It is one of many ocean mysteries: where do basking sharks - the biggest fish in UK waters - go in the winter? A basking shark has been tracked with a satellite tag and was found to travel farther and deeper than known before, swimming more than 5,903 miles from British waters to the Canadian coast and diving more than half a mile down.

Research scientists are not sure why the 26ft-long female made the incredible trip, speculating she could have been looking for a mate, for a better food supply, or returning 'home' after a summer in British waters. Another option is that she was on a previously undiscovered migration route, prompting debate among some experts that basking sharks in Britain, North America, southern Africa and as far afield as New Zealand could be part of one big family. 'We're asking the questions - these are just some ideas we're thinking about why she chose to do what she did,' said Mauvis Gore of Save Our Seas Foundation, one of the team who took part in the project.

Basking sharks are the second biggest fish in the world, growing up to 39ft long and reported in shoals of up to a few hundred at a time. They are a common sight in British waters in summer and are thought to be one possible source for the myth of giant sea serpents, because when males follow a female up to nine snouts and fins rise out of the water in a line.

But until 10 years ago experts had no idea where they went in winter. Recent tracking suggested many dive deeper into the ocean or move a little way off shore on to the continental shelf.

For this study two sharks were fitted with satellite tags last summer and tracked for 100 days. One shark got no farther than the west coast of Scotland; the other, shark A, stayed near Cornwall for a short time, then headed to Newfoundland, where it dropped its tag.

The journey was more than three times the longest recorded by the species, something researchers think was possible because she was big enough to survive with little food. It is almost certainly not a one-off, though, said Jackie Hall from Manx Wildlife Trust and Manx Basking Shark Watch on the Isle of Man, and another member of the project team. 'We just didn't know about it before,' said Hall.

The discovery has raised hopes, too, that protection for the world's estimated 8,000 remaining basking sharks will be raised from national to international level.

'They are one of the great wonders of the British Isles,' said Douglas Herdson, of the National Marine Aquarium in Plymouth. 'It's great we're finding out about them, and the more we know the more we can conserve them.'

Long-distance lives

· Bumblebees have been discovered on Mount Everest, more than 5,600 metres above sea level.

· This month a polar bear made a 900-mile trek to become the first to reach Iceland in 20 years.

· In 2007 Wallace the whimbrel, a type of large wading bird, was recorded by satellite technology as covering at least 4,300 miles after leaving a reserve in Yorkshire.

· The monarch butterfly winters in Mexico and flies north as far as Canada to breed, from where successive generations then fly south again.

Caroline White