‘Our new normal’: how marine heatwaves affect life in British seas | Marine life

‘YOIt sounds silly, but I thought it was a person in the water waving,” says Kurt Lander, skipper of the Brighton-based charter boat Yellowfin. But when he grabbed his binoculars, he realized that the fluttering movement he had noticed in the middle of the Channel shipping lane was a pair of sunfish.

The huge fish lay on their sides and flapped their fins. “Apparently they do it as a visual attraction for the birds,” he says. “They are trying to get the birds to eat the parasites they remove.”

Lander had seen these huge fish with their unusual round bodies before. But the difference is that the ones he detected this year are part of a rise in sightings of the species off the British coast this summer, probably as a result of a marine heatwave.

Marine heat waves often follow long periods of heat on land, as the seas also warm. Last year was branded “exceptional” by the Met Office, with parts of British waters experiencing heatwave conditions for three quarters of the year, and 2026 is already showing more of the same.

Extraordinary temperatures are causing changes in marine life far beyond an increase in sunfish. Some species are moving away from warmer waters, other “invaders” are appearing for the first time on the British coast, while some marine life is dying in the sun’s rays.

Lander notices the change in the species of fish he now lands. “I used to catch at least a couple hundred cod a year,” he says. Now, bringing cod is rare. If you catch any, he adds, “it’s probably one or two a year in the summer.”

Colder water species are simply not seen on the south coast, says Dr Zoe Jacobs, senior researcher at the National Oceanography Center in Southampton. Shifting their range to colder seas further north, they are now thriving in Scotland. “Temporarily or permanently, we don’t know,” he says.

Meanwhile, Lander catches more warmwater fish, such as bass, combers and triggerfish. According to last year’s South West Seas report, other species found in warmer waters, such as sardines and anchovies, are also thriving.

Gio Reale, a diver working in Falmouth Harbour, Cornwall, says: “Small changes in water temperature have huge effects on marine life.” It’s been weeks since he last saw his favorite species of shark: the small cat shark (also called dogfish). These small sharks, which grow no taller than a meter and are dotted with delicate freckles, prefer water temperatures of about 16-17°C (61-63°F).

“This summer, like every day, the water has been around 20°C,” he says, adding that he believes the sharks may have moved to deeper, colder waters, replicating the way they behave in the Mediterranean.

These sharks may be gone, but common octopuses have arrived in force since January last year, when the warmer waters of Devon and Cornwall created the perfect conditions for them. This species is native to the United Kingdom but is not commonly seen.

The bloom caused problems for some crab and lobster fishermen. “Everyone complains, ‘Oh, they’re taking all our crabs,’” Reale says. “But obviously, for the octopus, it’s a very easy meal. You can just get into a trap where the crabs can’t go anywhere and eat them.”

Some boats have shifted their focus to the now-abundant octopuses, with catches set to soar by 2026. Octopuses also provide food for predators, which could explain an increase in sightings of summer visitors such as gray dolphins.

As sea swimmers know, warmer waters also bring jellyfish, another food source. But these blooms “have up and down years,” Jacobs says, and interpreting sightings can be tricky: Reports can increase simply because there are more people on the beach.

Seeing lots of native moon, compass and barrel jellyfish is normal in the summer. However, the arrival of more hollyhock stingers from the Mediterranean would be “a different story,” he says. “That could well be due to warming.”

Warmer waters can bring in entirely new species. A few sand tiger sharks (an “exceptionally rare visitor” to local waters) have turned up on UK beaches in recent years, and sea slugs with hair curlers. , It is generally found in the Caribbean and the Mediterranean, They were first recorded in 2025.

Some outsiders can cause problems for native animals. For example, scientists are observing a newly arrived sea squirt, called Didemnum pseudovexillum, closely. It is almost identical to another invader called the carpet sea squirt (Didemnum vexillum) that can suffocate the seabed, but it is not yet known whether this new sea squirt will cause similar damage.

Elsewhere in the world, extreme marine heatwaves have caused extensive loss of seagrass and seaweed in countries such as Australia and the United States. “Large areas have been completely wiped out and haven’t come back,” Jacobs says.

Chris Williams, founder of the volunteer-run Hastings Kelp Project, is concerned. The UK may have lost around 92% of its seagrass beds since the 1930s. It is not clear exactly how much seaweed has disappeared, but, in Sussex, it could be more than 96%.

And this is hidden under the sea. “You don’t see a degrading kelp forest from the ground like you might see a forest burning or dying due to temperatures,” he says.

Kelp prefers waters between 8 and 12°C, and even Williams’ controlled lab is struggling. “Due to the heat wave, our refrigerators in the laboratory have not been able to keep the tanks below 12°C,” he says.

Your algae is growing, but you think it’s because you can control other variables, such as lighting.

Seagrasses and algae provide vital habitats for many animals, protect coastlines from erosion and flooding, and store carbon, which is why Williams hopes to grow algae that can withstand warmer seas. The Seagrass Ocean Rescue project also aims to restore 15% of the UK’s seagrass meadows by 2030.

Warmer waters can also trigger harmful algal blooms, which can harbor nasty pathogens that make animals and people very sick. In the Bay of Biscay, authorities closed beaches to protect bathers. While the same thing could happen here, our infrastructure is not yet ready for such closures, Jacobs says. “We’re just not prepared,” he says.

The impact of repeated heatwaves on fisheries may take a couple of years to become evident because, even if adults survived the high temperatures, eggs and juveniles may have been affected. And experts don’t know whether the cold-water fish that moved north will return when the heat waves end.

“It could be that our cod and mackerel fisheries are over,” Jacobs says. But southern boats could pivot to catch warmer water species such as triggerfish, while Scotland benefits from cod moving across its range.

Marine heat waves are not new, but climate deterioration is causing them to strike with greater frequency and severity. “I’m starting to call it our new normal because we seem to get hit in successive years,” Jacobs says.

As greenhouse gases emitted from burning fossil fuels raise baseline temperatures, “the extreme is going to get even more extreme,” he says.