Scientists were amazed to discover female sharks made such long-distance journeys because until now females were thought to stay within the region of the ocean where they were born.
Ramon Bonfil, of the Wildlife Conservation Society in New York, said that for 61 per cent of the journey, the shark swam within 16ft of the surface, indicating that it may have used visual cues to navigate. "I'm very excited about the possibility that she was using celestial cues such as the sun or the moon,' he said. "She travelled along a very straight track; she knew where she was going. It just blew my mind. Why else would a shark go to the surface in the middle of the ocean where there are no seals, penguins or other food? There is no reason unless she was trying to look at something."
Great whites may also be able to sense the direction of the earth's magnetic field, which they could be using as a compass for navigation, Dr Bonfil added.
The shark was tagged with a transmitter off South Africa in November 2003 and four months later, when the instrument was automatically released to float to the surface, Nicole was off the west coast of Australia. Data transmitted from the instrument to a satellite revealed her precise movements. The route is published in the journal Science. Five months later, on 20 August 2004, the scientists spotted Nicole back off the coast of South Africa, identified by the unique notches on her dorsal fin. Dr Bonfil said the shark, who is sexually immature, may have been making a dummy run to find a mate in Australia where there is a large breeding population of great whites. "There is a possibility she does this regularly, maybe to find a suitable mate off Australia then return home to give birth in South Africa," he added.
The shark performed the fastest transoceanic return migration recorded by marine biologists, travelling for nine months at an average of three miles an hour, the fastest sustained speed known among sharks, putting Nicole's trip on a par with fast-swimming tuna.
Sharks in general and great whites in particular are threatened by over-fishing and loss of habitats and the findings that they travel so widely suggest measures to protect them need to be global rather than local, Dr Bonfil said. "Our studies show the protection by a handful of countries - just five in total - is not enough if we want to protect this magnificent fish."
A second study in Science, by Barbara Block of the US National Marine Fisheries Service, revealed that salmon sharks also travel enormous distances, regularly migrating from Alaska to Hawaii. "Sharks are declining globally, yet the movements and habitats of most species are unknown," Dr Block said. Forty-eight salmon sharks were tracked from the sub-arctic with water temperatures of 2C to the tropics with temperatures of 24C. A specialised protein in their heart muscle allows them to live in very cold water which would interfere with the cardiac rhythm of other marine animals.Reuse content