Beneath the ocean’s surface lies a world teeming with diverse marine life – but scientists say some “rather unusual” changes are now taking place. Driven in part by a marine heatwave in May, waters off England’s south coast are warming and disrupting the delicate balance of 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, who runs a blog and Instagram page documenting marine species in Cornwall, has noticed increasing sightings of various jellyfish species, along with a notable rise in octopus numbers.
She described feeling a “mix of emotions” about these developments – from excitement to growing 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?”
Her observations align with what marine ecologist Dr Bryce Stewart, of the Marine Biological Association in Plymouth, describes as “dramatic changes” in the marine ecosystem.
Dr Stewart – who contributed to the most recent Sir David Attenborough documentary – said he was “very confident” that ocean warming is behind the environmental shifts being recorded.
In recent months, he noted a series of “rather unusual” developments off the coasts of Devon and Cornwall, where the marine heatwave was particularly intense in May.
Local fishermen have reported an “invasion” of octopuses, which are said to be having a severe impact on the Devon shellfish industry.
Species typically found in the Mediterranean are now turning up in lobster and crab pots along the south-west coast. It’s believed the octopuses may be migrating from Morocco, where fishers are reporting declining numbers.
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.”
Dr Stewart also pointed to a surge in sightings of blue sharks off England’s south coast, with current numbers said to be the highest anglers have ever encountered.
“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.
He said the rate and scale of change in the marine environment are “more dramatic” than anyone had foreseen.
Reflecting on a recent trip off the coast of Falmouth, Dr Stewart described it as “one of the greatest days at sea” he’d ever experienced – despite having travelled extensively across the globe.
That day, he encountered two fin whales – the world’s second-largest whale species – thousands of dolphins, and even snorkelled alongside blue sharks.
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.”
Meanwhile, the Devon Wildlife Trust has observed changes occurring closer to the shore, including the appearance of brightly coloured sea slugs.
Carli Cocciardi, marine nature recovery officer at the trust, said one of their citizen science projects involves monthly surveys of rock pools at Wembury beach in South Hams.
“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 on record. With high levels of sunshine and limited vertical mixing in the water due to persistent easterly and north-easterly winds, a marine heatwave was recorded in mid-May across waters west of Ireland and throughout the Irish and Celtic Seas.
Marine heatwaves are defined as periods when sea temperatures exceed the seasonal average for more than five consecutive days.
In the UK, the marine heatwave threshold for May is 11.3°C. On 19 May, the average sea surface temperature reached 12.69°C.
For Ms Hamilton, the sea is a “very special place”, and growing up in the Isles of Scilly, it was “almost inevitable” that she would develop a deep connection to 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 believes that the more people engage with the marine environment, the more likely they are to care about – and take action to protect – 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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