The zoo says Bismarck died Wednesday under anesthesia while he was being prepared for surgery to remove a testicle that hadn't dropped. The surgery was meant to help him avoid future health problems.
The sea lion was found near a pier in Newport Beach, Calif., in 2008 as an orphaned pup missing his rear flippers. He came to the Denver Zoo in 2009.
Bismarck was believed to be about 5 years old.
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