which feed on local lobsters, brown crabs and scallops.
For members of the South Devon and Channel Shellfishermen's cooperative, 2026 has been an "octopus apocalypse," according to its executive officer, Beshlie Pool. One recent fishing trip yielded a single crab all day, she said.
The MBA estimates the octopus bloom could account for about £10 million ($13.25 million) in lost shellfish catches against about £12 million in octopus landings between January and September 2025.
But the benefits are not evenly distributed. Some shellfishermen have switched to catching octopuses, but many do not have the necessary equipment, Pool said. Four of the coop's 90-some vessels have gone out of business in the past 12 months.
The unpredictability of the changes makes it hard to adapt, Stewart added.
"Will the octopus still be here next year?" he asked. "If they've eaten all the crabs and lobsters, and the octopus die off, then what do we do?"
