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.
California Fish and Game officials believe the fish are anchovies and sardines.
Experts had yet to determine what happened, but Gabrielli said the fish appeared to have moved into the harbor to escape a red tide, a naturally occurring bloom of toxic algae that can poison fish or starve them of oxygen.
High winds overnight might then have trapped the fish in the harbor, crushing them against a wall where they used up the oxygen and suffocated, she said.
The dead fish were so thick in some places that Garbrielli said boats can't get out of the harbor.
Fish and Game authorities arrived and began taking samples of the fish.
"We have no idea how they got here," said spokesman Andrew Hughan. "There are thousands and thousands and thousands of fish."
King Harbor is on the Santa Monica Bay coast, about 22 miles southeast of downtown Los Angeles. Source