Controversial Canadian waterpark Marineland has announced the death of one of its walruses, days after “demonstrators” were blamed for causing a stampede that led to the deaths of two deer.

The tourist attraction in Niagara Falls, Ont. announced Apollo’s death on Tuesday, confirming the 18-year-old animal died of a heart attack in late April.

"Even with the immediate intervention of multiple medical marine mammal experts, we are sad to report that Apollo passed away," a Marineworld release said.

"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 is now keeping a close eye on its last remaining walrus, Smooshi, which has been subject to "extensive additional checkups to confirm the status of her health."

“Our team is providing her with additional enrichment and care while plans for her future at the park are finalized,” the park said.

“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.”

Apollo is the fourth walrus to die at Marineland in two years.

Zeus died of natural causes on Boxing Day last year. Another walrus, Buttercup, died in the winter of 2017/18.

Female walrus Sonja died suddenly in May 2017 from a rare abdominal aneurysm, the park said.

Two deer killed in stampede

Meanwhile, Marineland said it had its busiest opening day in a decade, despite protests from animal rights groups.

The park claims two men deliberately started a deer stampede Saturday, resulting in the deaths of two of the animals.

“These individuals laughed in the face of staff as they tried to get them to stop,” a Marineland statement said.

“They refused all instruction by staff and resisted efforts to remove them from the Deer Park. We are all upset by this terrible act against innocent animals.

“In order to protect our animals, we are closing the Deer Park to make modifications to prevent this type of incident from ever happening again.”

Ontario SPCA and Humane Society has called for an overhaul to provincial animal welfare legislation, which it says is failing animals kept in captivity for commercial gain.

“The Ontario SPCA and Humane Society has formed a task force dedicated to developing ‎new provincial animal welfare legislation that reflects the need for both greater protection and social justice for animals,” the charity said in a statement.

“The task force is reviewing the need for animals to be recognized under law as sentient beings to acknowledge their ability to feel, to have subjective experiences and to be treated accordingly, rather than as property.”