Warming seas are pushing native fish species north while tropical visitors like sunfish and octopuses surge off UK coasts, threatening fisheries and marine habitats.
By Adam Makins · 
Kurt Lander thought he spotted someone waving in the English Channel. Through his binoculars, the skipper of the Yellowfin charter boat realized it was something far stranger: a pair of enormous sunfish lying on their sides, flapping their fins. The fish were putting on a show, apparently hoping seabirds would pick parasites off their bodies. While sunfish have always existed in British waters, this summer brought a surge in sightings, part of a much larger transformation rippling through our seas.
The culprit is becoming familiar to marine scientists: marine heatwaves. Last year was labeled “exceptional” by the Met Office, with some British waters experiencing heatwave conditions for three-quarters of the year. This year is shaping up to be more of the same. These aren’t just about slightly warmer water. They’re triggering seismic shifts in which species thrive, which vanish, and which arrive for the first time in living memory.
Lander has watched his catch transform dramatically. “I used to catch at least a couple of hundred cod a year,” he says. Now, bringing in cod is rare. On a good day in summer, he might land one or two. Colder-water species like cod and mackerel are vanishing from southern UK coastlines, according to Dr Zoe Jacobs, a senior researcher at the National Oceanography Centre. They’re not extinct; they’re migrating north to Scotland, seeking the cooler waters they need to survive. Whether that shift is temporary or permanent, nobody knows yet.
In their place, warm-water species are booming. Bass, comber, and triggerfish now fill nets where cod once dominated. Sardines and anchovies, according to last year’s State of the South-West Seas report, are thriving. The species composition of British fisheries is being rewritten in real time, and the economic implications haven’t fully landed yet.
A diver named Gio Reale, based in Falmouth, Cornwall, says small temperature changes have “huge effects on marine life.” His favorite sharks, the small spotted catsharks (also called lesser-spotted dogfish), vanished weeks ago. These delicate, freckled sharks prefer water temperatures around 16-17 degrees Celsius. This summer, water temperatures have hovered near 20 degrees. He suspects they’ve retreated to deeper, cooler waters.
But others have moved in. Common octopuses, rarely seen in UK waters historically, have arrived in force since January last year, when Devon and Cornwall waters warmed perfectly for them. Crab and lobster fishers have complained bitterly as octopuses raid their traps, but this abundance has created a new fishery. Catches soared in 2026.
The real crisis, however, lurks beneath the surface where nobody can see it. Seagrass and kelp forests are collapsing under heat stress. The UK has lost approximately 92 percent of its seagrass meadows since the 1930s. In Sussex, kelp loss may exceed 96 percent. These aren’t just pretty underwater landscapes. They’re vital habitats, carbon stores, and coastal protections.
Chris Williams, founder of the Hastings Kelp Project, is struggling to keep laboratory tanks cool enough for kelp survival. Kelp thrives at 8-12 degrees Celsius, but even with commercial chillers running constantly, his lab tanks can’t stay below 12 degrees during heatwaves. “You don’t see a degrading kelp forest from the land like you might see a woodland burning down,” Williams says, highlighting how invisible this marine collapse truly is.
The ecological consequences of these shifts won’t fully emerge for years. Young fish and juveniles may have died during peak heatwave temperatures, meaning fishery impacts could arrive on a delayed timeline. Scientists also can’t predict whether cold-water species will return once heatwaves subside, or whether the baseline temperature shift has permanently altered British marine ecosystems.
Dr Jacobs offers a sobering perspective: “It might be that our cod and mackerel fisheries are done.” Meanwhile, marine heatwaves are arriving “in successive years,” she adds. “I’m starting to call it our new normal.” As greenhouse gas emissions push baseline ocean temperatures ever higher, the extreme just keeps becoming more extreme.
Source: BBC Science Focus
If fish can’t return home when the heat breaks, what does that mean for the communities built around them?
