Image copyright N.Fisher/Vancouver Aquarium Image caption Activists are calling for a ban on marine animals in captivity after two whale deaths at the Vancouver Aquarium.

Two beluga whales at the Vancouver Aquarium have died under mysterious circumstances.

Aurora, a 29-year-old white beluga whale, died on Friday just nine days after her calf Quila died of a similar illness.

A necropsy found the mother probably died of a virus or toxin, but the cause of death is unknown.

The facility's head said it was possible they were deliberately poisoned.

"We're waiting for more test results in hopes they will provide an explanation," said Aquarium chief executive John Nightingale during a news conference. "Absolutely nothing is off the table."

Aurora's 21-year-old calf Quila was the first whale born in captivity in Canada. Aurora began exhibiting symptoms days after her calf died.

The aquarium conducted a necropsy on Saturday to determine the cause of death. The examination revealed Aurora had severe liver damage, but that the cause of death was impossible to determine.

"We have to assume their deaths are related," Mr Nightingale said.

Image copyright Getty Images Image caption Aurora with her second calf in 2002. Quila, Aurora’s first born, died nine days before Aurora herself.

Beluga whales can live up to 40 years in captivity, according to the Department of Fisheries and Oceans.

The deaths have fuelled calls by animal rights activists to make keeping marine animals in captivity illegal.

The Vancouver Humane Society said the aquarium is on the ''wrong side of history'' by continuing to keep marine animals in captivity.

''There's no way the aquarium can replicate their natural environment,'' humane society spokesperson Peter Fricker told the BBC.

Public opinion regarding whale captivity has turned, Mr Fricker said, since the documentary Blackfish, which was highly critical of the sea park industry.

Vancouver Park Board chair Sarah Kirby-Yung said there should be a city-wide referendum on the issue.

"I have a lot of heart for both sides of the issue," Ms Kirby-Yung told the CBC.

"I think as an institution, they do incredible work. I also have to listen to Vancouverites, that's what I was elected to do, and there's a growing discomfort that people have with having cetaceans there."

The aquarium has five other beluga whales in captivity, but Mr Nightingale said they won't take in any more until they can figure out what killed Aurora and Quila.