Marine Heatwave Reshapes Britain’s Coastal Ecosystems

A prolonged marine heatwave this summer has pushed warm-water species such as sunfish, bass, comber and triggerfish into southern English Channel waters, while cold-water fish like cod have become rare. Skipper Kurt Lander of the Yellowfin charter boat reports catching only one or two cod per summer, compared with “a couple of hundred” in previous years. At the same time, octopuses have proliferated, prompting some fishers to target them instead of traditional crab and lobster catches.

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The Met Office labelled the previous year “exceptional,” with heat-wave conditions persisting for three-quarters of the year in parts of British waters. Scientists note that such marine heatwaves follow extended periods of hot weather on land, and their frequency is increasing as greenhouse-gas emissions raise baseline ocean temperatures.

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• Seagrass loss: Dr Zoe Jacobs of the National Oceanography Centre estimates the UK may have lost about 92 % of its seagrass meadows since the 1930s.

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• Kelp decline: In Sussex, kelp cover could have fallen by more than 96 %, though exact figures are uncertain.

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• Species displacement: Small spotted catsharks, which prefer 16-17 °C water, have largely disappeared from shallow southern waters, likely moving to deeper, cooler zones.

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• New arrivals: Species such as the Mediterranean mauve-stinger jellyfish, small-tooth sand tiger sharks, and Caribbean-origin sea slugs have been recorded for the first time in recent years.

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• Habitat threats: The invasive sea squirt *Didemnum pseudovexillum* resembles the harmful carpet sea squirt, raising concerns about potential seabed smothering.

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Jacobs warns that the loss of seagrass and kelp removes critical habitat, shoreline protection, and carbon storage, and she emphasizes that the UK is “just not ready” for heat-related beach closures. Chris Williams, founder of the volunteer-run Hastings Kelp Project, notes laboratory chillers can no longer maintain the 8-12 °C temperatures kelp requires, underscoring the difficulty of cultivating resilient kelp strains. Both scientists describe the situation as a possible end to traditional cod and mackerel fisheries in southern waters.

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• “Everyone’s complaining, ‘oh, they’re taking all our crabs’,” — Gio Reale, a diver based in the Cornish port of Falmouth

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• “Huge areas have just been completely wiped out, and they’ve not returned,” — Jacobs. Photograph

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• “It might be that our cod and mackerel fisheries are done,” — Jacobs, a senior researcher at the National Oceanography Centre in Southampton

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• “I’m starting to call it our new normal because we seem to be getting hit by them in successive years,” — Jacobs. As

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