SAN FRANCISCO — After an abrupt disappearance that left tourists disappointed and experts baffled, sea lions are slowly returning to San Francisco's Pier 39.

The animals come and go regularly, leaving in the fall to breed and birth their young in the Channel Islands.

Last fall, however, something unusual happened. After ballooning to a population of 1,700 animals layered two and three deep on the docks, the sea lions departed as if following orders, leaving behind only a few stragglers.

Marine experts believe the animals might have traveled 500 miles north, looking for fish and colder waters along the Oregon coast.

Now they are coming back to San Francisco for equally mysterious reasons — but not in their usual numbers. An estimated 80 or so animals were in the area last week — a drop from the typical 200 to 400 sea lions this time of year.

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