Minnesota Zoo dolphin Semo came up to the glass for a close up for three-year-old Cipactli Acosta on Monday, May 14, 2012 in Apple Valley, Minn. The Minnesota Zoo closed its 15-year-old dolphin exhibit for good that year. (The Minnesota Star Tribune)

Semo, the oldest-known male bottlenose dolphin in the United States and a longtime star attraction at the Minnesota Zoo, has died at a theme park near San Francisco.

Semo died Tuesday at age 54 at Six Flags Discovery Kingdom in Vallejo, Calif., apparently of natural causes.

Semo came to the Vallejo park in 2012 from the Minnesota Zoo in Apple Valley, where he took up residence in 1991 and fathered other zoo offspring. Before that, Semo lived at SeaWorld in San Diego.

"He was a big goofball but so powerful and impressive," former Minnesota Zoo dolphin trainer Jenny Beem recalled Thursday. "We would end the show with Semo's 1½ flip. It was amazing to watch. The crowd would roar so much the building felt like it was vibrating."

Beem, who now works with other marine mammals at the zoo, described Semo as "a bit of a ham." She said he used to strike a pose on a slide, seemingly to allow zoo visitors to snap a good photo.

"He would hold that pose longer than any other dolphin. I can only imagine how many people took a photo of Semo on that slide out," Beem said.

The Minnesota Zoo had to move Semo in 2012 for major repairs to the Discovery Bay building caused by saltwater damage. The exhibit reopened with other types of sea creatures because other dolphins in captivity were not available and the capture of dolphins in the wild had been brought to a halt.

The average dolphin life span is 25 years in captivity and 12 years or more in the wild.

Beem said she visited Semo after he moved to Six Flags. "I guess I had to know that he was OK," she said. "At first he didn't have any idea who I was, but as I kept talking he swam past me more and more and then you could just see it. It clicked and he remembered who I was, and he stopped and really looked at me." He swam away to fetch another dolphin, then "swam by with her a few times to show me," she said.

"I left knowing they loved him as much as we did."

The Associated Press and staff copy editor Amy Kuebelbeck contributed. Paul Walsh • 612-673-4482