Another beluga has died at Canada’s Marineland, as questions mount over the future of both the controversial theme park and one of the world’s largest populations of captive whales.

The most recent fatality – the fifth this year – is the 17th beluga to die at the Niagara Falls aquarium since 2019.

Neither the Ontario government nor the park have disclosed the cause of the whale’s death.

But speaking to the Canadian Press, the province’s chief animal welfare inspector said the quality of Marineland’s water was “within the acceptable limits” and that a specialized unit of inspectors tested Marineland’s water weekly.

Melanie Milczynski also said enforcement officials had visited the park 205 times since the province took over animal welfare enforcement from the Ontario Society for the Prevention of Cruelty to Animals in 2020.

In late October, the whistleblower account UrgentSeas, co-founded by a former walrus trainer at Marineland, Phil Demers, published drone footage of veterinarians and trainers attempting to give medication and fluids to the sick beluga.

“I really don’t know how many days it has left,” Demers told the Guardian at the time. “But when you’re at this stage, just trying to keep the whale alive, it’s not good. Seeing this is absolutely heartbreaking. It just kills you inside.”

Marineland Canada is the last aquarium in the country to hold captive whales and made headlines last year when a captive whale named Kiska, dubbed the “world’s loneliest orca”, died from a bacterial infection after spending four decades at the park. In a video clip before her death, the 47-year-old whale, who didn’t encounter another orca for more than a decade, is seen drifting listlessly in her tank.

The park, which has the world’s largest beluga population, has defended the quality of its care, telling the Guardian deaths were a natural outcome. Marineland’s specialists “care for the animals when they are sick and every effort to save them is made” the park said in an email.

In August, Marineland was ordered to pay nearly C$85,000 (US$61,000) after it was found guilty of three violations of the province’s animal cruelty laws related to its captive American black bears.

News of the latest beluga death has prompted an outcry from the province’s politicians. The New Democrat leader, Marit Stiles, called the outcome “disgraceful” and threatened to shut down the park if elected premier. The Liberal leader, Bonnie Crombie, warned there was “no accountability” for Marineland and the care of “beautiful mammals”.

For Demers, whose public clashes with the park have resulted in a string of lawsuits from his former employer, the death reflects a long-running failure of the province to forcefully intervene in the park.

“We’ve been forewarning the public for over a decade that Marineland’s whales would be dying en masse unless someone intervened to fix the conditions,” he said. “Now it seems the government themselves are protecting Marineland. It’s difficult to have trust in your institutions when they continually fail.”