The number of jellyfish spotted in UK waters and on beaches increased by 32% in the past year, according to a survey by the Marine Conservation Society. The most commonly spotted were the huge barrel jellyfish – but rarer warm-water crystal jellyfish were also seen. Jellyfish populations vary naturally over time – but climate change warming the UK seas is creating favourable conditions.
A marine heatwave in June increased UK water temperatures by about 3-4C.
The world has warmed 1.1C compared to the pre-industrial period before humans began burning fossil fuels and a series of broken records this year is alarming scientists.
In August, oceans hit their highest global average temperature on record.
The Marine Conservation Society’s annual wildlife-sightings report is based on members of the public using an online form or app.
From October 2022 to September 2023, jellyfish sightings increased by 32% compared with the previous year.
Most were on the UK’s west coasts, particularly in Cornwall and Wales. And 11% were of large blooms of more than 100.
The barrel, or “dustbin-lid”, jellyfish can grow to more than 3ft (1m) in diameter. And rather than tentacles, it has eight thick, frilled arms. There were 467 sightings of these.
Lion’s mane, Portuguese man o’ war, moon and compass jellyfish were also seen.
The Marine Conservation Society has been asking the public to report sightings of jellyfish for 20 years.
“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,” Dr Peter Richardson said.
“But this survey gives us an indication of what’s happening in our seas with climate 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, including the crystal jellyfish which she had never before seen in the UK.
This creature normally lives in warmer waters but the hot weather in June created good marine conditions for it to thrive in the UK.
“The jellyfish this summer just did great, including the native ones,” she adds.
It “could be an indication that as climate change is happening, we are seeing tropicalisation of the oceans”, she said, referring to the higher temperatures of the water.
Scientists say that many marine species will move their range northwards over time as waters warm.
“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,” she says.
In the long-term, warmer UK seas are predicted to alter the country’s fish stocks as the food that feeds current fish like cod will change.
The Marine Conservation Society survey also reported 12 turtle sightings – and four of these were of the leatherback, the largest sea turtle.