How Europe's intensifying marine heatwaves threaten its fisheries

As drought spread and wildfires raged across Europe this summer, the extraordinary heat didn't stop at the shoreline.

European waters notched their hottest summer months since satellite sensors began providing reliable data in 1982, according to a Reuters analysis of temperatures recorded at the seas' surface.

Scientists warn that events like this year's marine heatwaves are becoming more frequent, more intense and more widespread as greenhouse-gas emissions drive up atmospheric temperatures around the globe. 

The world's oceans absorb the majority of the excess heat and retain it longer than the land or atmosphere.

That's transforming marine ecosystems, with catastrophic consequences for some European fisheries and the people who rely on them, according to a review of the scientific literature and interviews with more than a dozen researchers, fishing industry sources and analysts. 

Species that have sustained some coastal communities in Spain and southern England for generations are disappearing, as unfamiliar arrivals surge into newly hospitable waters.

Taken together, European seas averaged 21.6 deg C between June and August, the highest in 45 years of available data.

Few areas strayed further from the norm this summer than the Bay of Biscay, off the Atlantic-facing coast of France and Spain.

Stretching from Cape Ortegal in Galicia to the Pointe de Penmarch in southern Brittany, the bay's traditionally cool waters averaged 21.4 deg C between June and August, more than 2.4 deg C above the 1991-2020 average. 

Readings from the US National Oceanic and Atmospheric Administration peaked on Aug 13 at 23.2 deg C, 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 heatwave days per year. By the end of August, it had logged 185 this year, more than three out of every four days, Reuters found.

Disappearing kelp, lobster, crayfish

Jose Manuel Rico Ordas, a professor of marine ecology at the University of Oviedo in the northern Asturias region, has been spending summers on the Bay of Biscay 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 per cent 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.

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' 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.

The disappearance of kelp could lead to the decline of sea urchins and other species fishermen depend on, he said.

'Octopus apocalypse'

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.

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.

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's 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.

The MBA estimates the octopus bloom could account for about £10 million (S$17 million) in lost shellfish catches against about £12 million in octopus landings between January and September 2025.

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