A controversial theme park in Canada is facing renewed criticism following the deaths of three animals in recent weeks, triggering fresh calls from activists for the attraction to be shut down.

Officials at Marineland, which sits on the Canadian side of Niagara Falls in Ontario, said two deer were killed in a stampede allegedly caused by a father and son taunting the animals. The incident, which occurred last weekend when the attraction opened for the season, has prompted staff to temporarily close the deer park area.

Days after the stampede, the park also announced the results of a postmortem on one of its walruses which died in April, citing a heart attack as the cause of death for the 18-year-old animal named Apollo. Apollo is the fourth walrus to die in the park over the last two years, leaving it with only one remaining animal – a female called Smooshi.

Speaking of the deer deaths, Marineland said: “We are all upset by this terrible act against innocent animals. In order to protect our animals, we are closing the deer park to make modifications to prevent this type of incident from ever happening again.” It added that this was the first time such an incident had occurred.

Facebook Twitter Pinterest Deer sit around the perimeter of Marineland’s deer park. Photograph: Tara Walton/Toronto Star via Getty Images

But Phil Demers, a former animal trainer at the park who has become Marineland’s most vocal critic, rejected the park’s explanation, claiming that a move to reopen the deer park to visitors after years of closure could cause panic among the animals.

“There’s been countless incidences of patrons stressing out the animals. [Marineland] took a gamble. They opened it for the first time in years,” he said.

Marineland has increasingly become a target for activists, who argue the park has a moral responsibility to release the animals it keeps in captivity. The park dismissed protests outside its gates over the opening weekend as a “small group of annual demonstrators [who] continue to seek to damage Marineland at all costs”.

The park was the focus of a 2012 investigation by the Toronto Star, which interviewed a number of employees alleging incidences of animal neglect. The park is contesting the claims.

“Marineland is in what can only be described as a significantly worse condition than when I spoke out in 2012. There has been exactly no investment in improvement to the animal life support systems whatsoever,” said Demers.

Marineland has vigorously denied the claims and has been in a legal battle with Demers over the last seven years, alleging he attempted to steal a walrus – an allegation he calls “absurd”.

Despite pressure from critics, the park continues to house an estimated 51 beluga whales. It also has five bottle-nosed dolphins and a single orca named Kiska.

Facebook Twitter Pinterest A male beluga whale at Marineland. Photograph: Denis Cahill/AP

Proposed legislation in the Canadian parliament – dubbed the “Free Willy bill”– is likely to receive royal assent in the coming weeks and would ban the captivity of cetaceans, including orcas, dolphins and beluga whales across the country. The province of Ontario previously passed similar legislation, which banned the acquisition of large marine mammals, but allowed Marineland to keep its whales.

Ever since the park’s owner, John Holer, died last year, the company – Holer Family Amusements – has been at a “crossroads”, the Niagara Falls mayor, Jim Diodati, told the Toronto Star, as it explores a possible expansion of the park, or sale of its land to developers.