Head veterinarian Dr. Martin Haulena says the cause could remain a mystery until the results of toxicology tests on Spinnaker's body come in.
He suspects cancer or a bone marrow problem may have played a role in the death of the 26-year-old dolphin, affectionately known as Spin.
The dolphin had been suffering from a gastrointestinal illness but had recently appeared to be doing better.
Spin had a list of other health problems, including an enlarged pancreas, and had been on 24 hour watch but was found unresponsive at the bottom of his pool by aquarium staff on Thursday morning.
The dolphin was first rescued in 1991 and was moved to the Vancouver Aquarium 10 years later. (News1130)