He added: "The most likely animal to confuse a great white with is a porbeagle shark. Chaynee wasn't able to confirm whether the shark she saw had a white spot on its dorsal finif it did it may have been a porbeagle. Then again, you don't get porbeagles measuring 12 feet."
Williams pointed to another observation which, if accurate, would almost certainly signify a great white. Chaynee reported that the shark was "spyhopping." This behavior, where the shark raises its head above water to see what's happening on the surface, is unique to great whites.
Others are more skeptical.
Ali Hood, conservation officer for the Shark Trust, a conservation charity also based in Plymouth, said: "Although the girl's notes were remarkably detailed for a 15-year-old, these are incredibly rare animals, so to see them in our waters would be remarkable."
However, for other shark researchers the key question isn't whether there are great whites swimming off the U.K., but why they have not been seen before.
"This is the big question mark hanging over marine biologists' heads," said Williams. "When they look at sea temperatures and the food out there, they can't see a reason why they're not here."
The United Nations Food and Agriculture Organization has recently backed this view, having changed its distribution map for the species. This suggests its range could extend as far north as Scotland.
Shark biologist Ian Fergusson, who says just a single specimen has been caught in the eastern North Atlantic in the past century, added: "The enigma is all the greater, given that a center of abundance and reproduction for white sharks is located in the adjoining Mediterranean Sea."
Williams suggests one possible reason why great whites are not reported in U.K. waters is that few people know what to look for. He says even anglers who fish specifically for sharks often confuse porbeagles with mako sharks and other species.
Another possibility is that dwindling great white populations in the Mediterranean and off northwest Africa mean there are few individuals left to explore waters further north.
Marine biologists blame these declines on reduced numbers of favored prey species. For instance, Mediterranean tuna stocks have been cut dramatically due to over-fishing.
But if great whites are now prowling the English coastline, they follow in the wake of many other species that have headed north.
Seen by many as harbingers of climate change, octopus, sea bream, sunfish, turtles, and other marine creatures associated with warmer waters are turning up with increasing regularity. In 2001 one angler even caught a barracudaa razor-toothed predator usually found in tropical seas.
Although great whites often inhabit relatively cool waters, their presence around the U.K. could also be a response to climate change if their food is luring them to the North Atlantic.
And if they do start to turn up in numbers, the U.K.'s burgeoning seal population should give them good reason to stay. Already there have been reports of seals being attacked by large sharks, including one from a Cornish fisherman who claims to have seen a seal being bitten in half.
Despite such reports, Fergusson says he has yet to see any compelling evidence that great whites have arrived. But he added: "The possibility of these wide-ranging, globally distributed sharks sporadically roaming to U.K. shores as vagrants cannot be discounted, and we anticipate such records might well occur in the future."
While this may not be enough to stop nervous, seawards glances on England's beaches this month, it's worth remembering the great white's man-eating reputation rests largely with the bloodcurdling creature depicted in the Spielberg movie Jaws.
The reality is that people are at low risk of being eaten by a shark. Humans are thought to be rather too bony for a great white's taste.
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