A LARGE oil spill off the southern California coast, first reported on 2 October, has left fish dead, birds mired in petroleum and wetlands contaminated, in what local officials called an environmental catastrophe.

The US Coast Guard, heading a clean-up response, on 3 October announced an around-the-clock investigation into how the spill occurred.

An estimated 477,000 litres, or 3000 barrels, has spread into an oil slick covering more than 33 square kilometres of the Pacific Ocean.

Kim Carr, the mayor of Huntington Beach, has called the spill an “environmental catastrophe” and a “potential ecological disaster”. The beachside city, about 65 kilometres south of Los Angeles, was bearing the brunt of the spill.

A report by Reuters indicates that officials are investigating whether a ship anchor striking a pipeline could have triggered the leak. The pipeline is connected to an offshore facility owned by a unit of Amplify Energy Corp.

Amplify CEO Martyn Willsher said at a news conference on 4 October it was possible a ship anchor could have struck the pipeline. Earlier, he said the line was shut and its remaining oil removed.

Dozens of container ships have been stranded off the coast recently, awaiting their turn to enter the port, which could have transited over the pipeline.

Officials have deployed 625 metres of protective booms, which help contain and slow the oil flows, and nearly 12,000 litres had been recovered on 3 October, the U.S. Coast Guard said.

Among the areas threatened is the Magnolia Marsh, a wetland that was rehabilitated after a local conservancy bought the land in 2008. Up to 90 bird species use the area each year, including eight to 10 that are either endangered or threatened, officials have said.

Some 23 oil and gas production facilities operate in federal waters off the California coast, according to the US Bureau of Ocean Energy Management. Amplify’s Beta Offshore unit has three, including the Elly offshore platform, where the pipeline was connected.