Portuguese men o’ war plague beaches in Europe amid heatwave

Financial Times UK

19 Aug 2026

KENZA BRYAN — LONDON

Authorities along the Atlantic Ocean’s Bay of Biscay have warned in recent

weeks that the species typically found in tropical waters has been appearing on

European beaches that tourists and locals have used to cool down from heatwaves.

Meanwhile, warnings for traditional species of jellyfish are in place along the

Mediterranean coast. In northern France, an EDF nuclear reactor was still offline yesterday after a “massive arrival” of jellyfish last week clogged cooling

pumps. Sightings of unwelcome jellyfish in Europe this year have coincided

with a historic marine heatwave and increased interest from researchers in the

species’ ability to thrive in oceans warmed by climate change.

Scientists who collaborated on the well-respected World Weather Attribution

project said last week that high temperatures in European oceans would have

been “virtually impossible” without global warming.

Their modelling showed climate change caused by fossil fuel burning has last

month added 2C to Mediterranean temperatures, 1.4C to a region centred on the

Bay of Biscay and the Iberian peninsula and 1.3C to a Celtic region centred on

the Irish Sea and the English Channel. They also highlighted the jellyfish phenomenon.

John Bruno, a marine ecologist at the University of North Carolina, warned

extreme heat could lead to “mass mortality” events at sea for fish, birds and

ocean mammals. In contrast, jellyfish and related creatures could be more tolerant of temperature rises, leading them to “bloom” during some marine heatwaves even as other species decline, he said. The rubbery animals are also

more resilient to low-oxygen or low-nutrient conditions triggered by warmer

water, and benefit from overfishing of their predators.

A book chapter entitled The Rise of Jellyfish Dominance said last year that the

species’ proliferation even as other marine wildlife dwindles was a “growing

concern” among marine scientists. Its author, Albert Calbet, from Barcelona’s

Institut de Ciències del Mar, wrote that a jellyfish known as the “mauve stinger”

was now blooming in the Mediterranean on a yearly basis, compared to every

dozen years in the 1980s.

Uncertainties remain, however, about their response to heat. Scientists know

little about the population size of jellyfish at sea and often rely on citizens and

lifeguards spotting them on beaches during summer. Some types even thrive in

colder waters, while others appear on beaches months after a heatwave.

The Mediterranean region has been the hardest hit by climate change of the

European regions the WWA studied, warming at twice the global average, it

said. The Portuguese men o’ war are not typically deadly but about 10 per cent

of those stung need hospital treatment