The Vancouver Aquarium says an unknown toxin was the cause of death for two belugas last year.
Aurora, aged 30, and her calf, Qila, 21, died within nine days of each other in November 2016.
The aquarium says the determination followed a five-month investigation involving "dozens" of aquarium and external specialists.
"Extensive testing was unable to identify the exact substance involved, which is not uncommon due to the very limited time a toxin is traceable in the bloodstream," a statement from the aquarium reads. "The investigation also determined that the toxin was likely introduced by food, water, or through human interference."
The aquarium says in the months after the deaths, staff have worked to prevent risks that could have introduced a toxin to their habitat through:
- Enhanced food screening.
- Removing vegetation near their habitat that "could be considered problematic."
- An overhaul and replacement of mechanical water treatment systems in all habitats housing whales, dolphins and porpoises.
- Real-time testing and monitoring of water in tanks and of water treatment systems.
- Updating security to reduce "potential threats of human interference."
"On a personal level, this is the worst thing that's happened to me in my 25 years of being a veterinarian," head veterinarian Dr. Martin Haulena, who headed the investigation, said. "It's an absolutely devastating circumstance."
The whales' deaths caused intense debate about keeping cetaceans in captivity in Vancouver. In March, the Vancouver Park Board unanimously voted to end the display of live cetaceans by the aquarium.
The aquarium says their facility currently has 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.
Investigation essentially closed
Haulena says nailing down the exact toxin was complicated by not knowing how the animals may have come into contact with it.
He says when discussing human interference as a possible culprit, accidental and intentional interference was discussed.
"Our security has worked with outside experts to beef up how our perimeter is maintained and surveyed and how our camera systems work ... and how people move through the aquarium and how they have access to what areas," he said.
Haulena says without new information or new technology to analyze archived samples emerging, the investigation into the toxin is considered closed.