Kurt Lander, skipper of the Brighton-based charter boat Yellowfin, thought he was seeing a person waving in the middle of the Channel shipping lane. Turns out it was a pair of sunfish flapping their fins - apparently a move to attract birds to eat parasites off them. Lander has seen these odd, round fish before, but this year they're part of a surge in sightings off British shores, likely thanks to a marine heatwave.
Marine heatwaves typically follow hot spells on land, and last year the Met Office called British waters 'exceptional,' with some areas experiencing heatwave conditions for three-quarters of the year. 2026 is already following suit, and the extraordinary temperatures are shifting marine life in ways beyond sunfish sightings. Some species are fleeing warmer waters, others - like 'invaders' - are showing up for the first time, and some marine life is simply dying off.
Lander, who used to catch a couple of hundred cod a year, now lands one or two annually in the summer. Cold-water species are vanishing from the southern coast, says Dr. Zoe Jacobs of the National Oceanography Centre in Southampton. They're shifting north to cooler seas, thriving in Scotland - temporarily or permanently, we don't know. Meanwhile, warm-water fish like bass, comber, and triggerfish are increasingly common, and sardines and anchovies are doing well too.
Diver Gio Reale, based in Falmouth, notes that small temperature changes have big effects. His favorite shark, the small spotted catshark, prefers 16-17C water, but this summer it's been about 20C every day. He thinks they've moved deeper, like they do in the Mediterranean. But common octopuses have arrived in force since January last year, when warmer waters in Devon and Cornwall created perfect conditions. They're native but rarely seen, and they're wreaking havoc on crab and lobster traps - 'Everyone's complaining, oh, they're taking all our crabs,' says Reale. 'But obviously, for the octopus, that's just a really easy meal.' Some boats have switched to catching octopuses, with catches soaring in 2026, and the octopuses provide food for predators like Risso's dolphins.
Warmer waters also bring jellyfish - a food source. Native moon, compass, and barrel jellyfish are normal, but more mauve stingers from the Mediterranean would be 'a different story,' says Jacobs, possibly due to warming. New species are appearing: smalltooth sand tiger sharks, an 'exceptionally rare visitor,' have washed up, and hair curler sea slugs, usually Caribbean, were first recorded in 2025. Some outsiders are concerning, like the newly arrived sea squirt Didemnum pseudovexillum, which may smother the seabed like its relative, the carpet sea squirt.
Globally, extreme marine heatwaves have wiped out seagrass and kelp in Australia and the US. In the UK, we've lost about 92% of seagrass meadows since the 1930s, and Sussex may have lost over 96% of its kelp. Chris Williams of the Hastings Kelp Project says, 'You don't see a degrading kelp forest from the land like you might see a woodland burning down.' Kelp prefers 8-12C, but the heatwave has overwhelmed his lab's chillers, keeping tanks above 12C. Seagrass and kelp are vital habitats, protect shorelines, and store carbon, so Williams is trying to grow heat-resistant kelp, and the Seagrass Ocean Rescue project aims to restore 15% of UK seagrass by 2030.
Warmer waters can also trigger harmful algal blooms with nasty pathogens. In the Bay of Biscay, beaches were closed, but the UK isn't ready for that, says Jacobs. 'We're just not ready.' The impact on fisheries may take years to show, as eggs and juveniles may have been affected even if adults survived. Cod and mackerel fisheries might be done, but southern vessels could pivot to triggerfish, while Scotland benefits from cod's northward shift.
Marine heatwaves aren't new, but climate breakdown is making them more frequent and severe. Jacobs calls it 'our new normal' because we're getting hit in successive years. As greenhouse gases push up baseline temperatures, 'the extreme is just going to be more extreme.'