The Spanish Red Cross has already treated more than 15,000 swimmers for nasty stings.

Nearly all the Spanish coastline is affected, and lifeguards and Red Cross volunteers have imposed extensive restrictions on popular beaches in areas such as the Costa Brava. Swimmers who ignore the red flags frequently end up in the queues outside the first aid tents.

Last week, city authorities in Alicante, on the Costa Blanca, held an annual race round the port for 500 swimmers. More than 200 were stung and many had to be rescued.

Marine biologists say that the number of jellyfish has at least tripled on the Costa Brava, around Barcelona, on the Costa Dorada, in the Mar Menor, at La Manga, and at Motril and on the Costa del Sol. The Balearic Islands — Majorca, Minorca and Ibiza — are not quite so badly affected. Mechanical diggers were brought in to Ceuta, a Spanish enclave on the North African coast, to remove two tonnes of jellyfish, which were mainly the mauve stinger (Pelagia noctiluca) variety.

Scientists say that this year’s drought has reduced the amount of cold, fresh water flowing into the sea from rivers. They also blame overfishing, which has reduced the numbers of fish that feed on jellyfish.

Sebastián Losada, who is responsible for Greenpeace’s ocean campaign, said: “The disappearance of their natural predators such as shellfish, fish and marine turtles, as well as pollution and overfishing in a sea as enclosed as the Mediterranean, which warms up easily, generates the ideal conditions for the proliferation of opportunist species such as jellyfish.”

On the Costa del Sol, Juan Antonio Camiñas, director of the regional marine institute in Fuengirola, blamed the very high temperatures this year, with a constant easterly Levante wind driving the jellyfish, which are usually far offshore, towards the beaches and the warmer water where they thrive. “If the wind changes to a westerly or northerly, then they will be moved further offshore,” he said.

The Mar Menor is always one of the worst-affected areas. It is an inland sea on salt flats near the resort of La Manga, where fertilisers from neighbouring farms aggravate the problem. This year there are so many jellyfish — Cotylorhiza tuberculata, known locally as “fried eggs” — in the water that its colour has turned a milky blue.

Benidorm, one of Spain’s biggest resorts, is still sting-free. Montserrat Gasco, the spokesperson for Benidorm town hall, said that the beaches were currently safe but, as in past years, September is the known season for medusas, as jellyfish are called in Spanish.

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