A killer whale was euthanized Tuesday after a long treatment for an infection, SeaWorld San Diego officials said.

Kasatka, 42, was considered the matriarch of the orca family at the embattled amusement park in San Diego. She had been fighting a lung infection for years, and in the past several days her health and appetite had significantly declined, park officials said.

SeaWorld San Diego officials say the lung disease is a common cause of death for orcas living in the wild and at zoo facilities.

“All of us at SeaWorld are deeply saddened by this loss, but thankful for the joy she has brought us and more than 125 million park guests,'' park officials said in a written statement.

This is the third orca death at a SeaWorld park in 2017.

Tilikum, the killer whale whose story was documented in the movie "Blackfish," died in January at the company's park in Orlando, Florida.

The last killer whale born in captivity under SeaWorld's former orca-breeding program died in June at the company's San Antonio, Texas, park.

The People for the Ethical Treatment of Animals, an organization that has been critical of SeaWorld’s treatment of orcas, claims the park artificially inseminated Kasatka in 2011 even though she was being treated for an infection.

“The dead bodies at SeaWorld are stacking up about as fast as its stock is falling,” PETA Executive Vice President Tracy Reiman said in a news release.

PETA plans to hold a memorial for Kasatka outside of the park at noon Wednesday.

SeaWorld San Diego plans to conduct a necropsy to examine the extent of the illness.