In the Japanese town infamous for its bloody and controversial dolphin hunts, four dolphins have escaped their enclosure.

A fisherman spotted the four bottlenose dolphins swimming outside their netted outdoor enclosure at DolphinBase center in Taiji this week, AFP reports. Members of staff from the facility believe the netted enclosure had been slashed open by a sharp object. Police are now investigating the case on suspicion of criminal damage.

The dolphins managed to escape through holes in the net. According to the DolphinBase blog, three returned to their enclosure on their own accord, but one remains outside and is swimming nearby.

DolphinBase added that dolphins rarely swim too far from their pod, especially if they are in an established group. They also described the actions of the culprit as a “selfish and irresponsible act” that made them “intensely angry.”

Taiji was put under the spotlight in the Academy Award-winning 2009 documentary The Cove, which investigated the mass hunting practices of both the town and Wakayama province. The annual hunt takes place between September and March, and involves fishermen aboard boats striking the water with mallets to draw the dolphins to the shore. After they net the surrounding area, over a thousand dolphins and small whales are either captured or killed by the slice of a knife, leaving the water red with blood. They are then sold as meat.

You can see raw video footage of the hunt in Taiji from the Oceanic Preservation Society below.

Following the film’s success, grim photographs showing the blood-red shores of Taiji during the dolphin hunts often circulate online. The practices of this western Japanese town have also become the subject of protests by conservationists and condemned by a host of famous faces, including musician Sting and Game of Thrones actress Maisie Williams.

Activist group Ric O'Barry's Dolphin Project denounced the recent episode, saying "while we are against keeping dolphins in captivity, we do not condone illegal behavior. Our Cove Monitors operate fully within Japanese law, documenting Taiji’s dolphin drive hunts for the Japanese people and the rest of the world to see – including the horrific capture methods and continued slaughters."

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