Bathers are being warned to watch out for jellyfish after an unusually high number of early sightings, ahead of the usual summer peak.

The Marine Conservation Society has had reports in the last fortnight from Cumbria of moon jellyfish which has a mild sting, seen in blooms more than 500-strong.

Richard Harrington, an MCS spokesman, said: "The Irish Sea does seem to be a hot spot at the moment. The warm weather has undoubtedly triggered blooms around the coast.

"Jellyfish bloom when there is some kind of environmental change that triggers them to do so. That might be a change in sea temperature or changes in plankton."

Blooms of jellyfish covering 62 square miles (160 sq km) – or the size of 22,167 Wembley football pitches – have already been spotted off the coast of Wales.

The MCS is conducting a monitoring project in British waters, and as well as Cumbria has had reports from West Scotland, County Down and Devon of moon jellyfish.

Experts believe the number of jellyfish in UK waters has increased sharply in recent years, possibly as a result of warming seas linked to climate change, or a decline in some fish stocks which reduces competition for food.

Several studies have been launched to establish what is behind the blooms – or sudden, massive increase in numbers – which usually occur in the summer months.

Researchers have now warned that blooms are expected to be particularly abundant this year, even compared with recent years, after an abnormally large number of recent sightings.

Dr Victoria Hobson, a marine biologist at Swansea University, said: "We are seeing a lot this year.

"It is definitely going to be a good year for jellyfish and people should be wary and steer clear of them where possible.

"It is hard for people to imagine the scale of some of these blooms. They go somewhere else in the winter but they are suddenly all back in their millions. We have lots of reports of strandings coming in.

"We are studying why this might be. It could be that the sea has warmed up a bit quicker this summer."

As well as the barrel jellyfish, lion's mane and moon jellyfish, other species occurring in UK waters include blue jellyfish (Cyanea lamarckii), and compass jellyfish (Chrysaora hysoscella), which have a relatively mild sting.

Others which have been found here include the mauve stinger (Pelagia noctiluca), which has a more serious sting.

Also found is the Portuguese man-of-war (Physalia physalia), which is actually a siphonophore – a colony of organisms resembling jellyfish – but which can be fatal to humans.

Another creature, the By-the-wind-sailor, which is a harmless relative of the Portuguese man-of-war, has been washed up on beaches in the south west in its thousands this year.

As well as stinging swimmers, jellyfish pose a serious problem to some fisheries.

In 2007, a bloom of mauve stingers, covering an area of up to ten square miles (27 sq km) and up to 35ft (13.6m) deep, swamped a fish farm off the coast of Northern Ireland, killing more than 100,000 salmon, valued at more than £1 million.

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