Tag first arrived at the Vancouver Aquarium as a two-week-old pup in 1993. (Vancouver Aquarium)

Staff at the Vancouver Aquarium are mourning the loss of a male Steller sea lion that died Tuesday morning after battling with cancer.

Tag, 15, was diagnosed with oral cancer last winter and underwent treatment that included chemotherapy, a veterinarian with the aquarium said Tuesday.

"Experts from around the world helped us to develop a very sophisticated treatment plan aimed at ensuring that Tag would have the best quality of life possible for his remaining months with us," Martin Haulena said in a news release.

'He will always be remembered for his noble heart, playful character and gentle soul.'— Clint Wright, senior vice-president of the Vancouver Aquarium

Tag showed abnormal bleeding in the mouth Monday evening and a surgery revealed major problems in his mouth caused by the invasive and progressive form of cancer, Haulena said.

Tag failed to recover from the surgery and died shortly after 2 a.m.

"Tag was truly one of the most unique marine mammals that any of us have had the pleasure to work with," said Clint Wright, the aquarium's senior vice-president for operations and planning.

"He will always be remembered for his noble heart, playful character and gentle soul. He allowed us incredible insight into the world of Steller sea lions," Wright said.

Tag first arrived at the aquarium as a two-week-old pup in 1993. He was being studied by researchers at the University of British Columbia's marine mammal research unit.

More than 80 per cent of the world's Steller sea lions — a species named for 18th-century German naturalist Georg Wilhelm Steller — have disappeared since 1980; fewer than 75,000 of the mammals currently exist, Haulena said.

The studies Tag was part of tried to explain the population decline.