MYSTIC, Conn. (AP) — Mystic Aquarium staff and visitors are mourning the loss of a 33-year-old female beluga whale that died after a prolonged respiratory illness.

Aquarium officials say Naku the beluga died Saturday night despite extraordinary efforts by aquarium veterinarians and animal care staff.

Tracy Romano, the aquarium’s executive vice president of research and zoological operations, says Naku had battled the illness since last fall. Aquarium officials are working with University of Connecticut experts in performing a necropsy on the beluga,