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Aurora, a Beluga whale, swims with her newborn calf after giving birth at the Vancouver Aquarium in June, 2009.ANDY CLARK/Reuters

The deaths of two beluga whales – a mother and calf in the same tank – within two weeks at the Vancouver Aquarium have sparked a global mystery, with police being consulted, and marine experts examining possible causes that include the animals' food, an infection or even poisoning.

The facility has kept whales since the 1960s, when it acquired its first orca, and several have died in that time, but never two so close together.

Necropsies have so far failed to pinpoint a cause of death, and aquarium staff said they had not ruled anything out and had been in contact with the police.

Andrew Trites, a University of British Columbia marine mammal researcher visiting the French coastal city of La Rochelle, said he mentioned the B.C. situation to scientists there.

"They said, 'C'est incroyable. None of us have heard of such a thing happening,'" Dr. Trites said in an interview on Monday from Europe.

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Neither has the Vancouver Aquarium in its 60-year history. On Monday, officials said they have launched a massive scientific investigation into what happened to 30-year-old Aurora and her 21-year-old calf, Qila.

"The guiding principle is absolutely no stone unturned," aquarium CEO and president John Nightingale told a news conference.

Martin Haulena, the head veterinarian for the facility, said the most likely cause was a virus or toxin. He said "toxin" is a broad term that includes toxins from bacteria, inorganic toxins or "possibly introduced toxins as well."

Commenting on the possibility of poisoning, Dr. Nightingale said the aquarium reviewed surveillance footage, changed security procedures and has had "a couple of conversations" with the Vancouver Police Department.

A spokesman for the force in an e-mail statement played down any contact with the aquarium. "I am not aware of any evidence to suggest that the unfortunate demise of the beluga whales were as a result of a criminal act," Staff-Sergeant Randy Fincham said.

Qila, the first beluga born in captivity in Canada, died two weeks ago, and Aurora died late last week with similar symptoms. Aurora had shown signs of lethargy, a loss of appetite and abdominal cramping.

"I don't have to tell you it's perplexing how you can lose two belugas so closely together with no explanation," Dr. Nightingale said a few metres from the now-empty pool where Aurora and Qila had delighted tens of thousands of visitors.

Jay Barlow, president of the Society for Marine Mammalogy, which includes some of the world's leading experts, said from California on Monday that aquariums around the world will be watching for the solution to this mystery.

"As far as I know, this is unprecedented," Dr. Barlow said from San Diego. "It seems like to me that they died of the same cause, whatever that could be."

He said he wonders whether it was a common food source or a disease to which both were exposed.

Dr. Nightingale said there is no evidence of something nefarious "except for a whole bunch of weird coincidences."

Last Thursday night, someone broke into a marine mammal rescue centre operated by the aquarium on the Vancouver port, spreading debris, and draining water from a pool containing seal pups. The seals were not harmed. Police are investigating.

Dr. Nightingale also said social-media "trolls" have been "systematically harassing" some aquarium staff. Then came the two beluga deaths. "[It's] a number of circumstances that, taken together, paint a pretty clouded and confusing picture," he said.

While animals have died in aquariums, Dr. Trites said he could not think of a case where two related animals had died so closely together with similar symptoms yet no known cause.

"It could well be something that we have never seen before. That's a possibility in the same way, for humans, that when AIDS first came, nobody knew what it was," he said, speculating that it could be a virus from a bird or a unknown fungal infection.

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