In the wild, Apollo the walrus could have lived more than 30 years.

But this week at Marineland in Niagara Falls, his life was cut short when he suddenly had a heart attack at the age of 17. He didn’t have any known health concerns — and his death follows a mysterious string of walrus deaths at that aquarium.

Within the past two years, four of the marine park’s walruses have died. Walruses are social animals, and now there’s only one left: a female named Smooshi, who has probably never lived alone.

“While the loss of Apollo is truly devastating for all of us who knew him, we are comforted in knowing he passed very quickly and without obvious pain,” the park said in a statement.

Phil Demers, a former Marineland trainer turned animal advocate, said he worked closely with Apollo — and that while the walrus never showed signs of illness, he never seemed fully comfortable living in captivity.

“Apollo was a sweet animal,” Demers told CBC. “He wasn’t your alpha male type. He was a real sweetheart. He was never comfortable with his life in captivity, but he was never an aggressive animal.”

Demers will remember Apollo fondly from his time working there, although now the experience is bittersweet. Apollo was a special walrus — always a big teddy bear, Demers said — and his sweet personality makes the loss even harder.

Over time, the stress of losing family members in captivity has impacted marine mammals like Apollo in the worst way — and the public is starting to notice.

In the wild, walruses typically live in large herds. The only remaining walrus at Marineland is now living alone and still being used in performances until further notice, the aquarium said.

Now, Demers wants to encourage the public to support the move of the aquarium’s only walrus to a different facility where she can live with others.

“Her personality is such that she needs other walruses, she needs companionship,” he said. “The situation is absolutely critical.”