Two exceptionally large Arctic sharks ply northern waters -- Greenland sharks and the Pacific sleeper sharks. During the last few years, scientists have documented Greenland sharks using the St. Lawrence Seaway, lending further credence to the hypothesis that some sharks can survive in freshwater. Bull sharks are also known to swim in fresh water, but this species needs warmer waters.
The idea of sharks possibly using Loch Ness is not new; that's long been one of the hypotheses explaining the Loch Ness Monster. But until now, nobody has suggested sleeper sharks, perhaps because they're secretive and so rarely seen.