The fine summer weather has made St Ives a popular hot spot for residents and tourists alike, but the sighting of the blue shark means it could be a little less busy until it leaves the area.
Experts have said the animal – which was caught on camera by a couple of teenage workmates – has likely suffered an injury or lost its way while chasing prey.
Archie Pickin and Harry Hocking, both 16, who captured the footage, said the shark looked “really big”.
Harry told Cornwall Live: “It was swimming along for a good two to three minutes and went towards west pier before leaving the harbour.
“A lot of people noticed and by the time it left there were dozens of people watching from the harbour wall.
“People have already been saying they won’t be swimming now.”
Cornwall was named the shark capital of the UK in a study released on Tuesday, followed by the Scilly Isles and Devon, with at least 20 species found off the coast
Around 10 million small sharks and 100,000 larger ones – from 40 different species – are found in the seas around the country, with warmer seas tempting them to travel from the Mediterranean and the coast of Africa.
The research – carried out to mark National Geographic’s Shark Week – said great hammerheads, blacktips, sand tigers and bigeye threshers were among the species that could appear in the UK in the coming years as a result of the changing temperature.
John Richardson, of the Plymouth-based Shark Trust, told Cornwall Live that the presence of the blue shark – which normally inhabit deep waters in temperate and tropical oceans – in Cornwall was “a real privilege”.
“They are predominantly an oceanic, open water species, and not commonly found close to shore,” he said.
“But it is certainly not unprecedented to see one in such shallow waters. To see a free-swimming blue shark close to shore like this is a real privilege.
If this shark remains in St Ives harbour, the Shark Trust advises people to give it plenty of space until it moves back offshore to its usual habitat.
“Due to the confined nature of the harbour, which could hinder the shark’s ability to naturally move away from disturbance, the trust strongly advises people not to get into the water with the shark.”