The mammal's death was announced Friday by Marineland in Niagra Falls, Ontario where she's been living for the past several decades. The theme park said in a statement, "Marineland's marine mammal care team and experts did everything possible to support Kiska’s comfort and will mourn her loss."

Kiska was captured back in 1979 when she was only 3 years old ... living with a few orcas at an aquarium in Iceland before being sold to Marineland. While in Iceland, she was a tankmate with an orca named Keiko -- future star of the hit film, "Free Willy."

During her time at Marineland, Kiska gave birth to 5 calves, but they all tragically died at a young age before her. She's been by herself since 2011, hence the sad nickname -- and studies have shown orcas are social mammals, and isolation like this is considered torture.

Marineland has been criticized for mistreatment of its animals since the '70s, but a viral video of Kiska banging her head against the glass picked up steam in 2021, and an investigation into the facility was held ... although prosecutors later said they wouldn't pursue animal cruelty charges.

Another clip from earlier this year showed her doing constant circles in her tank ... still swimming solo after over a decade of solitude.