Los Angeles, California - A bike rider who decapitated a sea lion over Christmas and rode off with the animal's head in a clear plastic bag was being hunted on Thursday.

Officials offered $20,000 for information leading to the conviction of the man who sliced the creature's head off on a California beach.

A notice from the US National Oceanic and Atmospheric Administration (NOAA) said the intact body of a dead sea lion was spotted at Doran Regional Park, about 60 miles north of San Francisco, on December 25.

But a short time later the animal's corpse was horrifically mutilated.

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"An eyewitness described the suspect as a tan-complexioned male, approximately 30 - 40 years old, dressed in all black and riding a black fat-tire e-bike," NOAA said.

"The suspect was seen using a black 8-inch knife to remove the sea lion's head, placing it in a clear plastic bag, and riding away."

Under federal law, it is illegal to harass, hunt, capture, or kill sea lions and other marine life in the US.

California sea lions are one of six known species and a common sight along the western coast of North America, including around Fisherman's Wharf and the Golden Gate Bridge in San Francisco, where their distinctive barking can frequently be heard.

Known for their intelligence, they are sometimes trained to perform tricks in circuses and aquariums, and have been used by the US Navy to carry out military operations.