A toxin killed two beluga whales that died suddenly in November 2016, according to a five-month-long investigation by the Vancouver Aquarium.
An investigation involving dozens of veterinary pathologists, toxicologists, genome specialists, medical doctors, and field research scientists failed to find a specific toxin.
It's not uncommon for a toxin to be hard or impossible to identify, he said, because it could have been metabolized quickly by the animals, leaving the substance undetectable.
The toxin was likely introduced by food water or human interference, the investigation found.
The aquarium's head veterinarian, Dr. Martin Haulena, says the investigation has helped them understand what happened and how they can help ensure the safety of the mammals in their care.
In the wake of the deaths, the Aquarium says it has brought in new measures to "test, evaluate, and reduce risks in the Arctic habitat".
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Significant security updates have been deployed to monitor perimeter access and reduce potential threats of human interference. "Their loss is felt profoundly by our staff, members, supporters, and the public", Haulena said.
They also include a new food screening process and the removal of vegetation next to the habitat, as well as an overhaul of water treatment systems and new real-time water testing.
In 2010, Aurora's calf Nala died just two weeks after her first birthday when her airway became blocked, and in 2005, three-year-old Tuvaq died from heart arrhythmia.
Vancouver Aquarium is now home to a false killer whale, a Pacific white-sided dolphin and a harbour porpoise - all receiving long-term care as part of the Aquarium's Marine Mammal Rescue Centre Program.
The future of whales and dolphins being held at the aquarium remains uncertain after the Vancouver Park Board voted last month to ask staff to create a bylaw that would prohibit the importation and display of cetaceans at the facility.
The debate over whether animals, like whales, should be kept in captivity continues with the Vancouver Park Board. The aquarium now has five belugas on loan to US marine parks for breeding.