Marineland has officially announced the death of another walrus, citing the "circle of life" as a "natural and ever-present fact" faced by staff every day.

The Niagara Falls amusement park and zoo says an independent necropsy shows a walrus named Apollo died of a heart attack in late April.

"Apollo was in the care of marine mammal staff and veterinarians when he collapsed due to a sudden and unexpected heart attack," reads a statement released Tuesday, adding lifesaving efforts by staff and "marine mammal experts" were unsuccessful.

"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," stated the park.

"Apollo inspired generations of guests and staff to care and learn about this remarkable species."

'He was a real sweetheart'

Phil Demers, who worked as a trainer at Marineland more than a decade before becoming a whistleblower and activist, said he worked closely with the walrus during his time at the park.

"Apollo was a sweet animal. He wasn't your alpha male type. He was a real sweetheart," said Demers. "He was never comfortable with his life in captivity, but he was never an aggressive animal."

Demers said Apollo was a sweetheart who was never aggressive, but added he was never comfortable in captivity. (Supplied by Phil Demers)

The animal advocate said he can't comment on the reported cause of death, but added he estimates Apollo was 17-18 years-old when he died, while walruses in the wild can live for more than 30 years.

"He was always the picture of health," he said. "This was a big, beautiful animal."

Demers described the enclosures for walruses at Marineland as too small for the large animals and "substandard."

He pointed to the recent deaths of multiple walruses, including Zeus who died at the end of December, as evidence animals at the park are under threat.

Marineland also publicly acknowledged the "surprise" death of a walrus named Sonja in May 2017.

"Four walruses have died in less than two years," he said. "It's an atrocity."

Just 1 walrus left

Apollo's death means Smooshi, the walrus Demers was closest with, is the last remaining walrus at the park.

Marineland says Smooshi has received "extensive additional checkups" for her health and a team is providing her with "additional enrichment and care while plans for her future at the park are finalized."

"Smooshi continues to show her love and adoration for her favourite marine mammal trainers and appeared to be in good spirits when taking to the stage at Marineland's educational presentation on Saturday's opening day," according to the park's statement.

But Demers said walruses are social animals and the fact Smooshi is now alone means she needs to be moved somewhere with other animals as soon as possible.

"Her personality is such that she needs other walruses, she needs companionship," he explained. "The situation is absolutely critical."