Semo, the country's oldest known male bottlenose dolphin living under human care, died Tuesday at Six Flags Discovery Kingdom in Vallejo.
The cause of Semo's death is pending the results of a necropsy, but it's likely that old age was a contributing factor, officials with Six Flags Discovery Kingdom said.
The average dolphin's lifespan under human care is 25 years, and often 12 more years in the wild, according to data by the Alliance of Marine Mammal Parks and Aquariums.
Semo lived 54 years and spent the last six years as an animal ambassador to millions of guests at Six Flags.
Six Flags Animal Care Director Dianne Cameron said Semo was a "ruggedly handsome, infinitely joyful and playful large and imposing male.
"We feel so fortunate to have been a part of his life," Cameron said.
Semo arrived at Six Flags in 2012. His previous home was the Minnesota Zoo and for many years before that, he lived at Sea World.
At Six Flags he was integrated into a primarily female dolphin social group for a few years before the arrival of a pair of males and a calf born at the park.
Six Flags officials said Semo kept mentally and physically stimulated by participating in the park's Dolphin Discovery education program, which led to a new generation of fans.
Six Flags Discovery Kingdom also cared for a 51-year-old female bottlenose dolphin that died in 2011 and cares for older dolphins that are now in their late 30s. With the proper geriatric care, a lack of predation and a healthy environment most animals can live longer expected.
More information and photos of Semo are available on Facebook.