Repeated marine heatwaves are changing what lives in and around Britain’s seas. Cold-water species such as cod are becoming scarce off southern England, while warm-water fish, octopuses and occasional visitors from farther south are appearing more often.
Last year was described as “exceptional” by the Met Office, with some British waters experiencing heatwave conditions for three-quarters of the year. Marine heatwave conditions are continuing into 2026, according to the source report, with effects extending from fisheries and diving to coastal habitats and public health.
Fish are moving as water temperatures rise
Kurt Lander, skipper of the Yellowfin charter boat in Brighton, said he used to catch at least a couple of hundred cod a year. In summer 2026, he said, catching one or two was more typical. Dr Zoe Jacobs, a senior researcher at the National Oceanography Centre in Southampton, said cold-water species were no longer commonly seen off the southern coastline and were thriving farther north in Scotland. It is not yet known whether that shift is temporary or permanent.
Lander is now catching more warm-water fish, including bass, comber and triggerfish. Sardines and anchovies were also reported to be doing well in last year’s State of the South-West Seas report. Southern fishing vessels may be able to target these species, while Scotland could benefit from cod moving north, but the longer-term effects on fisheries may take years to emerge because heat can affect eggs and juveniles as well as adult fish.
Sunfish, octopuses and southern visitors are more visible
Sunfish sightings have surged off British shores this summer. Lander saw two in the English Channel, where they were lying on their sides and flapping their fins, apparently allowing seabirds to remove parasites. Warmer waters have also been linked with an increase in common octopuses in Devon and Cornwall since January last year. Their abundance has caused problems for some crab and lobster fishers because octopuses can enter traps and eat the trapped crabs; some boats have switched to catching octopuses instead.
The octopuses may also be providing food for predators, potentially helping explain more sightings of summer visitors such as Risso’s dolphins. Warmer water can bring jellyfish blooms, although sightings are difficult to interpret because more people may be at the coast in summer. Native moon, compass and barrel jellyfish are normal seasonal sightings; more mauve stingers from the Mediterranean would be a stronger indication of warming-related change.
Other unusual arrivals include a few smalltooth sand tiger sharks, described as exceptionally rare visitors to local waters, and hair curler sea slugs, first recorded in Britain in 2025. Scientists are monitoring the newly arrived sea squirt Didemnum pseudovexillum because it resembles the invasive carpet sea squirt, Didemnum vexillum, which can smother the seabed. Its potential impact is not yet known.
Kelp, seagrass and coastal health face pressure
Some species are disappearing from familiar areas rather than arriving. Cornish diver Gio Reale said he had not seen a small spotted catshark for weeks during a summer when water temperatures were about 20C; the species generally prefers 16-17C and may have moved into deeper, cooler water. Elsewhere, extreme marine heatwaves have caused extensive losses of seagrass and kelp, including in Australia and the United States.
Chris Williams of the volunteer-run Hastings Kelp Project said laboratory chillers had struggled to keep tanks below 12C, even though kelp prefers roughly 8-12C. The source report says Britain may have lost about 92% of its seagrass meadows since the 1930s; the scale of kelp loss is less certain but could exceed 96% in Sussex. These habitats support marine life, protect shorelines from erosion and flooding, and store carbon. Seagrass Ocean Rescue aims to restore 15% of UK seagrass meadows by 2030.
Marine heatwaves can also trigger harmful algal blooms containing pathogens that make people and animals seriously ill. Beaches were closed in the Bay of Biscay during such an event, but Jacobs said UK infrastructure was not yet prepared for similar closures. She also said repeated heatwaves are becoming a “new normal” as greenhouse-gas emissions raise baseline temperatures, making extreme conditions more severe.
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