The blubbery sea lions at Pier 39, one of San Francisco's smelliest and most famous tourist attractions, are gone. During the last week of November, they left the wooden docks on which they've spent the last 20 years and no one knows if they'll be coming back.

"We have no idea where they moved on to or why," said Shelbi Stoudt, who manages a team that helps stranded animals in the San Francisco Bay from the Marine Mammal Center in Sausalito, California.

The sea lions' disappearance is as strange as their initial colonization of the pier about 20 years ago, in late 1989. They just started showing up one day and as their numbers increased, their traditional hang out, Seal Rocks, became less populated. There are all sorts of theories about why the pier became a favorite haul-out spot for the sea lions, but no one knows for sure why the animals' behavior changed.

Stoudt said the officials at the Marine Mammal Center weren't worried about the animals' disappearance from their favored location. The sea lions are migratory animals, after all, and it's natural for them to move around.

So, even though no one has found them, "there really isn't a reason to be looking for them," Stoudt said.

The disappearance is unusual, though. The animals' numbers usually peak in late fall and many stick around during the winter months before heading south for the summer. According to the Marine Mammal Center's FAQ on the animals, "from late summer to late spring, 150 to 300 sea lions haul out here," though their numbers can run much higher.

This year saw a massive influx of sea lions. In fact, a Marine Mammal Center survey conducted in the fall found 1,585 mammals hauled out on the spot, an all-time high. Some of them invaded a neighboring area, the Hyde Street Pier, where they may have been scared away by an itinerant fisherman's dog.

Their disappearance drew the attention of San Franciscans like local blogger Gary Soup, who posted the photo above on Twitter of the deserted docks. The animals had become a major tourist and education locus on the otherwise highly commercial strip known as Fisherman's Wharf. The Marine Mammal Center sends docents to the area to answer questions about the creatures.

On the other hand, fishermen and others who work the waters of the Port of San Francisco have far less friendly relations with the animals. One recently told a local radio station, "They're cute when they're in here lying on the docks by Pier 39, but they're not too cute out in the ocean when they're stealing your livelihood."

It doesn't appear that local weather conditions could have influenced the animals. The weather in San Francisco has been normal, according to National Weather Service meteorologist Rick Canaepa. "It's pretty typical winter conditions," Canaepa said.

This is an El Niño year, but the local impacts of that warming of the Pacific have been moderate. "I don't know if that would be enough to make them change their minds and leave the area," he said.

The Mammal Center's Stoudt said they hadn't detected signs of something unusual going on with the fauna of the Bay, either.

While it's appealing to think that the animals may have just returned to their previous home at Seal Rocks, locals contacted by Wired.com didn't think there had been much of a change in the sea lion population there.

"Nothing unusual has happened," said Jennifer Valencia, who takes reservations at the Cliff House, which overlooks the Rocks.

So, for now, no one knows where they've gone or whether they'll ever head back to their perch amid the clam chowder shops and street performers.

Image Composite: Left: flickr/wallyg. Right: Twitter/@GarySoup.

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