Media playback is unsupported on your device Media caption The BBC's Rajesh Mirchandani explains what scientists think caused the fish to die

US officials fear a public health hazard after an estimated one million sardines that inexplicably washed up in a California marina begin to rot.

The carcasses coated the surface of the King Harbor Marina in Redondo Beach, near Los Angeles, and piled as high as 18in (46cm) on the marina floor.

Officials said the fish had sought shelter from rough seas and soon exhausted the water's oxygen supply.

They said they would "recycle" the fish carcasses into fertiliser.

At the scene It's quite an astonishing sight. The water is carpeted in places with the carcasses of dead fish. They are jammed up in between the rocks and some of the pleasure boats, and underwater they are lining the shallow sea floor. There are dozens of people pulling up nets of dead fish but it's going to take several days to clean up the marina. The fish are being dumped into wheelbarrows and taken to giant containers. Officials say they believe six of these containers could be filled, totalling 150 tonnes of dead fish. From here, the carcasses will be taken to a landfill site, where they will be turned into fertiliser. Some residents say they are quite upset. One woman woke overnight on her boat and saw fish filling the harbour. As the night wore on, they started to die in increasing numbers.

"We need to get rid of them," Redondo Beach Police Sgt Phil Keenan was quoted by the local Daily Breeze newspaper as saying.

"This is going to create a terrible pollution and public health issue if we don't."

With the temperature on Tuesday already passing 68F (20C), the dead fish were beginning to smell.

Roughly 1,400 boats are moored at the harbour, including about 500 with full-time residents.

In some places, the dead fish were so thick boats were unable to leave the marina, officials said.

Local officials said they had found no traces of toxins or pollutants, and that they believed the fish had used up all the oxygen in the marina.

"We're confident that it is a naturally occurring, but unusual event," said Andrew Hughan, spokesman for the California fish and wildlife department.

He said it seemed likely that the fish had got lost after entering the marina and had been unable to find their way out. "They get confused easily," he told the Associated Press news agency.

Earlier estimates put the number of dead fish at millions but the state fish and wildlife department later said it believed there were about one million.

Redondo Beach is about 16 miles (26km) south-west of Los Angeles.