Tilikum, the world's largest captive marine mammal, is nearing the end of his life.

According to SeaWorld officials, the 35-year-old, 12,000-pound killer whale has been diagnosed with an incurable bacterial infection in his lungs and has become increasingly lethargic.

“It has been our duty and passion to make sure we give him the utmost care we possibly can,” Daniel Richardville, an animal training supervisor, said in a statement on SeaWorld's website.

Tilikum was the focus of the 2013 documentary "Blackfish," which highlighted his deadly history and the controversy surrounding keeping killer whales in captivity. Since 1991, the bull orca has been responsible for three deaths, including SeaWorld trainer Dawn Brancheau in 2010.

According to activists, Tilikum's severe confinement to a "small concrete tank" over the last several decades likely contributed to his violent tendencies.

"The constant stress and deprivation have driven him to kill three human beings and to wear his teeth to the nubs from chewing on the underwater bars of his prison," People for the Ethical Treatment of Animals (PETA) said in a statement. "Since SeaWorld can no longer capture orcas from the wild, it has used Tilikum as its chief breeding machine, churning out more performers for a miserable life that no one would ever willingly choose."

Reaction on social media has been predictably sympathetic to Tilikum's plight, with many shaming SeaWorld for denying the killer whale his chance at freedom.

News of Tilikum's declining health comes as SeaWorld is facing increasing legal and public pressures to end its captive whale program and release its orcas to sea pens. In Orlando, one activist is currently in the middle of a month-long protest outside the Miami Seaquarium, spending her days in a bathtub to highlight the plight of Lolita, the "world's loneliest orca."

"By embarking on this challenge, I will experience the same feelings of captivity, separation from my family, boredom and stress that Lolita endures every day," Danielle Daals said earlier this year. "I don't believe anything like this has ever been done before, and I'm hoping it will bring worldwide attention to Lolita's suffering."

As for Tilikum, SeaWorld officials say they will continue to monitor his illness "in a way that makes him comfortable and creates an enriching life."

The irony of those last two words when talking about a whale that has been stuck in captivity the last 30 years should be lost on no one.