Redondo Beach Police Sgt. Phil Keenan said authorities were confident about test results indicating oxygen deprivation caused the fish kill because another part of the same sardine school was doing well at the mouth of King Harbor, CNN reported.
"All evidence points to oxygen deprivation as cause of death," California Department of Fish and Game spokesman Andrew Hughan said. "There is no oil sheen, nor is there a chemical sheen."
Keenan said clean-up boats would spend the next few days removing the dead fish by net.
"Part of the sardine school is out in the channel of the harbor and they're doing fine," Keenan told CNN. "For some reason, this large school of sardines got chased into the harbor -- and they died off."
While the large number of dead fish is unusual, it isn't unprecedented, Hughan said.
Redondo Beach City Manager Bill Workman said there were no visible signs of toxins that could have caused the fish kill, the Los Angeles Times reported.
"This is similar to what we experienced five years ago but that was distinctly a red tide event but there's no discoloration of the water, no associated foaming in the waves," Workman said.