Members of the Mounts Bay Pilot Gig Rowing Club said they saw a 7ft long blue shark swim under their boat as they practised near Penzance on Wednesday.
Richard Peirce, chairman of the Plymouth-based Shark Trust, said blue sharks were commonly found in British waters. He added: "Blue sharks have been here for millions of years. They are summer visitors. Sadly they are becoming unusual. The rate at which we are killing them off means that sooner or later we won't be seeing them."
The rowers said they were about 500 yards off the beach when they saw the creature, but Mr Peirce said this meant it was unlikely to have been a blue shark. "Generally speaking you wouldn't see a blue shark unless you were six miles plus - and more commonly seven or eight miles - offshore, he said. "It would be rare for a blue shark to be that close to shore. Porbeagles are far more commonly found inshore than blue sharks."
In the 1960s the Shark Angling Club in Looe, Cornwall, used to catch up to 6,000 blue sharks a year, but this year they only caught just over 100, Mr Peirce said. Blue sharks had been known to attack humans, but never in British waters.
There has been a series of suspected but unconfirmed sightings of potentially dangerous sharks off the coast of Cornwall this summer. A surfer at Sennen Cove, near Land's End, reported seeing a bull shark last month and there were reports of possible mako sightings off the north coast at Bude and Hayle.
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