Millions of giant jellyfish invading Britain's coastline

Unprecedented numbers of jellyfish have been found in the seas and washed up on the shores of the south coast leading to concern that some beaches may have to be closed.

The barrel or dustbin-lid jellyfish, which are the largest in British waters and can grow up to 90cm long, have been found on Hampshire and Dorset beaches as well as the coastline of Wales in recent days.

ITV News Science correspondent Alok Jha reports.

However, experts are not unduly worried at the levels or the dangers these jellyfish pose. Their stings are akin to being stung by a nettle and it is two other types - the blue and compass jellyfish - that will cause greater concern.

"This is very uncommon," said Steve Trewhella, a conservationist who has been working in the waters in the Dorset area for 30 years. "The numbers we have now are off the scale, there are vast, vast amounts of these animals in the water.

"I was out snorkelling the other day and I was completely surrounded by them."

Jellyfish are not often seen in the waters around Britain although their number have increased in recent years. The Marine Conservation Society reported 2013 as a record year for jellyfish sightings with high numbers also found in 2014.

However, this year has already proved to have smashed previous records and there has been much speculation as to why they have made an appearance - and in such huge numbers.

One theory is that over-fishing may have depleted their predators but it's thought the most likely cause is a combination of warm summer days combined with rain washing nutrients into the sea which have created good conditions to help the jellyfish grow, a theory with which Dr Simon Boxall, an oceanographer at the University of Southampton, agrees.

"Jellyfish will thrive if the conditions are right," he said. "They are always there in the seas but they can grow very quickly and at the moment we have had a nice mix of bit of rain, sun and a bit of warmth.

"The rain is washing nutrients off the land into the sea which feeds the plankton and the algae grow meaning the jellyfish have lots and lots of food which means they are swarming at the moment which is helped by the fairly calm seas."

It's expected that the number of jellyfish in British coastal waters will increase further this year but whether or not 2015 is a one-off remains to be seen.