They did it on porpoise.

It took 13 years to make “Avatar: The Way of Water” and mere minutes for animal lovers to get upset about a stunt involving live, captive dolphins at the Japan premiere Saturday, Yahoo News reported.

Director James Cameron and actors Zoe Saldaña, Sam Worthington and Sigourney Weaver attended the Tokyo event, which featured dolphins performing tricks in a pool to orchestral music.

“I’m sure everybody asked their permission to be in the show,” joked Cameron in a video of the affair. “I love these animals, I love their intelligence.”

Dolphin fans expressed varying degrees of “heartbreak” and “anger” over the spectacle. Some even threatened to boycott the film entirely.

“I would love to see James Cameron pledge to never attend a dolphin show ever again and to denounce the cruel capture and slaughter of dolphins happening in Taiji, [Japan],” activist Bailey Mason urged in an Instagram.

The Post reached out to Cameron, 68, for comment.

Animal welfare activists say they initially believed the mammals were digitally produced because Cameron is vegan, and he creates movies with strong environmental messaging.

Lisa Lange, senior vice president of communications for People for the Ethical Treatment of Animals, said she thought the dolphins were animatronics, or some kind of “movie magic” was used — but the more she watched, the more she realized they were real.

“To see James Cameron, Sigourney Weaver and Sam Worthington sitting there applauding was shocking,” Lange told Yahoo News. “The trainers were treating those dolphins like circus clowns. They were riding on the noses of the dolphins, I mean, that’s as bad as it gets.”

Ric O’Barry, who was featured in the Oscar-winning documentary “The Cove” — which explored the horrors of the Japanese dolphin industry — called the promotional performance “crazy.”

O’Barry trained dolphins for the 1960s television show “Flipper” before quitting and advocating against dolphin captivity.

The World Animal Protection group said the stunt was “confoundingly out of step” with Cameron’s reputation.

“Captive dolphins are confined to a tiny, barren, artificial environment 200 thousand times smaller than their natural habitat. They endure severe suffering and distress and live much shorter lives on average than their wild counterparts,” US programs director Cameron Harsh said in a statement.

“Avatar: The Way of Water,” meanwhile, continues to make waves at the box office. The film earned $15.8 million in 15 overseas markets on its opening day, far surpassing the original “Avatar” and “Top Gun: Maverick,” reported Variety.