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Rocky, a California sea lion originally rescued off the West Coast with a pellet in his head and gas bubbles in his brain, died last week at Utah's Hogle Zoo while undergoing surgery.

Zoo officials said they believe Rocky died of an infection, based on swollen lymph nodes discovered throughout his body during a necropsy.

A news release Monday from the zoo said the animal autopsy also determined Rocky had evidence of past strokes, "possibly associated with trauma he endured before arriving at Hogle Zoo."

He had been immobilized last week for X-rays to determine why he was lethargic and apparently not feeling well, said zoo spokeswoman Erica Hansen.

"This is one of the challenges with rehabilitated animals," said Christina Castellano, the zoo's assistant director of programs. "We love being able to offer them a good home and we make sure they have everything they need, but you never know what health issues you'll encounter."

Added senior veterinarian Nancy Carpenter: "It's certainly possible that due to Rocky's condition, he didn't have everything he needed mentally and physically to fight off infection like a normal sea lion."

After the injured Rocky was found near Oakland, Calif., in 2012, doctors removed the pellet from his head. He was only the second sea lion known to have bubbles on the brain — the other was found dead three weeks after it was released into the wild.

Working with The Marine Mammal Center, Hogle Zoo gave Rocky a second chance.

"Every step of the way, Rocky has overcome the odds. … We never knew how long we would get with him," said Michelle Hanenberg, keeper at the zoo's Rocky Shores exhibit, where the sea lion lived. "He challenged us to look at things differently. What works for other sea lions did not work for Rocky, so we had to rethink our approach.

"He was so eager to do anything asked of him," Hanenberg added. "Even when we were frustrated, Rocky was all in — he was right there trying hard. He would try and try. He never gave up. He was really inspiring in that way."

Hogle Zoo still has one male sea lion, named Maverick, and three harbor seals. Zoo officials are trying to decide how best to deal with them in light of Rocky's death, Hansen said.