Octopus Invasion In English Channel Shows Risks From Warming Seas

By Joe Wertz

May 30, 2025 (Bloomberg) –Some English fishermen have hit the jackpot this season, hauling in tons of octopus, but the invasion of the eight-legged molluscs also highlights the threat from marine heat waves.

Persistent high-pressure weather patterns this spring accelerated the heating of an unusually warm North Atlantic, blocking westerly winds and smothering currents that stir up colder water layers deep in the ocean, said Paul Moore, a climatologist with Ireland’s Met Éireann. 

Sea surface temperatures west of Ireland soared 4C above normal in May, with waters near the UK rising by as much as 2.5C to the highest on record, according to the country’s Met Office.

Those warmer waters have suited the octopus, but when one part of the natural ecosystem changes, there can be knock-on effects. The surge in octopus has come at the expense of the shellfish populations they prey on. For now, Neil Watson, whose firm operates out of Brixham Fish Market on the English Channel, has been one of the winners. 

“They’re pulling up the crab pots and it’s just a happy octopus and a load of shells where he’s had his lunch,” said Watson, after his boats unloaded 48 tons of octopus on May 27, a 240-fold jump on the previous year. “I’ve never seen what I’m seeing today.”

Other beneficiaries of the warm seas include jellyfish and sea bass, but it’s hurting cod — a traditional staple of British fish and chips. And crucially, it’s also hitting plankton, the foundation of marine food chains, said Georg Engelhard, a senior marine scientist at the UK’s Centre for Environment, Fisheries and Aquaculture Science.

A recent shift in weather has brought more low-pressure systems and unsettled conditions, lowering ocean temperatures from their highs. But the sea remains warmer than the norm, increasing the chances that the marine heat wave will worsen toward a summer peak in August, said Met Éireann’s Moore.

“It won’t take much to push it up again over the next few months,” he said.

Further high-pressure systems — which brought onshore temperature records, drought and slumping wind-power generation earlier this year — are forecast over the summer. The heating from those short-term weather patterns is occurring on top of global warming, which is raising ocean temperatures and intensifying marine heat waves.

While Brixham Trawling Agents is enjoying the octopus boom — getting good prices from major European seafood buyers — Watson is concerned about the long-term damage to fisheries.

Warmer seas are changing migration patterns and ensuring higher survival rates among octopus eggs and larvae, said Engelhard. But the plummeting shellfish numbers show the uneven impact on the ocean.

“It’s just quite unprecedented,” he said.

The threat posed by marine heat waves goes beyond winners and losers among ocean species. It can trigger algae blooms that starve waters of oxygen, creating “dead zones” with mass fish die-offs. It can also boost toxins and other pathogens, which can sicken humans too.

“That can impact the beaches, whether people go swimming or because it affects things like mussels and oysters that we would like to eat,” Engelhard said.

© 2025 Bloomberg L.P.

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