Spinnaker the dolphin likely died from an unpredictable stroke, according officials at the Vancouver Aquarium.
The male Pacific white-sided dolphin was found dead in his tank by staff on March 8. He was estimated to be at least 26 years old.
On Monday, aquarium veterinarian Martin Haulena said an examination has concluded Spinnaker had recovered from a recent gastro illness, but had arteriosclerosis — hardening of the arteries.
"It is likely that Spin was back to being a very robust and active animal following his illness and had a stroke-like event immediately preceding death," Haulena said.
"The incident is being considered spontaneous and was not something that could have [been] avoided, predicted, or treated differently."
The examination also found a lesion on Spinnaker's brain but it was not believed to be related to his death, although it may be related to changes in his behaviour, said Haulena.
"Spin's death remains a very sad event for our staff and volunteers, and especially our veterinary and animal care team, as it was such a surprise, particularly since he had been doing so well that same day."
Spinnaker was one of three Pacific white-sided dolphins rescued from fishing nets that were living at the aquarium in Stanley Park. The remaining two are Hana, a 14-year-old female, and Helen, a 21-year-old female.