Gone fishing? One theory is the sea lions have gone to find food elsewhere

The famous sea lions of San Francisco's Fisherman's Wharf have disappeared after 20 years - leaving experts baffled as to why.

Last month, Pier 39 groaned under the weight of some 1,500 of the animals.

But now all but a few have swum off bringing to an end a two-decade long sojourn - and one of the world's smelliest tourist attractions.

The first bull sea lions arrived in late 1989, but the animals began to depart in November.

Officials hope the departure is only temporary - as a 20th anniversary party had been planned for 15 January.

Favourite food

The director of public relations for Pier 39, Sue Muzzin, told The Times newspaper: "With animals, you never know. If they are not back on January 15, we'll party with the ones who are here."

It may be a small party, as only 20 or so animals are left.

Marine experts suspect that the animals originally came to Fisherman's Wharf chasing their favourite food - sardines and anchovies.

And they suspect that is why they have gone.

"Most likely, they left chasing a food source," said Jeff Boehm, the executive director of the Marine Mammal Center in Sausalito.

"It's probably what kept them here in the first place," he added.

Pier 39 also provided a friendly home because it is out of the reach of their natural predators - great white sharks and orcas.

Although the hunt for food seems the likeliest reason for their disappearance, experts admit it is a mystery.

One more outlandish theory is that their departure is the sign of an imminent earthquake.

The animals arrived soon after the Loma Prieta quake of October 1989.