|Environmental campaigners visit Taiji, Japan, every year to protest an annual slaughter of dolphins AFP/Toshifumi Kitamura|
A fisherman in the city of Taiji noticed that four bottlenose dolphins from the facility were swimming outside their netted enclosure in the early morning, police said.
"We are investigating the case on suspicion of criminal damage," a police official in the nearby city of Shingu told AFP.
He said two of the nets at the DolphinBase park were apparently cut with a sharp object, allowing the mammals to escape, before three of them returned to the enclosure of their own volition. A fourth dolphin was swimming nearby.
Taiji, a small port in western Japan's Wakayama prefecture, was thrust into the global spotlight after the 2009 documentary "The Cove".
The Oscar-winning film depicted an annual dolphin slaughter in the area, where some of the animals are also captured and sold to aquariums.
Environmental campaigners visit Taiji every year to protest the slaughter and authorities have boosted security to prevent clashes between locals and activists. Defenders of the hunt say it is a tradition and point out that the animals are not endangered, a position echoed by the Japanese government.
In September 2010, a net owned by a local fishermen's union in Taiji was cut, with an Amsterdam-based environmental group claiming responsibility for the act, but no arrests were ever made in that case.
DolphinBase officials could not immediately be reached for comment on Thursday's incident, but expressed outrage on their blog.
"We feel furious about this egoistic and irresponsible act," they said.
Ric O'Barry, the director of "The Cove", was detained in Japan last year for nearly three weeks after being denied entry to the country, and was eventually deported.
O'Barry's Dolphin Project, a dolphin protection and campaign group, criticised the alleged cutting of the nets.
"While we are against keeping dolphins in captivity, we do not condone illegal behaviour," it said on its web site.