Is a real shark used or is it just computerised?

Three mechanical sharks were built, all nicknamed "Bruce", after Spielberg's lawyer.

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How Steven Spielberg's Malfunctioning Sharks Transformed the Movie Business

Producers wanted Spielberg to hire someone to train a great white — an impossibility.

The director toyed with rubber props before ultimately deciding the only real answer was to build a remote-controlled mega-shark—a 25-footer that could swim, leap in the air, and munch on human prey.

Every special effects company in Hollywood called the task impossible. Undeterred, Spielberg lured effects guru Bob Mattey out of retirement. Famous for designing the giant squid in the 1954 film 20,000 Leagues Under the Sea, Mattey assured the director he could build the perfect monster.

The mechanical nature of the shark caused some problems, which actually changed Spielbergs approach to the movie.

When Bob Mattey finally delivered Bruce, Spielberg began to panic.

On its first day on the job, the shark promptly sank to the bottom of Nantucket Sound. Within a week, saltwater had eroded Bruce’s electric motor, and he had to be refitted with a system of pneumatic hoses. Every night, Bruce also had to be drained, scrubbed, and repainted.

Even by diva standards, Bruce was high-maintenance.

Spielberg also called the shark "the great white turd".

• “I had no choice but to figure out how to tell the story without the shark,” Spielberg said. “So I just went back to Alfred Hitchcock: ‘What would Hitchcock do in a situation like this?’ ... It’s what we don’t see which is truly frightening.”

The idea of the unseen enemy completely changed the film’s direction.

Hunting Bruce, Or, On The Trail Of The 'Jaws' Shark

Bruce the mechanical shark had a dirty little secret: It didn't work very well.

Stories abound of the animatronic shark stalling the moment it hit water and sinking to the seafloor. In fact, the finicky mechanics ended up pushing the film both over budget and over schedule.

As it turns out, there is actually a fourth Bruce.

... this fourth shark was never used in the film, but it does appear to have been cast from the original Bruce mold at the time of the film.

It was made for use at the Universal Studios Theme Park in Hollywood, where it hung by its tail from 1975 to 1990.

Most of the scenes feature a mechanical shark.

Some footage of a real shark was also used:

Footage of real sharks was shot by Ron and Valerie Taylor in the waters off Australia, with a smaller-framed actor in a miniature shark cage to create the illusion that the sharks were enormous.

Computer-generated imagery was not used - Jaws was released in 1975, computers were much less powerful back then.