The silver-colored fish appeared overnight at King Harbor in Redondo Beach, where the surface of the water was covered with a layer up to a thick deep in places, said officials.
Locals suggested that high winds might have pushed the fish -- initially reported to be anchovies, but later identified as sardines -- into the marina south of Los Angeles.
But police spokesman Phil Keenan suggested they were likely chased in by other fish.
"We believe the sardines were chased in, probably by some other type of predator fish," said Keenan, adding that millions of fish remained alive outside the marina, with seagulls and sea lions feeding on them.
"But these particular sardines got into this basin here, which is a smaller area -- and a confined area -- and we believe they died of oxygen deprivation."
Local official Bill Workman said the water was not polluted.
"There are no visible signs of any toxins that might have caused (the deaths)... There are no oil slicks or leaking of substances into the water," he said.
"It looks like what happens to goldfish when you don`t change the water in the tank, mouth open and belly up," he told the Los Angeles Times.
Keenan added, "Sardines consume a lot of oxygen, and there was not a lot of oxygen in the confined area where they were, and so they died of oxygen deprivation.
"It`s like putting too many fish in a small aquarium," he said.
(U.C003/P003)
Editor: Aditia Maruli Radja
Copyright © ANTARA 2011
