The sardines apparently depleted the water of oxygen and suffocated after getting lost in the marina, officials said.
"All indications are it's a naturally occurring event," said Andrew Hughan, a California Fish and Game spokesman at the scene.
The die-off was unusual but not unprecedented.
"In the world of fishing this is an afternoon's catch," he noted.
Boaters awakened to find a carpet of small silvery fish surrounding their vessels, said Staci Gabrielli, marine coordinator for King Harbor Marina on the Los Angeles County coast.
Authorities said there was also a 12- to 18-inch layer of dead fish on the bottom of the marina.
The scale was impressive to locals at King Harbor, which shelters about 1,400 boats on south Santa Monica Bay. More news at: http://news.yahoo.com/s/ap/20110309/ap_on_fe_st/us_fish_die_off