Record numbers of octopuses found off the south-west coast of England last year have now spread as far as Scotland and Wales and are transforming the fishing industry and the marine ecosystem, according to a study.

The surge in sightings of one of the world’s most intelligent invertebrates was first recorded in 2025 off the south coast of Devon and Cornwall.

A new study, based on scientific surveys, underwater monitoring and observations from recreational divers and snorkellers, has found octopuses have spread along the north coasts of Devon and Cornwall, with sightings as far afield as Wales, Dorset, East Sussex and Scotland.

“It is pretty extraordinary,” said Bryce Stewart, a senior researcher at the Marine Biological Association and lead author of the study. “We have had blooms before but everything I am seeing is telling me this is the biggest bloom we have seen, it is quite different.”

The common or Mediterranean octopus, Octopus vulgaris, is native to UK waters but ordinarily in such small numbers that it is rarely seen. A sudden increase in the population – a bloom – is caused by a combination of a mild winter followed by a warm breeding season in the spring, and researchers say the surging numbers in UK waters are likely to be linked to warming seas and wider changes in the marine environment.

“Now we have warmer waters much more suited to these animals, we are seeing a huge increase in numbers,” Stewart said.

The current bloom has sparked growing interest from the public, with hundreds of divers and snorkellers helping scientists to survey the burgeoning octopus population.

The bloom has been mixed news for fishers. Those relying on traditional shellfish have been badly hit as octopuses, which are highly effective predators, target crabs and lobsters often from fishers’ pots. However, others have cashed in with record hauls of octopuses.

Stewart said the octopus catch increased by 7,700% in 2025 and just last week at Brixham market in Devon, where most of the catch is sold, a record 100 tonnes of octopus was sold in one day.

“Some fishermen have had to sell their boats because of the impact on crab and lobster populations, while others are doing extremely well,” he said.

Stewart said the influx of octopuses was also upending the marine ecosystem. As well as preying on shellfish and some other fish, the octopus were providing food for seals, conga eels and the rare risso’s dolphins. “It is a shake-up of the whole ecosystem,” he said.