FROM the Costa del Sol to the French Riviera, an infestation of jellyfish is forcing seaside resorts to set up defences, repel the invaders and protect the tourist industry.

The city of Cannes on the Côte d’Azur will use a floating barrier to block the swarms of jellyfish that inflicted painful stings on thousands of bathers around the Mediterranean last year.

In Spain, a network of volunteers with boats has been recruited for a preemptive strike against the primitive creatures, which will be scooped up and deposited in deeper water.

The French, for their part, are trying out technology for keeping the jellyfish away from the beaches.

“Last year we were defenceless against the threat,” said a spokeswoman for the mayor of Cannes, who worries about tourist revenues falling victim to the translucent pests.

“The jelly fish sting can be very dangerous, especially to people with allergies. We are taking it very seriously.” This has meant deploying a floating “boom” which holds a net stretching just over two yards beneath the surface and cordons off an area of sea into which no jellyfish will enter.

“We’re not talking about encircling the whole coast,” said Guy Tramoy, designer of the system, which is similar to one used to prevent oil slicks spreading. “The idea is simply to create a zone where people can swim in safety.”

Beaches had to be closed to swimmers from Spain’s Costa del Sol to Sicily during last year’s invasion of Pelagia noctiluca, also known as the mauve stinger, a poisonous invertebrate that uses long stinging tentacles for fishing and emits a yellowish glow at night, hence noctiluca, which means night light.

In humans, the sting can cause painful sores and even death from heart failure in rare cases: a couple on the French Riviera had to be taken to hospital with multiple stings last year after swimming into a large shoal of jellyfish. They were lucky not to have drowned.

Besides assaulting the beaches of southern Europe, a similar, although slightly less toxic, species has turned up off the Irish coast and salmon farmers from Scotland to Norway have suffered heavy losses from a variety of jellyfish.

The Mediterranean invasion has been blamed by some on climate change. “As the water temperature rises, it allows the jellyfish to live in areas where they could never go before,” said Jacqueline Goy, a jellyfish specialist at the Oceanographic Institute of Paris.

Jellyfish have thrived due to a decline in the population of tuna and turtles, which prey on them, and also because of the overfishing of anchovies, which compete with jellyfish for plankton.

The result is that recent infestations have turned parts of the Black Sea, the Baltic and the Mediterranean into what Goy, only half-jokingly, referred to as “jellyfish soup”.

Spain has launched a national “jellyfish plan” to tackle the menace. The environment ministry has organised a network of recruits among fishermen and pleasure craft operators to inform the coastguard of jellyfish sightings. A “spotter plane” is also on hand for the summer campaign.

More than a dozen boats normally used for scooping up rubbish at sea can be deployed to suck jelly fish into their holds. Under a plan drawn up by Catalonia’s environment ministry, the creatures will then be taken far from the coast to be released.

“To kill them en masse would be an ecological error,” said Francesc Baltasar, Catalonia’s environment minister, not wanting to incur the wrath of environ-mentalists who regard jellyfish as an important feature of the marine environment.

Marine biologists recommend the restoration of Spain’s turtle population to combat the jellyfish scourge, but that could take many years and by then the tourist beaches could be deserted.

Some have seen in the jellyfish plague a timely warning about environmental changes.

“The fact that the jellyfish are making it to the coast is a sign of how badly we are treating the sea,” said Josep-Maria Gili, a marine biologist in Barcelona.

“It is a reflection of how we have changed the sea more than we thought.”

Jellyfish, which have been around much longer than humans, spend most of their lives in the open sea. Recent drought, however, has meant that less fresh water enters the sea from rivers, resulting in warmer, more jellyfish-friendly coastal waters.

Researchers claim that jellyfish are also thriving on an increase in the pollution-related nutrients that have been detected in Mediterranean seawater. This, too, has been blamed for the scourge of “toxic algae”.

Italy, at least, is breathing a sigh of relief: experts there say that the country, badly hit in last year’s invasion of the jellyfish, will be spared the plague this year through a quirk of wind and sea current.

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