Invasion of the jellyfish: Mediterranean on alert as hundreds suffer from stings

As thousands of tourists head to the Mediterranean, the spectre of jellyfish ruining holidays looms large after French emergency services received more than 500 calls for help in a single day along a 10-mile stretch of coast from Nice to Cannes.

Paddlers suffered painful stings and wanted something to treat the pain while swimmers reported that they had found themselves totally surrounded by a species commonly known as the mauve stinger.

It is a pattern being repeated along the shores of Mediterranean. As well as the Côte d'Azur, the coast of Liguria on the west coast of Italy, the Costa Smeralda in Sardinia, parts of the Adriatic on Italy's east coast, and much of the southern – and even northern – coastlines of Spain have been hit.

Jellyfish have no autonomy of movement and are swept around the oceans by wind and tide. In the past they came billowing into the beaches once every 10 or 12 years. They stayed for three or four years then disappeared as mysteriously as they arrived. But not any more. This is the eighth year straight that they have stormed the smartest resorts in the Mediterranean.

Spaniards hoping to avoid the invasion by heading north have had to think again. The Portuguese Man o' War (Physalia physalis), whose sting can be fatal, is marauding the coasts of Cantabria and Asturias, which until now have managed to escape the seasonal plague. Winds have blown the creatures ashore in recent days, prompting warning flags to be flown.

At Nice and Cannes, the jellyfish menace vanished as quickly as it arrived, but scientists are in no doubt that they will be back, perhaps before the end of the season. The species haunting this 10-mile stretch of coast is the Pelagia noctiluca, the mauve stinger. Its sting can cause severe burns, in some cases scarring their victims. Despite warnings to keep out of the water, many swimmers were caught out last week, prompting the flood of calls to the French emergency services.

Fearful of the effect on the tourist trade, Cannes and Monaco have installed booms and nets on several beaches. But hundreds slipped through and many more invaded unprotected beaches.

In Antibes a 30ft catamaran which has been described as a "jellyfish hoover" now patrols the coastline, ready to suck up any returning jellyfish.

"I can't say that the jellyfish will definitely return," said Jacqueline Goy, the leading jellyfish expert at the Institut d'Oceanographie in Paris. "At the moment the mistral has blown them offshore but a change in wind direction could well bring them back later this year."

The phenomenon is by no means a new one said Mme Goy, known to colleagues as la Dame aux Méduses ("Jellyfish Lady"). The earliest report of a "jellyfish soup" in the region dates back to 1802. In recent years, however, the frequency and persistence of the swarms has increased.

Marine biologists believe that overfishing has killed off both the fish that hunt jellyfish, and their young which compete with jellyfish for plankton.

Fabrizio Bulgarini, biodiversity expert with WWF Italy, said there was also another theory. "The rate of reproductivity of the jellyfish appears to be related to the heat of the water," he explained, and thanks to global warming the sea is getting hotter.