CHICAGO (CBS) — Puiji, 25-year-old Beluga whale at the Shedd Aquarium, died Wednesday evening.

Aquarium officials said Puiji had been suffering from health problems and, on Tuesday, she had begun to show signs of a rapid decline in health.

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Wednesday evening, her posturing and movement became erratic for about 10 minutes before she experienced a seizure. Veterinarian and marine mammal specialists tried to resuscitate Puiji for about 45 minutes, but she died.

“This is an extremely sad day for the Shedd Aquarium family,” said Ken Ramirez, executive vice president of animal care and training. “We had been monitoring Puiji for the last several months, keeping a close eye on a health issue that was being treated. … Based on our knowledge of her behavior throughout her long life, we knew something was wrong. ”

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A full necropsy (animal autopsy) has been performed and doctors found that Puiji had experienced changes in several organs, including her spleen and lymphnodes. Those problems had not been evident in previous health assessments, according to a statement from the aquarium.

Further evaluations will be conducted to determine if those issues were tied to Puiji’s advanced age or other factors, but officials said the exact cause of death might never be known.

According to the aquarium, the life span of Beluga whales in the wild is unknown, but in zoos and aquarium, they can live to be 25 to 30 years old.

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Puiji had been at the Shedd Aquarium since 1989, before the Oceanarium opened. She gave birth to two calves: Bella, in July 2006; and Nunavik in December 2009.