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Hogle Zoo's Rocky Shores has a leadership void.

California sea lion Big Guy, who died Monday after undergoing sedation for an ultrasound, was "the undisputed boss" of his exhibit, according to his keeper.

"He just had an 'I don't care' attitude," said Rocky Shores' Michelle Hanenburg. "If he wanted to go where another sea lion was, he would just displace them."

The blind sea lion, thought to be between age 15 and his early 20s, was found washed up on the shore in Southern California in 2010. He arrived at Hogle's $18 million Rocky Shores exhibit via airplane and FedEx truck in spring 2012 after two years of rehabilitating at the Fort MacArthur Marine Mammal Care Center in California. Hogle's staff thought he'd be unruly after all his years in the wild, but he defied those expectations.

"We were just kind of astounded at how mellow he was," Hanenburg said. "He didn't have bad days. It was shocking that our naive, blind adult male was our most reliable animal."

The absence of a 650-pound sea lion is hard to ignore, and Hogle Zoo's staff was in a somber mood Tuesday morning. Keepers first noted out-of-character behavior from Big Guy last Thursday, when he showed a lack of interest in eating and training. He had colic and indications of an intestinal issue, and didn't want to leave the water.

Staff herded Big Guy inside and eventually brought him to South Valley Large Animal Clinic in South Jordan. During the past week, he had antibiotic injections, painkillers, X-rays, an endoscopy and ultrasounds. It was all for naught: Vets had no answers and Big Guy was still ailing.

Zoo officials consulted with national marine mammal experts and reached the consensus that he likely had cancer, which afflicts 20 percent of sea lions. His sharp decline — a total of five days, from first symptoms to death — is abnormal, but not unprecedented.

"Sometimes in exotic animals, they hide what they're going through until the very end," said Nancy Carpenter, Hogle's associate director of animal health. Natural instincts tell animals to subdue symptoms and trick would-be predators, she said. "When we figure out what they've got, we're already behind the eight ball."

Big Guy quit breathing Monday, an hour into sedation for an ultrasound and blood work. A necropsy confirmed the experts' suspicions: He had a large, cancerous tumor in the lymph nodes near his pelvis that had metastasized and would have been untreatable.

Knowing that Big Guy's illness was terminal helped ease the pain of his death from anesthesia, said Hanenburg. Still, "it's been kind of a sad day realizing how much of what we do was impacted by his presence. Everybody's still kind of slightly in shock. … He was fine on Wednesday."

The Rocky Shores exhibit still features two younger sea lions, Maverick and Rocky, and three harbor seals. They're enough to keep staff busy, but their massive pool mate had big flippers to fill.

"Every time somebody looks into that great big pool and doesn't see him, they're going to miss him," Carpenter said. "He won a place in all of our hearts."

Twitter: @matthew_piper