VALLEJO, Calif. — Semo, the oldest known male bottlenose dolphin in the United States, has died at a Northern California theme park where he was a fan favorite.

The San Francisco Chronicle says Semo died Tuesday at age 54.

Officials at Six Flags Discovery Kingdom in Vallejo say Semo apparently died of natural causes but a necropsy is planned.

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

Semo came to the Vallejo park in 2012 from the Minnesota Zoo. Before that, Semo lived at SeaWorld in San Diego.

Nellie, the world's oldest captive dolphin, died at 61 in Florida's Marineland park in 2014.