A tourist swims with a harmless variety of jellyfish in Jellyfish Lake, Micronesia. However, outbreaks of many stinging jellyfish species are causing problems for swimmers in oceans around the world.

A tourist swims with a harmless variety of jellyfish in Jellyfish Lake, Micronesia. However, blooms of many stinging jellyfish species are causing problems for swimmers in oceans around the world. Photo: Reinhard Dirscherl

Dr Lisa-Ann Gershwin, a US scientist and author of the book Stung: On Jellyfish Blooms and the Future of the Ocean, told CNN that overfishing and changing water temperatures were responsible for huge increases in "bloom" activity.

"The French and Spanish Rivieras, Chesapeake Bay, the Great Barrier Reef, Hawaii ... some of the numbers are staggering," said Dr Gershwin, who's now based in Australia.

In Spain or Florida, it's not uncommon in recent years for a half a million people to be stung during an outbreak 

"Jellyfish and tourism are not happy bedfellows. In Hawaii there have been times that 800 or 1000 people have been stung in a day. In Spain or Florida, it's not uncommon in recent years for a half a million people to be stung during an outbreak. These numbers are simply astonishing," she added.

Two incidents have highlighted the problem in recent weeks.

On October 13, operators at the Oskarshamn nuclear plant in Sweden had to shut down a reactor as jellyfish clogged the pipes that move cool water to the plant's turbines.

A week later, up to 20,000 farmed salmon were killed by a jellyfish bloom off the coast of County Mayo in Ireland.

This summer has seen a jellyfish infestation in southern Europe so severe that the Foreign Office issued a travel warning, while in July the Marine Conservation Society reported "huge numbers" of several species in British seas, including the Lion's Mane, which has a powerful sting.

A report by Fernando Borea for the General Fisheries Commission for the Mediterranean and the United Nations claimed the problem in the Mediterranean has been growing for several years.

"In the last decade ... the media are reporting on an increasingly high number of gelatinous plankton blooms," it said. "The reason for these reports is that thousands of tourists are stung, fisheries are harmed and even impaired by jellyfish."

A study by the University of British Columbia last year found that jellyfish populations had increased by up to 62 per cent in areas including East Asia, the Black Sea, Hawaii and Antarctica.

Dr Gershwin's book warns that jellyfish are able to overrun ecosystems because of their adaptability and resilience as an organism.

"Hermaphroditism. Cloning. External fertilisation. Self fertilisation. Courtship and copulation. Fission. Fusion. Cannibalism. You name it, jellyfish [are] doing it," she writes.

One species, the mnemopsis, is able to lay eggs without needing a mate at just 13 days old, she notes.

It can eventually lay up to 10,000 eggs a day.

She also warns the damage may be permanent: "I think I underestimated how severely we have damaged our oceans and their inhabitants. I now think that we have pushed them too far ... without us knowing the precise moment it all became irreversible".

Contrary to popular opinion, if you are stung, urine is not the best remedy.

The British Red Cross has issued health advice for beach-goers in response to the sightings.

"A sting from a jellyfish can be extremely painful, but trying to treat it with urine isn't going to make your day any better," said Joe Mulligan, British Red Cross head of first aid.

"Urine just doesn't have the right chemical make-up to solve the problem."

He advised slowly pouring seawater over the area.

"Doing the same thing with vinegar can be even more effective as the acid helps neutralise the sting. But, unless you're near a chip shop, seawater will probably be easier to find."

The Telegraph, London