BLUE whales, the world’s largest animals, are reappearing in parts of the oceans where hunting once wiped them out, signalling that they may finally be returning from the brink of extinction.

Marine scientists have recorded the animals roaming migratory routes and feeding grounds in the Pacific from which they had vanished for much of the past six decades.

Research also suggests that the Antarctic population of blue whales may now be growing at 6% a year. In the Atlantic, sightings are also increasing.

“The overall numbers are still tiny compared with the original populations before whaling started, but the trend is at last in the right direction,” said John Calambokidis, a marine scientist whose research on whale movements and populations has just been published in the journal Marine Mammal Science.

Blue whales, which can exceed 100ft in length and weigh up to 200 tons, were once common, with an estimated global population of between 350,000 and 400,000 in 1900.

Previously, they had not been targeted by whalers because they were too large and fast for the ships and harpoons available.

After the invention of steam-powered whaling ships and exploding harpoons, fleets came to favour blue whales because a single animal could provide 120 barrels of valuable oil along with vast amounts of meat. The animals’ tongues alone can weigh as much as an elephant.

By the 1960s, when blue whale hunting was banned, there were only around 5,000 animals left.

For most of the past five decades since then, blue whale numbers have hardly changed.

This has baffled most researchers, because other species such as humpbacks saw populations surge once they were protected. Some researchers feared blue whales might become extinct.

One problem was that the remaining blue whales seemed to have split into separate populations whose numbers risked being too small to be viable. In the Pacific, these included one group in the Gulf of Alaska and another off California.

Calambokidis tracked these groups using photo-identification to spot individual blue whales from fin shapes and other markings. In recent years, however, he was surprised to see the populations growing and mingling.

“This may represent a return to a migration pattern that existed in earlier periods for the eastern north Pacific blue whale population,” he said.

One reason for the increase in sightings could be growing competition for food on existing routes, driving whales further afield. Alternatively, changes in ocean currents may have shifted the concentrations of krill, the tiny shrimp-like animals on which blue whales feed.

Either change would force the animals to start moving around more, recolonising the same migratory routes and feeding grounds favoured by earlier generations.

Other researchers have recorded similar trends. Richard Sears, founder of the Mingan Island Cetacean Study in Canada, who studies blue whale populations in the north Atlantic, said sightings there had risen in the past few years.

About 200 animals have been recorded in the eastern Atlantic and 440 in the west, including large numbers off Iceland. These are likely to be just a fraction of the total.

Sears is cautiously optimistic, but warns that the increase in sightings may be partly due to more people looking for whales. “There is still no room for complacency,” he said.

There may be a more sinister reason for the failure of whale numbers to recover after the ban was imposed. Files handed to the International Whaling Commission by Alexey Yablokov, environmental adviser to Boris Yeltsin, the Russian president from 1991-9, showed that the Soviet Union illicitly killed more than 9,000 blue whales from the time of the ban until 1972.

Dan Bortolotti, author of Wild Blue, a new book looking at blue whale populations worldwide, said: “These revelations go some way towards explaining why blue whale populations stayed low for so long.

“It also suggests that they may now have a chance to recover — but only if the ban on hunting all large whales stays in place.”

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