Blue whales – thought to be the largest animals ever to have inhabited the Earth – have returned to the seas off Canada and Alaska for the first time since hunting them ceased more than four decades ago. New research suggests that they appear to have rediscovered an old migration route that they abandoned at the height of the slaughter.

The research, by US and government scientists and a private research institute focussing on marine mammals, comes as whales face their greatest ever danger in over 20 years, as key governments threaten to breach the international moratorium on commercial whaling.

It has so far spotted 15 of the blue whales, which can weigh up to 200 tons, in the Gulf of Alaska and off British Columbia, and identified four of them as having been previously seen off southern California. Long ago, before commercial whaling began, they used to migrate between the two areas, heading north in summer in search of food – they can each consume four tons of tiny crustacean krill a day. But they were hunted close to extinction, with their numbers reduced from some 200,000 world wide to between 5,000 and 12,000.

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When their slaughter was banned in 1965, the seas off southern California were one of their last remaining strongholds, and there are now some 2,000 there, forming the largest population in the world. Environmentalists hope that the more northerly sightings suggest they are recovering so well that they are able to spread into new areas.

However, some scientists fear that it could just as easily be a bad sign: global warming, they say, could be causing their prey to shift northwards in search of cooler waters, forcing the whales to follow. Others suggest that so many of them may have been killed during the decades of slaughter that they simply lost the collective memory of the feeding grounds off Alaska and Canada.

Key nations are preparing a deal with Japan, the main whaling nation, which could open the way to resuming commercial whaling since a moratorium on all species of the leviathans was introduced in 1986. Japan has continued the hunt off Antarctica by exploiting a loophole that allows whales to be killed for "scientific research".

The deal – which, as first reported in The Independent on Sunday has been thrashed out at unpublicised meetings behind closed doors – is expected to be unveiled at the annual meeting of the International Whaling Commission in Portugal next month. It would allow Japan to hunt minke whales near its coasts and South Korea has already intimated that, if it goes ahead, it will require similar treatment and conservationist fear that the floodgates will open.

Mark Simmons, of the Whale and Dolphin Conservation Society says the threat of new whaling is "dreadful and terrifying" and "fulfils our worst expectations".