The pod were freed from the Dolphin Base, a captive dolphin facility in Taiji, western Japan, a town known for it controversial annual hunt of the marine mammals.

Three of the bottlenose dolphins returned to the centre on their own, while the other member of the pod remains free just outside of the enclosure.

"We are enraged by the heinous act which can easily lead to the dolphins dying," they wrote.

"They think that once out of their pen, dolphins will swim far away but that is not true. Dolphins will not stray far and they will not leave their group."