Story highlights Unna was 18 year old and weighed 4,600 pounds

Attendance at SeaWorld's theme parks has waned in the aftermath of the "Blackfish" documentary

(CNN) An 18-year-old killer whale has died after a months-long illness at the SeaWorld park in San Antonio.

Unna, a 4,600-pound orca, died Monday.

"We are saddened to share the passing of Unna," SeaWorld said. "Unna had been under the constant care of the SeaWorld veterinary team and outside experts for the past several months."

She suffered from a resistant strain of a fungus called Candida.

A necropsy will be performed to determine the cause of her death.

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