The popular orca at SeaWorld Orlando and star of the documentary Blackfish, Tilikum, is dying, the entertainment park company announced.

“We are saddened to report that over the past few weeks, Tilikum’s behavior has become increasingly lethargic, and the SeaWorld veterinary and animal care teams are concerned that his health is beginning to deteriorate,” the company said on Tuesday in a post on their website.

“It has been our duty and passion to make sure we give him the utmost care we possibly can,” said Daniel Richardville, Animal Training Supervisor, in the post.

Tilikum is believed to be 35 years old, which is near the high end of an orca’s average natural lifespan – though some studies imply that in the wild they can live considerably longer – and is suspected to have a treatment-resistant bacterial infection in his lungs.

Tilikum was brought to SeaWorld from another unconnected marine park, Sealand of the Pacific, in Vancouver, Canada, which captured him off the coast of Iceland in 1983. He has been at SeaWorld in Orlando for 23 years.

In February 2010, Tilikum dragged one of his trainers, Dawn Brancheau, to her death during a live show in front of a horrified audience – the third time he had been involved in a person’s death.

The incident, which was caught on video, inspired filmmaker Gabriela Cowperthwaite to investigate the treatment of captive orcas at places such as SeaWorld.

“Tilikum killed Dawn Brancheau, Daniel Dukes and Keltie Byrne. He suffered a miserable life both before and after these events,” Cowperthwaite said on Wednesday. “But none of the conditions that led to these tragedies have truly changed for him, for the trainers, or for other orcas. Only public pressure can really turn this from a tragic story, to a tragic story with a lesson.”

Her documentary, Blackfish, which debuted at Sundance in 2013, shone a none-too-flattering light on what she saw as near-coverups of attacks by orcas in captivity on humans.

The documentary implies that the orcas are driven to psychosis – and attacking humans – by the cruelties and indignities of their imprisonment.

SeaWorld entertainment strongly denied the allegations made by Blackfish, saying in a statement that “the film conveys falsehoods, manipulates viewers emotionally and relies on questionable filmmaking techniques to create ‘facts’ that support its point of view.” The statement called the film “propaganda.” They especially disputed the contention that Tilikum had been “driven crazy” by his time in captivity.

But their response had little effect. The public turned on SeaWorld after Blackfish’s release, especially after an estimated 20 million people saw the film when it was aired on news network CNN. This led to plummeting visitor numbers and a $10m drop in profits for SeaWorld. Several musicians who had been scheduled to perform at concerts at SeaWorld locations, including Willie Nelson, cancelled.

Last year, in response to the extremely negative response people had after watching Blackfish, SeaWorld chief executive Joel Manby announced that the controversial killer whale shows would come to an end at the end of 2016, to be replaced with “an all new orca experience focused on the natural environment.”

In a column for the Guardian in 2015, Cowperthwaite wrote: “People have stopped going to SeaWorld not simply because of a movie but because, it seems, we’re recalibrating how we feel ethically about animal welfare.”

“Faced with uncomfortable truths about animal welfare,” she continued, “we’re clarifying what it means to be humane.”