03/09/2012 04:14 EST | Updated 05/09/2012 05:12 EDT

Cause of Spinnaker the dolphin's death unknown

The Vancouver Aquarium says a necropsy conducted on Spinnaker the dolphin was unable to determine why he died.

The male Pacific white-sided dolphin was found on the bottom of the dolphin tank around 4 a.m. PT Thursday.

The dolphin was around 26 years old and had been sick since the fall, but was doing much better in the past few weeks, staff at the aquarium said on Friday morning.

They will be running more tests to try determine why the animal died.