In 2010, Dawn Brancheau, an experienced 40-year-old trainer at Florida's SeaWorld, was killed by the Orlando theme park's star attraction, a 12,000lb bull orca named Tilikum. SeaWorld officials called it an accident and were keen to identify Dawn's ponytail – a potential plaything for a killer whale, it was claimed – as a contributing factor.

The tragedy, which unfolded in front of visitors just after a performance of the park's signature Dine with Shamu show, provides a springboard for director Gabriela Cowperthwaite's gripping new investigative documentary, Blackfish.

“I heard about the story when it was in the news and thought it was so tragic,” recalls the filmmaker. “I couldn’t imagine how a beloved trainer could end up being killed by a highly intelligent animal that she loved and who presumably loved her. That was my image of SeaWorld. I thought it was a happy place where killer whales were happy and would never be driven to kill.”

As soon as she started asking questions, Cowperthwaite, who had no previous history of animal activism and had no ambition to make an advocacy film, found herself shocked by what lay behind the glitzy theme-park facade.

“I’m a documentary filmmaker,” she says, “but I’m also a mom who took her kids to SeaWorld. I never imagined I’d end up making this kind of controversial film. I never imagined I’d end up questioning SeaWorld’s practices and entire belief system.”

Brancheau's death, as the award-winning Blackfish reveals, was not an isolated occurrence. To date, Tilikum has been involved in the deaths of three people. The first incident dates back to 1991 when Tilikum and two female whales mauled and pulled a young trainer named Keltie Byrne to her death. Following Keltie's death, Tilikum was moved to SeaWorld, Orlando, where, seven years later, the animal was involved in a second fatality. In July 1999, SeaWorld employees found the body of Daniel P Dukes (27) draped across Tilikum's back, with multiple wounds.

SeaWorld Parks and Entertainment, a publicly traded company since last year, has been quick to dismiss claims that its trainers were not "adequately informed about Tilikum". Blackfish, though, presents various testimonies from former employees to the contrary.

“SeaWorld is claiming now that every trainer gets the quote-unquote ‘Tilly talk’ – that they explain that you do not want to be in the water with this animal; that the outcome is uncertain,” says Cowperthwaite. “However, if they had been open about the details of the previous cases, people would understand that they don’t even have to be in the water to be in danger. Tilikum has the ability to grab a trainer and pull them in much like he did with Dawn.”

Loving marine animals
Each of the SeaWorld trainers tells a similar story about their time at the park: they take the job at SeaWorld because they love marine animals, then, ultimately, they leave the job at SeaWorld because they love marine animals.

"I think that's right," says Blackfish's director. "The experience makes you start to examine what it is to truly love an animal. You can love it up close and want to touch it and imagine that the animal is benefiting tremendously from your touch and your hugs. But with whales, once you learn how much space they need, you know they're not benefiting from captivity. They swim a hundred miles a day in the wild. They are social animals who need their families, not you. You're getting something out of the interaction that they will never get."

Footage depicting the moments just before Dawn's death – as with Werner Herzog's Grizzly Man, Blackfish cuts away before the violence begins – sees Tilikum behaving petulantly as his bucket of fish gets close to the end. His visible strop soon turns to murderous rage.

“With these animals – much like a lot of intelligent predators – there are two sides to the story,” says Cowperthwaite. “Killer whales are capable of social interaction. They are tremendous parents. They appear to be capable of something like love and empathy: although we need to be careful with those very human words. They display highly evolved emotional behaviours. Then there is the other side.

“That’s not as convenient for us to think about. This is a top predator capable of taking down a great white shark. They eat dolphins. They become incredibly bored in captivity and are capable of becoming very aggressive, of enacting punishment, of killing. But that side of the equation certainly does not sell Shamu dolls.”

Tilikum was captured off the Icelandic coast in 1983. His biography, to date, has been characterised by boredom and bullying in a series of relatively tiny concrete pools. He is currently the largest bull orca in captivity and has sired at least 11 whales in marine parks around the world. In terms of a genetic legacy, is this sensible?

“It is pretty unprecedented in terms of the breeding industry,” says Cowperthwaite. “It’s unheard of to be actively breeding an animal who has displayed this kind of aggression toward human beings. In the cases of canines, for example, when an animal kills or attempts to kill, the usual response is to euthanise the animal.”

Free-roaming beasts
Is Tilikum simply a rogue whale? Hardly. Dawn Brancheau's death occurred just two months after a 6,600lb orca killed trainer Alexis Martínez at a marine park in the Canary Islands. Could it be that keeping huge free-roaming beasts as aquatic clowns for our amusement is just a bad idea?

“The SeaWorld PR machine is so powerful and their parks are so glossy,” says Cowperthwaite. “You’re almost anaesthetised when you’re there. The park hits you with every seduction and stimuli it can. The music, the colours. And then the whale splashes you and breaks the fourth wall and you feel connected to him. It’s happiness. And it has just gotten the better of us.

“We forget that this can’t be sustainable, that we can’t keep a 12,000lb free- roaming predator in a concrete pool. You can even forget about ethical if you want. It is unsustainable.”

Happily, there is an alternative to bored, angry, colossal sea mammals living in cramped swimming pools. Cowperthwaite and many of her interviewees favour the idea of a sea pen.

“I am tremendously hopeful that SeaWorld could play a big role in evolving past the current model,” says the film-maker. “The alternative we’re suggesting is turning their parks into rehabilitation and release facilities. Another alternative is the sea pen, which involves cordoning off an ocean cove with a net and retiring your cast of whales.

“The captive whale at SeaWorld would not survive the ocean. A lot of the animals are on antibiotics. They don’t know how to eat live fish or chase food down, but in an open sea pen, trainers can monitor their health and the operators can even charge admission to view them. This could be a profit-driven endeavour and the whales would be allowed to be whales for the first time in their lives.”

Blackfish opens today at the Light House cinema in Dublin