New tagging technology is enabling scientists to follow for the first time the migration paths of the largest marine animals - fish, mammals and reptiles - for thousands of miles across the open oceans.

It turns out creatures such as tuna and turtles tend to follow "marine highways" where currents create the best feeding and swimming conditions.

Results of the tagging show it would be possible to protect endangered and over-exploited species by setting up a new type of open ocean reserve varying as sea conditions change, marine biologists told the annual meeting of the American Association for the Advancement of Science yesterday.

Reserves on land and along coasts are defined by specific geographical features, said Larry Crowder of Duke University Marine Laboratory in North Carolina. "In a terrestrial system you can just draw a boundary around an important place and you have a park. In the open ocean it's different - the habitats are three-dimensional and moving."

But over the past few years, radio tags about the size of a small mobile phone have been attached to several thousand animals, including tuna, turtles, sharks and seals. These transmit each animal's position by satellite.

The scientists use the data to map migratory highways and feeding hotspots. Andrew Read, also of Duke University, said examples included the edge of the Gulf Stream across the North Atlantic and the Sub- Antarctic Convergence in the Southern Ocean.

Biologists told the AAAS meeting that satellite data, showing the position of tagged marine wildlife, combined with oceanographic measurements such as sea temperature and the flow of currents, would soon make it feasible to define transient marine reserves thousands of miles from land.

Here, rare species would be protected. "The United Nations has the authority to set up marine conservation zones in the high seas," said Elliott Norse of the Marine Conservation Biology Institute in Washington.

One of the biggest tracking programmes is for bluefin tuna, the world's most valuable fish. A giant tuna recently fetched $173,000 (€135,839, £91,889) in Tokyo's fish market.

"Doing surgery [to implant a tag] on a 500lb tuna on the deck of a rolling vessel in the middle of winter is as hard as it gets," said Barbara Block of Stanford University, a leader of the project. "But when you see a track from three years of a bluefin's life, moving back and forth across an ocean, it makes all the hard work worthwhile."

Satellite tagging of 800 Atlantic bluefin has revealed their migratory corridors. "We are recognising the tunas we see off the coast of the Carolinas in the winter might be at the Flemish Cape off Canada by spring and all the way to the Mediterranean by summer," said Prof Block. "And then they'll make the journey back by the end of that year to the western Atlantic."