SeaWorld lost another orca Monday when Unna, an 18-year-old killer whale, died from a bladder infection at SeaWorld's San Antonio park, making her the third killer whale to die at SeaWorld in the past six months, according to the Daily Mail. Unna died in Texas from a fungal infection called candida that she has been battling since September.

SeaWorld also lost a baby beluga whale in July at the San Antonio park and a 2-year-old beluga whale in November who died of gastrointestinal problems, Business Insider reported.

"We are saddened to share the passing of Unna today," SeaWorld's website said Monday, adding that it is a "difficult time for the SeaWorld team and all of Unna's many fans."

Unna had been treated by experts from around the country, SeaWorld officials said. "While there were some indications that the treatment was having a positive effect, Unna had remained in serious condition and under 24/7 care," SeaWorld added, stating that a necropsy will also be conducted.

"Unna had been under the constant care of the SeaWorld veterinary team and outside experts for the past several months," SeaWorld said, according to San Diego's Fox 5. 

SeaWorld has been under severe criticism following the release of "Blackfish," a 2013 documentary film showing the exhibition and captivity of killer whales as cruel and inhumane. Declining revenues since the release reflects the public view of orcas in captivity.