The whale named Kasatka was put down Tuesday evening after battling a lung disease for years.
SeaWorld says that veterinarians at its San Diego park had to euthanize an orca estimated to be almost 42 years old.
Regarding the question about the decision to euthanise, the post says: "Kasatka's veterinarians and caretakers made the difficult decision to humanely euthanize her to prevent compromising her quality of life" and said that she had had a harder time "fending off the illness" as she had aged.
Kristi Burtis, an orca behaviorist, spent years working with Kasatka. "Although I am heartbroken, I am grateful for the special time we had together and for the difference she has made for wild orcas by all that we have learned from her".
'I adored Kasatka and loved sharing her with millions of people. "I will miss her very much".
Kasatka was diagnosed with a lung condition in 2008, and caretakers had tailored a treatment plan for her, which included a custom-made inhaler that pumped medicine into her lungs, SeaWorld said.
Kasatka, who was captured off the coast of Iceland in 1978, became the mother of four orcas, grandmother of six, and great-grandmother of two. "Kasatka had a dedicated team of veterinarians and care staff providing critical care".
Veterinarians made the hard decision to euthanize Kasatka after her health started to decline in recent days despite treatment, which included a custom-built inhaler that allowed the medicine to go directly to her lungs.
She was one of the entertainment company's last killer whales to come from the wild.
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In January, Tilikum, the well-known killer whale that killed a SeaWorld trainer, also died after a battle with a lung infection.
In late July, the last killer whale to be born in capitivity at the park died just three months after its historic birth.
Meanwhile, PETA has insisted SeaWorld's remaining animals should be moved to seaside sanctuaries in the wake of the death.
Kasatka was the company's second oldest orca and the second death of a killer whale in a month at one of its marine parks.
In California, lawmakers also increased pressure by banning orca breeding and captivity programs in 2016, and parks were prohibited from featuring killer whales in performances.
They intentionally forced the almost 18 month gestation and almost two years of nursing knowing Kasatka was chronically ill. Most famously, she dragged trainer Ken Peters to the bottom of a tank during a show and almost drowned him in 2006.
The trainer, Ken Peters, fought for air for more than eight minutes before Kasatka finally let him go.
The local group of the People for the Ethical Treatment of Animals planned to hold a gathering near the San Diego park Wednesday as a memorial to Kasatka.
The park said the 10 remaining orcas are believed to be healthy, but any changes in their behaviour will be closely monitored.