Twelve to 18 inches of dead sardines blanketed the water's bottom in Basin 1 off Marina Way. Another thick layer of dead fish coated the surface from the breakwall to the inner docks, surrounding boats and walkways, reported The Daily Breeze.
Authorities with the California Department of Fish and Game, along with other ocean biologists at the scene, declared the mass death a natural event. The fish, they said, sucked every drop of oxygen from the water and couldn't breathe.
"They are in every slip and every dock," said Fish and Game spokesman Andrew Hughan. "It's a whole lot of fish."
Mr Hughan said authorities believe the huge school of sardines, perhaps blown in by the night's 40-mph winds and crashing waves, swam into King Harbour and became disoriented.
Those who chose other basins were fine, but the schools that headed into Basin 1 "backed themselves into a corner" and were unable to find their way out, he said.
They quickly depleted the oxygen in the water and died.
"If they turned right instead of left, they'd be fine," Mr Hughan said. "It's just bad luck."
Brent Scheiwe, director of the Los Angeles Conservation Corps' SEA Lab They quickly depleted the oxygen in the water and died.
"If they turned right instead of left, they'd be fine," Mr Hughan said. "It's just bad luck."
Mr Scheiwe said tests revealed the water contained virtually no oxygen.
"Someone reported it this morning at 7:30. We were astounded when we went down there," he said.
The aquarium received reports that there was a large number of fish in the harbor Monday night, "numbers we had not seen before," Scheiwe said.
He said the spike could have occurred for several reasons - one being rough ocean currents that could have driven the fish to one place.
King Harbour residents said they noticed the increase in fish the night before, swimming among their boats and bouncing off their hulls.
Dayn Schultz, who lives in a houseboat on the marina, said it is typical for fish to spawn in the evening, the action roiling the surface in small patches and giving the water the appearance of boiling.
At about 9:30 Monday night, Mr Schultz jumped on deck after hearing a noisy racket. He discovered the entire marina "boiling" around him.
"It was the most I have ever seen," Mr Schultz said.
When the sun came up, residents found the marina's ocean surface coated with dead fish.
"There were millions and millions of them floating everywhere," said Walter Waite, who lives aboard his boat at the harbor.
Mr Waite said he was struck by the fact that birds weren't feeding. Apparently, they had already gorged.
Onlookers were stunned by the spectacle. Hundreds of people arrived to gawk at the scene.
"It's just incredible," said Gene Wilson, who was vacationing in Redondo Beach from Kansas City. "We've never seen anything like this. I don't know how they're going to remove their carcasses."
In addition to testing the water's oxygen level, biologists also checked to make sure no pollutants, oils or other chemicals could have killed the fish. Mr Hughan said the water was clean, but sea gull droppings washed off the breakwall by the strong waves could have contributed to erasing the water's oxygen level.
Fish and Game authorities collected some of the dead fish and packaged them for express delivery to their laboratory in Rancho Cordova. Necropsies will be performed to determine if any other factors beyond the water's oxygen level contributed to their deaths.
David Caron, a professor of biological sciences at USC, has for the past few years monitored sensors that were placed in the harbor waters following a 2005 red tide that killed thousands of fish.
The plankton that caused that phenomenon - Lingulodinium polyedrum - isn't poisonous, but cast a red-brown tinge over the harbour. The fish died when the blanket of algae starved the water of oxygen.
Workman and Mayor Mike Gin said the city had prepared for such an event after the 2005 red tide.
Mayor Gin said the harbour was cleaned up through a massive effort by volunteers, marina representatives and city staff.
"Through that ... we were able to form a red-tide response task force to handle situations like this one," Mr Gin said.
