Aquarium staff say Kavna had been displaying some behavioural changes recently, and a series of tests were being done to find out why.
At 46 years old, Kavna was the oldest beluga at the aquarium.
The aquarium says Kavna was perhaps nearing the end of her life, but the cause of death won't be known until a full necropsy is done.
Last September, three-year-old Tiqa died from heart failure that was induced by pneumonia.
She was the third beluga calf to die at the aquarium since 2005.