Zoo officials said Orkney, a native of Prince Edward Island in Eastern Canada, was put down after undergoing medical treatments for months, the San Francisco Chronicle reported. His breathing had become deep and labored by Friday.
"Unfortunately, we had to make the decision to say goodbye to Orkney," said Tanya Peterson, the zoo's executive director. "He had been our own Lance Armstrong by beating the odds for so long. He'll be greatly missed by staff and visitors alike."
Orkney is survived by multiple offspring he sired in the late 1970s and early 1980s.
Gray seals had been hunted commercially until the 1972 Marine Mammal Protection Act banned it. Their populations have rebounded in their habitats on both sides of the North Atlantic Ocean.