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Spokeswoman Ali Hood said the sharks should be seen as wild animals and are due to be “treated with respect”.
However, she added: “If this shark remains present in St Ives harbour, due to the restricted nature of the harbour, the trust strongly advises people not to get into the water with the shark.”
Possible reasons for the shark coming so close to the shore include injury or illness, or perhaps disorientation after following prey inshore.
”Blue Sharks are predominantly an oceanic, open water species, and not usually found closely connected to shore, ” said Ms Hood.
“That said, it is certainly not unprecedented to watch one in such shallow waters with similar instances recorded here in the UK and overseas from time to time.”
Blue shark attacks are rare and they are nomadic beings, only appearing in British waters during the summer.
Research from the University of Southampton suggests that new kinds of shark could migrate to UK waters as the oceans warm.
Dr Ken Collins, from the university’s National Oceanography Centre, said 10 species of shark found in warmer regions of the world could occupy UK water by 2050 because of climate change.
Blue sharks 😛 TAGEND They live in temperate, tropical and subtropical water up to 1,150 ft( 350 m) deep They are nomadic and follow a clockwise migration pattern in the ocean from the Caribbean following the Gulf Stream Their number of offspring scopes from about 25 to 50 Twelve unprovoked assaults and four boat assaults have been documented by the International Shark Attack File