Dozens of tons of dead fish have been cleaned up at King Harbor in Redondo Beach near U.S. city Los Angeles, authorities said on Thursday.

The work was about half way done with an estimated 35 tons of fish scooped up from the surface, Redondo Beach police Sgt. Phil Keenan said.

Crews were ready to return to Redondo Beach Thursday morning for a third day to cleanup the remaining dead sardines packed inside King Harbor, said Keenan, who has been coordinating cleanup efforts.

He estimated another 35 tons of fish were on the bottom of the marina.

About 60 volunteers and 70 city employees were taking part in the cleanup, Keenan said.

A gigantic school of sardines got trapped in the harbor late Monday or early Tuesday and depleted the confined waters of all the available oxygen, before going belly-up about midmorning Tuesday.

Keenan said the city was experimenting with a vacuum truck normally used to clean muck out of sewer lines to get the fish in the shallow water near the marina walls. The city is also evaluating another "soft vacuum" method for sucking up fish from the bottom, but officials don't want to use dredging equipment because "that would disturb the bottom."

Keenan said the fish were being sent to company in Victorville, where they would be made into fertilizer.

The smell around the harbor was "fishy, but not horrific," Keenan said. "In the next couple of days, it'll be horrific."

Experts from the California Department of Fish and Game (CDFG) said tests showed the fishes were suffocated by lack of oxygen in the water.

Driven by wind gusts reaching 45 mph, swarms of sardines, along with mackerel and perch, massed at the harbor in the past few days, leading to insufficient oxygen and lack of water movement, according to experts.

Sardines usually move in gigantic schools, sometimes called "bait balls".

Initial testing revealed no sign of natural toxins, such as domoic acid, which sometimes kills marine life, the CDFG said.

The cleanup is expected to take a week and may cost about 100, 000 U.S. dollars, said Mayor Mike Gin of Redondo Beach.