A Patagonian toothfish, a species thought to be exclusive to Antarctica, has been captured off Greenland in the Arctic circle - suggesting the cold-loving deepwater fish may migrate between polar regions.

In this week's issue of the journal Nature, Dr Peter Rask Møller and colleagues at the University of Copenhagen in Denmark, report the capture of a large specimen of the fish by a commercial vessel in November 2000 in waters off Greenland.

The ship's captain kept the strange fish in the freezer while he tried to find someone who could identify it.

"I couldn't believe it," Møller told ABC Science Online. "It thought, this is a real scoop." Møller, a specialist on polar fish evolution, is currently on a research visit to the Australian Museum in Sydney.

The Patagonian toothfish is known to migrate around the Antarctic, and has previously been found as far north as the southern Atlantic Ocean off Uruguay, 10,000 km from its usual home.

The fact that a specimen has been found in the northern polar region is a puzzle, since the fish cannot live in waters warmer than 11°C. So how did it get there through the warm equatorial waters?

There are a number of fish species found in both north and south polar regions, and two main theories to explain this, said Møller. One is that there was originally one big population of fish which separated as the tropical waters warmed; another that fish migrated from one polar area to the other.

However, so far there has been no evidence to support either theory, Møller said. The fact that the species is unknown in waters around Greenland, despite intensive fishing in the area, strongly suggests that the fish has migrated from the south, he said.

Considering that the species lives in deep cold waters suggests that it might have dodged the warmer tropical waters by using the cool currents that lurk 1,000 metres or so below the surface.

Although Møller admits the discovery is not complete proof of migration - for that one would need to tag and track fish - it's the best proof yet that fish travel these kinds of vast distances, he said.

The specimen caught was 1.8 metres long and weighed 70 kg, indicating it was quite old and is estimated to have swum at least 10,000 km.

"We think it's a pretty old, but also quite a strong and fit specimen, Møller told ABC Science Online. "It's often like that in a population: it's either the very weak or the very strong that do strange things like migrating to explore new territories."

The Patagonian toothfish is a prized catch by trawlers, and the fisheries around Antarctica are protected by catch limits enforced by members of the Antarctic Treaty. However, poachers using factory ships often raid the region and catch the fish illegally, imperilling the species.

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