How warming seas are altering our underwater world

Beneath the surface of the ocean is a world filled with a vast array of fauna and flora but scientists say "somewhat unusual" changes are taking place. Intensified by a marine heatwave in May, they say waters off the south coast of England are getting warmer and disrupting the food chain.

"I'm not a scientist or anything like that," said Cornish snorkeler and underwater photographer Heather Hamilton.

"But because I've been in the sea so much over the last 20-odd years, I have seen changes, especially in the last two years."

Ms Hamilton runs a blog and Instagram page documenting the different species she sees in Cornwall, including different species of jellyfish as well as increases in octopus numbers.

She said the changes had also left her with a "mixture of emotions" from excitement to concern.

"You've got in the back of your mind, 'Is this a good thing?'" she said.

"It's a little bit worrying to think why is this happening all of a sudden in the last few years?"

Ms Hamilton's observations in the sea are part of what marine ecologist Dr Bryce Stewart, from the Marine Biological Association in Plymouth, said were "dramatic changes" to the marine ecosystem.

Dr Stewart, who helped create the latest Sir David Attenborough documentary, said he was "very confident" changes in the marine environment were being caused by the warming of the oceans.

In recent months he said there had been some "somewhat unusual" changes off the coast of Devon and Cornwall, where a marine heatwave was most intense in May.

Fishermen have reported an "invasion" of octopus which is "decimating" the Devon shellfish industry.

The octopus, usually found in the Mediterranean, are being increasingly found in lobster and crab pots off the coastline.

It is thought the octopuses are coming from Morocco, as fishermen there are reporting a shortage of the creatures.

Dr Stewart said: "So there's a lot of species either turning up that we haven't had before or increasing abundance... and then of course we have other species that were more adapted to cooler water decreasing in abundance... and we see those changes right from the base of the food chain in the plankton to right to the top, to the tuna and sharks."

He said the number of blue sharks off the south coast of England was currently the highest anglers had ever seen.

"We've been working with anglers, and they have datasets going back to the 1950s about their catch rates of blue sharks, they all let them go so don't worry about that, but the numbers now are the highest we've ever seen," he said.

Dr Stewart said changes were happening in the ocean "more dramatically" than anyone had predicted.

Dr Stewart said a trip off the coast of Falmouth a couple of years ago was "one of the greatest days at sea" he had ever had – having travelled all over the world.

During the trip, he saw two fin whales, "thousands of dolphins", and also snorkelled with blue sharks.

The fin whale is the second-largest species of whale.

Dr Stewart added: "It's the biggest creature I have ever seen, we estimated probably 70ft long, which is enormous right?

"These things are now just off the coast, and I would encourage as many people as possible to go out and see them."

The Devon Wildlife Trust said it was seeing changes closer to the shore, including colourful species of sea slugs.

Carli Cocciardi, marine nature recovery officer at the trust, said one citizen science project involved surveying rock pools at Wembury beach, in the South Hams, on a monthly basis.

"That gives us a good indication of what changes year-on-year and month-on-month," she said.

"So for example, what we're seeing more in Wembury is... furrowed crabs, and sometimes hermit crabs - and those are more Mediterranean species, that are seen here more and more often."

This spring has been the second sunniest spring on record.

With sunshine levels high, and the vertical mixing of water limited by the predominance of easterly and north-easterly winds, a marine heatwave was recorded in the waters to the west of Ireland and through the Irish and Celtic seas in mid-May

The phenomenon is defined as sea temperatures which exceed the seasonal threshold for more than five consecutive days.

In the UK the marine heatwave threshold for May is 11.3C (52.34F). On 19 May, the average sea surface temperature reached 12.69C (54.84F).

For Ms Hamilton, the sea is a "very special place" and growing up in the Isles of Scilly meant it was "kind of inevitable" she would be captivated by the ocean.

"Going in the sea and snorkelling is kind of like an escape from the world above, you're kind of putting your head under the water and you forget about all your worries in life," she said.

"It's just very calming and peaceful."

Dr Stewart said if people engaged with marine environment more, they would be more likely to become passionate about protecting it.

"I'm going to echo the recent Ocean with David Attenborough film here," he said.

"What that showed us and what I've seen as well in my own research is that when we look after the marine environment, it has an amazing ability to bounce back."

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