Dead sardines are seen in the harbor area in Redondo Beach, California March 9, 2011. Millions of fish washed up dead early on Tuesday, puzzling authorities and triggering a cleanup effort. [Photo/Agencies]

REDONDO BEACH, California - Thousands of slimy, reeking sardines gurgled to the surface of an increasingly murky Southern California marina on Thursday as crews kept scooping and vacuuming tons of fish that perished in a huge, unexplained die-off.

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Volunteers and city workers scrambled for a third day to remove the bloated fish that bobbed to the surface of King Harbor.

It could take about a week to clean up the mess.

"The virtue is we can get them easier," police Sgt. Phil Keenan said. "The vice is they smell."

An occasional breeze carried the stench from the shallow marina where the fish died late Monday.

By Thursday evening, 85 tons of fish had been removed, Mayor Mike Gin said.

Sunny, hot weather made finishing the cleanup a priority before the smell became any worse and the decomposing fish corpses feed bacteria that could reduce oxygen levels in the marina water and kill other sea life, officials said.

The water was already beginning to look brackish with tiny bubbles, scales and scum floating on the surface with the decomposing fish.

Volunteers and city workers netted fish and picked them by hand from the marina rocks. Keenan said several techniques were being used, including vacuuming the bottom.

"Depending on where the fish are, we have to use a different technique," Kennan said. "There's not a catch-all method - no pun intended."

The cleanup came after the enormous school of sardines apparently suffocated in the harbor, possibly while seeking shelter from a predator or simply becoming lost near a breakwater,

Instead of leaving, the fish crowded toward the back of the marina and used all the oxygen in the water, marine experts have said.

California Department of Fish and Game officials have estimated that at least a million fish died.

Fish and Game experts are confident the sardines suffocated, but about a dozen fish were sent to a laboratory in Rancho Cordova, where they'll be examined to see if a disease or toxin killed them, department spokesman Andrew Hughan said.

"Luckily, they're little small fish so it probably won't take long," he said.

The reason the fish gathered and died in the marina could remain a mystery.

"What this isn't is an oil spill or a chemical spill or an environmental disaster of any kind," Hughan said. "It's a natural fish die-off. It's kind of natural selection. It's sad but it happens."

He praised Redondo Beach for its fast response to the fishy tragedy. The city created a procedure for a rapid response after a toxic algae bloom known as a red tide killed millions of fish in 2005.

"It's the best disaster management that I have ever seen," Hughan said.

The fish are being sent to a composting center in Victorville. They are being buried and in 90 to 120 days will be ready for use as compost, said Gary Clifford, chief operating officer of Athens Services. The waste hauling company is considering selling the fish-based fertilizer to farms under the name "King Harbor Blend," he added.

Some residents had other plans for the sardines, though.