The 18-year-old man bobbed from side to side on his toes near a roaring beach in San Diego and kicked a sea lion’s chin, causing the animal to recoil.

It scampered away on a concrete barrier but the man raised his hands, poised as if ready for a boxing match, and lobbed another kick at the sea lion’s hindquarters, according to a video of the July 22 episode that was posted on social media and gained widespread attention.

The man, Tyler Muehl, pleaded guilty on Thursday in federal court in San Diego to violating the Marine Mammal Protection Act, which prohibits harassing or hunting oceanic animals.

He faces a year in prison and a $100,000 fine when he is sentenced on Oct. 26, the authorities said.

Robyn Davidoff, chair of the Sierra Club’s San Diego Seal Society chapter, said she had seen tourists throw rocks at pups, touch sea lions, creep close to snap pictures of them and block them from slipping into the ocean.

But she has never seen in La Jolla — a neighborhood in San Diego known for beautiful beaches, seaside cliffs and marine wildlife — a case where a sea lion was kicked.

“When we see something like this, an intentional and repeated harassment, it’s hard to comprehend,” she said.

Mr. Muehl admitted he kicked the sea lion four times near the Ellen Browning Scripps Park and chased the animal. He referred to himself as a professional mixed martial arts boxer, the authorities said in a news release.

He only ended his pursuit when the sea lion heaved itself over a concrete barrier and flopped onto a beach, the authorities said.

Kelly Thorton, a spokeswoman for the U.S. Attorney’s Office for the Southern District of California, declined to disclose Mr. Muehl’s motive in the attack.

Dana Grimes, Mr. Muehl’s lawyer, said in an email that her client is a teenager who quickly accepted responsibility. “We will present all of the mitigating circumstances at the appropriate time in court,” she said.

Mother sea lions often come ashore to give birth around La Jolla. Mr. Muehl’s actions happened during the animals’ mating season, Ms. Davidoff said.

Ms. Davidoff and her organization work to educate the public about sea lions. “Don’t approach them, don’t touch them, don’t feed them,” she said.

Rob DiGiovanni, the executive director and chief scientist for the Atlantic Marine Conservation Society, said federal law recommends staying 50 yards away from marine animals.

Sea lions can weigh anywhere from 300 to 600 pounds he said, and people can be seriously injured if they provoke one.

“Why do we need to stress them out?” he said.