Portuguese men o’ war plague Europe’s warming beaches
More sightings of jellyfish-like species have coincided with high ocean temperatures caused by climate change
Kenza Bryan in London
Their translucent blue, purple or pink inflated crests evoke the sails of 18th-century Portuguese warships.
Their barbed, venomous tentacles extend up to 30 metres below the ocean’s surface.
And the jellyfish-like creatures known as Portuguese men o’ war are swarming Europe’s beaches.
Authorities along the Bay of Biscay in the Atlantic Ocean 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 a series of heatwaves.
Warnings over jellyfish species traditionally seen in European waters are also in place along the Mediterranean coast, while in northern France an EDF nuclear reactor was still offline on Tuesday after a “massive arrival” of jellyfish clogged cooling pumps, following a similar incident last year.
Sightings of unwelcome jellyfish in Europe this year have coincided with a historic marine heatwave in the region and increased interest from researchers in the species’ ability to thrive in oceans warmed by climate change.
Scientists who collaborate on the World Weather Attribution project said on Wednesday that high temperatures in European oceans would have been “virtually impossible” without global warming.
Their modelling showed that climate change caused by the burning of fossil fuels 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 drew attention to the jellyfish phenomenon.
John Bruno, a marine ecologist at the University of North Carolina at Chapel Hill who participated in the study, warned that 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 published last year said the species’ proliferation even as other marine wildlife dwindles was a “growing concern” among marine scientists.
Its author Albert Calbet, from Barcelona’s Institute of Marine Sciences, wrote that a jellyfish known as the “mauve stinger” was now blooming in the Mediterranean on a yearly basis, compared with every dozen years several decades ago.
Uncertainties remain about their response to heat, however.
Scientists know little about the population size of jellyfish at sea and often rely on citizens and lifeguards spotting them on beaches during summer months.
Some types even thrive in colder waters, while others can appear on beaches months after a heatwave.
The Mediterranean region has been the hardest hit by climate change of the European regions studied by the WWA, warming at twice the global average, it said.
But around Marseille, variations in the north-westerly Mistral wind this year mean few jellyfish have been spotted, said Delphine Thibault, a professor at Aix-Marseille University.
Despite this, huge shoals could be lingering out at sea, she added.
“There are few direct links between climate change and jellyfish but definitely many indirect links, via changes to currents and temperatures.”
In Arcachon, a team of scientists is studying whether changing wind, ocean currents and ocean temperatures linked to climate change may be responsible for suspected population increases.
The basin around the resort town in south-western France is known for its oysters and was recently hit by wildfires.
Along the wider French Atlantic coast, Portuguese men o’ war have started to appear yearly rather than every seven or eight years, said Elvire Antajan, who researches zooplankton — animals that are carried by ocean currents — at France’s Ifremer, a state-funded oceanographic institute, in Arcachon.
“We are seeing more and more of them,” she said.
“We are a little perplexed.”
The past two years have also seen huge arrivals of smaller specimens that may be baby men o’ war, she said.
Warmer temperatures in the north Atlantic and Bay of Biscay could make it easier for them to reproduce locally.
Authorities in Arcachon briefly banned swimming on its main beach earlier this month because of the presence of Portuguese men o’ war.
On France’s Basque coast, there have been more than 1,000 stings from the creatures during two recent weeks, Antajan said.
The Basque city of San Sebastián in northern Spain and France’s Pyrénées-Atlantiques region have also issued warnings, with some beaches raising white flags with purple illustrations to warn swimmers away.
The Portuguese men o’ war are not typically deadly but about 10 per cent of those stung need hospital treatment, she said.
Incidents this summer include a young woman receiving scars across the face, and an experienced lifeguard being rescued by colleagues after encountering one during his morning swim.
Each Portuguese man o’ war is made up of a colony of clones that work together to float, capture prey and reproduce, sometimes travelling in groups of more than 1,000.
The creatures have in recent years been spotted in the UK as far north as Cumbria, according to the Royal Society of Wildlife Trusts.
Global sea temperatures were at a record monthly high in July.
The US’ National Oceanic and Atmospheric Administration said this week that the ongoing El Niño warming phenomenon in the Pacific Ocean was expected to persist through to next May.
El Niño, which can temporarily turbo-charge the effects of global warming, was likely this year to be the strongest observed since the 1950s, it added.
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