Huge rise in 'unusual' jellyfish spotted in UK waters – and no one is sure why

03 November 2023 , 15:28

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A crystal jellyfish (Image: Getty Images/iStockphoto)

Jellyfish have been spotted off the British coastline 32% more than normal, according to marine experts, but the reason for their sudden boost is not yet clear.

The Marine Conservation Society collected the data between October 1, last year and September 30, this year and had 1,737 separate sightings with barrel jellyfish - sometimes called Dustbin Lid Jellyfish as they can grow up to one metre in diameter - being the most common with 437 of the sightings.

Lion's mane, Portuguese man o' war, moon and compass jellyfish were also seen and there were also a few crystal jellyfish seen, which are much rarer in the UK as they are normally in more warmer waters. Although jellyfish populations are known to vary naturally over time, it is thought climate change, with seas warming could be creating favourable conditions for the creatures.

In August, oceans hit their highest ever global average temperature since records began, and a 'marine heatwave' in June saw an increase in the UK water temperatures by about 3 or 4 celsius.

A compass jellyfish (Getty Images)

The Marine Conservation Society's annual wildlife-sightings report asked members of the public to fill in a form if they spotted anything, with people on the west coasts, particularly in Cornwall and Wales seeing the most lively seas.

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The Marine Conservation Society has been asking the public to report sightings of jellyfish for 20 years. Dr Peter Richardson said: "These numbers could be part of a 20-year long boom-and-bust cycle but there are very few surveys out there which show what's happening. But this survey gives us an indication of what's happening in our seas with climate change."

The crystal jellies are usually seen in warmer waters (Getty Images/iStockphoto)

He added: "This year seems to have been a particularly good year for barrel jellyfish - one of our chunkiest jellyfish species that can occur in mind-boggling numbers when conditions are favourable. It's only by observing trends over many years that we can start to suggest reasons for change."

Dr Abigail McQuatters-Gollop, a plankton expert at the University of Plymouth, agrees that there is an abundance of jellyfish this year. She went diving every day in August and says she had never seen so many jellyfish. She told the BBC: "The jellyfish this summer just did great, including the native ones.

"But we don't know if the high numbers this summer are a longer-term natural trend or linked to the marine heatwaves. There is a lack of research - we have to do more studies."

Kelly-Ann Mills