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Scientists in the US believe they may have solved the riddle of San Francisco's vanishing sea lions.

The Californian city's famous colony of sea lions all but disappeared over the past month, baffling experts.

But now large numbers of the animals have been spotted further north, off the coast of Oregon.

Scientists say the animals have probably migrated in search of food during the winter, although in unusually high numbers.

The sea lions of San Francisco are almost as famous as the city's cable cars or even the Golden Gate bridge, says the BBC's Peter Bowes in Los Angeles.

Twenty years ago, for no apparent reason, the smelly, noisy animals took up residence in the docks at Pier 39.

Their numbers grew rapidly to about 1,700 animals, and they became a popular tourist attraction.

But then most of them disappeared.

Initially, marine experts were baffled.

One outlandish suggestion was that they were fleeing the bay because of an imminent earthquake, our correspondent says.

But now the sighting of large numbers of sea lions off the coast of Oregon may have solved the mystery.

Scientists say it is normal for the animals to move north in search of food during the winter but it is extremely unusual for them to migrate in such huge numbers.