NEAR SCOTTSDALE (3TV/CBS 5) - Dolphinaris Arizona announced Thursday evening that another of one its dolphins has died.

Kai, a 22-year-old male, is the fourth dolphin to die at the facility since it opened amid controversy on reservation land adjacent to Scottsdale.

[SLIDESHOW: The dolphins]

[READ MORE: Third Dolphinaris Arizona dolphin dies (Dec. 31, 2018)]

"Immediately after Kai started showing signs of health decline two weeks ago our team made every effort to save his life, including bloodwork testing, ultrasounds, x-rays, and engaging external specialists and submitting diagnostic samples to outside university veterinary laboratories," Christian Schaeffer, the general manager at Dolphinaris Arizona, said in a statement sent to media outlets. "Kai initially seemed to be responding, but his health suddenly declined last night around 11:30 p.m. After the veterinary team administrated hours of critical care, including providing him oxygen, medicine and x-ray testing, Kai's condition continued to decline. We made the extremely difficult decision to humanely euthanize Kai ensuring he would pass peacefully."

[READ MORE: Discrepancy in reported cause of death at Dolphinaris raises new concerns (Nov. 17, 2017)]

Kai's death comes a month after Khloe, an 11-year-old female Atlantic bottlenose dolphin, died after battling what Dolphinaris Arizona officials described as a chronic illness.

In May 2018, Dolphinaris Arizona lost another female dolphin named Alia. She was 10 years old.

In September 2017, a dolphin named Bodie died of "a rare muscle disease."

Bodie died just shy of Dolphinaris Arizona's first anniversary.

[AND THIS: Activists rally outside Arizona aquarium after federal report on dolphin death (Nov. 18, 2017)]

Schaeffer said the facility has launched an investigation to review the dolphins' death.

"We recognize losing four dolphins over the last year and a half is abnormal," said Schaeffer. "Over the last several years we have worked with a team of external experts in the fields of animal behavior, water quality and veterinary care to ensure our dolphin family remains healthy. We will be taking proactive measures to increase our collaborative efforts to further ensure our dolphins' wellbeing (sic) and high quality of life."

[RELATED: General manager of Dolphinaris responds to opposition (May 4, 2016)]

Dolphinaris said it has already contacted a third-party pathologist to conduct a necropsy, which is an animal autopsy, to help determine the source of Kai's health problems.

Dolphin Free AZ, with support from Dolphin Project, is planning to hold a protest in front of Dolphinaris on Saturday at 11 a.m.

“With four out of eight dolphins dying inside of 16 months, the situation has reached critical mass. For the safety of the public and the remaining dolphins, all activities should cease at Dolphinaris Arizona until an independent investigation takes place," said Lincoln O’Barry with the Dolphin Project.

Dolphinaris, which is part of the OdySea In The Desert complex on the Salt River Pima-Maricopa Indian Community near Scottsdale, opened in October 2016.

[RELATED: Trainers keep dolphins safe in 119 degree heat (June 20, 2017)]

On Friday, PETA released the following statement about the latest dolphin death:

"As the National Aquarium in Baltimore prepares to move dolphins to seaside sanctuaries, the Parliament of Canada considers a bill that would ban dolphin captivity, and two belugas will soon move to the first beluga sanctuary, Dolphinaris Arizona's deadly dolphin prison is out of touch with public sentiment—and there's no excuse for keeping it open. PETA urges Dolphinaris to send surviving dolphins to seaside sanctuaries, where they would never again be forced to haul tourists on their backs in the sweltering Arizona desert."

PETA supporters will join Dolphin Free AZ in partnership with Ric O'Barry's Dolphin Project in calling on Dolphinaris to send the dolphins to seaside sanctuaries at a memorial protest on Saturday, February 2, from 11 a.m. to 1 p.m., at the west corners of E. Via de Ventura and N. Pima Road in Scottsdale.