San Francisco ? Last month, marine scientists counted more than 1,500 sea lions on fabled Pier 39, a record number that delighted tourists and baffled experts.

Why so many? Why were they sticking around?

But now, almost all of the sea lions are gone, leaving the experts guessing where they went — and why.

“Most likely, they left chasing a food source,” said Jeff Boehm, executive director of the Marine Mammal Center in Sausalito, which runs an information center and gift shop at Pier 39. “It’s probably what kept them here in the first place.”

On Tuesday, 10 sea lions lounged and swam and dove from the docks, spreading themselves out where the animals were stacked three and four deep just a month ago. The bulk of the herd probably followed their favorite foods, sardines and anchovies, Boehm said.

The animals began leaving in droves the day after Thanksgiving, almost as if someone had issued an order. But Boehm said the fact that so many sea lions stayed for so long is even stranger than their disappearance.

“They do move off,” Boehm said, adding that in the fall, older sea lions head to breeding colonies in the Channel Islands, off the coast of Southern California along the Santa Barbara Channel. Younger sea lions, he said, “don’t mind those rules and tend to travel far and wide.”