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The seas surrounding the United Kingdom have warmed to unprecedented levels in 2025, sparking dramatic shifts in marine species and affecting the livelihoods of coastal fishing communities. Provisional data from the Met Office, analysed by the BBC, shows that from January to July this year, average sea surface temperatures were more than 0.2°C higher than in any other year since 1980.
Although the rise may appear modest, experts say it reflects a persistent long-term warming trend linked to human-driven climate change, largely caused by the burning of fossil fuels.
Newcomers to UK Waters
Warmer waters have paved the way for species rarely seen in UK seas to appear in growing numbers. Observations from scientists, divers, and recreational fishers point to an increase in octopus, bluefin tuna, mauve stinger jellyfish, and salps gelatinous, chain-forming organisms that glow faintly in the water.
Dr Bryce Stewart, a senior research fellow at the Marine Biological Association, likens these changes to “the canary in the coal mine,” warning that such species respond quickly to environmental shifts and are signalling deep changes in marine ecosystems.
Bluefin tuna, for instance, have been appearing off the south-west coast in larger numbers, aided by rising sea temperatures and improved fisheries management. Anglers now regularly spot schools of tuna feeding at the surface in dramatic frenzies. Off Cornwall, divers have reported swimming through vast blooms of salps — once considered rare in these waters.
Cold-Water Species Under Pressure
While some marine life is flourishing, others are struggling to survive in the warming seas. Cold-adapted species such as cod and wolf-fish are shifting northwards in search of cooler habitats, threatening long-established fisheries.
On the north Kent coast, common whelks, a vital cold-water shellfish for local fishers, suffered a mass die-off in 2022 during a prolonged marine heatwave. Ben Cooper, a fisherman from Whitstable, said the losses hit his livelihood hard, as whelks once made up around 75% of his earnings. Cod, which were abundant in the 1980s, have already retreated north due to sustained warming.
Marine Heatwaves Becoming Routine
Much of 2025 has seen marine heatwave conditions prolonged stretches of unusually warm sea temperatures around the UK. Buoys managed by the Centre for Environment, Fisheries and Aquaculture Science (Cefas) recorded exceptional readings in multiple locations. These conditions follow similarly high temperatures in 2023 and 2024.
According to Professor John Pinnegar of Cefas, the UK’s seas have been warming for more than a century, but extreme heat events that were once rare are now “very, very common.” While short-term weather patterns such as calm, sunny conditions can intensify surface warming, the dominant factor is the steady build-up of heat in the oceans due to greenhouse gas emissions.
The world’s oceans absorb about 90% of the excess heat generated by these emissions. Scientists warn that this makes marine heatwaves more frequent and intense, a trend expected to continue for decades without major reductions in global emissions.
Climate Ripple Effects
The impacts of warmer seas extend beyond marine biodiversity. Hotter ocean temperatures can weaken sea breezes that normally help cool the land, potentially intensifying heatwaves onshore. They can also hold less carbon dioxide, reducing the ocean’s role as a carbon sink and accelerating global warming.
Additionally, warmer seas may increase atmospheric moisture, raising the likelihood of heavier rainfall events, another challenge for the UK’s climate resilience.
Adjusting to a New Marine Landscape
For the UK’s fishing industry, these changes demand adaptation. Shifting species distributions may require targeting different catches, while consumers might need to adjust their preferences. Developing sustainable practices, diversifying target species, and investing in monitoring systems will be critical for resilience.
Marine experts emphasise that while some warming effects are unavoidable, limiting greenhouse gas emissions is essential to slowing the pace of change. Protecting marine habitats, improving fisheries management, and restoring degraded ecosystems can help safeguard biodiversity and livelihoods alike.
An Ocean in Transition
The record sea temperatures of 2025 are part of a continuing upward trend rather than an isolated event. The arrival of bluefin tuna and other warm-water species, alongside the decline of cold-water mainstays such as cod and whelks, shows that the UK’s marine environment is undergoing a profound transformation.
As the oceans continue to absorb heat from the atmosphere, the challenge will be balancing ecological health with economic survival ensuring that marine ecosystems remain vibrant while coastal communities can adapt and thrive in a warming world.
References:
https://www.bbc.com/news/articles/c05enyryqvmo
https://www.un.org/en/climatechange/science/climate-issues/ocean
https://www.fao.org/4/i2389e/i2389e.pdf
https://www.sciencedirect.com/science/article/pii/S0308597X17304682
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