The 2009 documentary The Cove makes engrossing viewing, combining a conservation message with undercover surveillance of a small cove at the town of Taiji in Wakayama Prefecture, Japan. The filmmakers are not welcome in Taiji, as the face of this man shows. "Go home!" he shouts at the camera.

Like Chinese plainclothes police who manoeuvred their umbrellas in front of Western journalists who visited Tiananmen Square last year for the 20th anniversary of the political demonstration that culminated in bloodshed, the guy in this photo and others like him did everything possible to prevent Ric O'Barry and his co-conspirators from filming in the area of the dolphin hunt.

Although it's not really a hunt, in any sense of the word. The fishermen merely harvest dolphins in industrial-scale boats. The dolphins are speared with short harpoons, are loaded onto transports, and are removed for butchering.

O'Barry is passionate about saving these dolphins. He holds a special place in dolphin lore as he was the trainer of the dolphins used in the TV series Flipper and has been actively working to free dolphins wherever possible for many years.

The group of specialists recruited for the purpose of making the film worked for months preparing high-tech equipment including special rocks for hiding high-definition cameras, underwater sound-recording equipment, and even a small dirigible for filming from the air. Getting the gear onto the plane and into Japan was a fraught process.

They were so sure, after their arrival, that they were being followed they organised decoy vehicles to draw the police away from active group members, on the night the team set out to plant the fake rocks, set up the cameras, and finalise the equipment. The next day, the fishermen did not know they were being filmed.

A special moment happens in the film near the end with a representative of the fisheries department. The dolphins are killed with special knives, he tells his interviewer, that are plunged into the creature's spine, killing it instantly. A video camera is produced and the shocked functionary watches the footage captured only a few days beforehand. "Where and when did you take this?" he asks.

When will the slaughter end? Japan now has seen, in limited release, a film that to make took copious ingenuity and boundless goodwill on the part of many individuals. Japan also now knows that dolphin meat contains high levels of mercury. My guess is that it is only a matter of time before the practise of killing dolphins is officially banned in Japan.