The epic voyage of Terry the bluefin tuna - the equivalent to circumnavigating the Earth - has been witnessed by scientists using electronic tags to track fish.

A few months ago a great white shark named Nicole completed the first known ocean crossing by a lone shark over a distance of more than 12,500 miles from South Africa to Australia and back in nine months, the fastest known return journey.

Now another epic migration, this time of a 200lb bluefin tuna (Thunnus orientalis) nicknamed Terry, has been followed by marine scientists tagging fish to reveal marine highways and to help to preserve endangered stocks.

The tag broadcast Terry's trans-Pacific wanderings - three crossings in 20 months, a distance of 25,000 miles. Why the fish did this is a mystery.

Prof Ron O'Dor, from Dalhousie University in Nova Scotia, one of the scientists working on the project, said: "I don't know anything that approaches this when it comes to fish."

The coastal migrations of fish and other animals are among the highlights of the 10-year global census of marine life, now at its 2005 mid-point, in which 1,700 experts from 73 countries are compiling the marine equivalent of the Domesday Book.

The Topp (Tagging of Pacific Pelagics) project has discovered that bluefin tuna once believed to live separate lives in Japanese and Californian waters may belong to a single population. "Before, this tuna would have been counted twice and we would have thought there are twice as many tuna as there are," said Prof O'Dor.

This has important implications for managing fish stocks, he said, adding that similar findings have also been made by Prof Barbara Block of Stanford University in the North Atlantic where the two separate groups are also thought to intermingle when migrating.

The tags track individual fish as they travel thousands of miles across the sea, to depths below 3,000 ft, in search of food and mates.

Each tag records the tuna's migration pattern, diving behaviour, body temperature and the temperature of the surrounding water.

Census scientists have increased by more than 50 per cent from 2004 the number of reporting devices on large open ocean animals that typically venture from the shallow shelves into the deep Pacific.

Today, 1,838 animals of 21 species, including sharks, bony fishes, birds, turtles, seals and sea lions, carry tags that, each time they surface, call information into scientists via satellite. Tagging of baleen whales show that they use the mid-ocean ridge as a feeding area and north-south migration route in the North Atlantic.

The Topp project also discovered that many salmon sharks (Lamna ditropsis) from Alaska have an attraction to warmer winter destinations and frequently migrate to destinations such as Hawaii. Topp's tags have also allowed marine turtle researchers to determine how much energy a leatherback turtle uses at sea.

James Baker, the president of the Academy of Natural Sciences of Philadelphia, said: "The new data reveal for the first time those zones of the ocean where we have the highest leverage for conservation and thus smarter fishing."

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