Puiji, a 25-year-old beluga whale at the Shedd Aquarium, died Wednesday evening after having a seizure, officials said.
"We had been monitoring Puiji for the last several months, keeping a close eye on a health issue that was being treated," said Ken Ramirez, Shedd's executive vice president of animal care and training.
He didn't provide specifics on the existing health issue.
"We became more concerned late Tuesday, as she began to show signs of a rapid decline in health. Based on our knowledge of her behavior throughout her long life, we knew something was wrong," he said.
Ramirez said the whale's movement "became erratic" about 10 to 12 minutes before she had an arresting seizure.
Preliminary information from a necropsy showed changes in several organs, including her spleen and lymph nodes, that hadn't been evident in previous health assessments, said Shedd veterinarian Caryn Poll.
Puiji had been part of Shedd Aquarium’s animal collection since 1989, before the opening of the Oceanarium. She gave birth to two calves at Shedd: Bella, in July 2006, and Nunavik in December 2009.
Shedd Aquarium currently cares for six beluga whales. One of them, Naluark, was recently transported to Connecticut. It's hoped he'll mate with one of two females there. He's sired three calves in Chicago.