Tokyo (CNN) -- Japan on Friday handed over to Australian authorities three anti-whaling activists held aboard a Japanese vessel after illegally boarding the ship to protest Japan's annual whale hunt in Atlantic waters.
The three Australian men were transferred onto Ocean Protector, an Australian customs ship, a spokesman for the Japanese Fisheries Agency said, declining to be identified as is customary in Japan. The activists were released without charge.
The men had been questioned by Japan's Coast Guard after boarding the Shonan Maru #2, a patrol vessel supporting the whaling mission in Antarctic waters, on Sunday.
Australia engaged in diplomatic discussions with Japan to ensure the well-being and release of the three men. Australia opposes Japan's whale hunt, but the incident did not occur in Australian territorial waters, meaning that the country's law did not automatically apply to the case.
Japan is pressing on with the hunt despite the incident. It annually hunts whales despite a worldwide moratorium, utilizing a loophole in the law that allows for killing the mammals for scientific research.
Each year, environmental groups like Sea Shepherd face off with Japan's hunters in a high seas drama that has led to collisions of ships, the detaining of activists and smoke bombs fired back and forth between the groups.
Japan has lodged a protest with the Australian government through diplomatic channels, asking it to prevent such boardings in the future, a Japanese foreign ministry official said Tuesday.
Sea Shepherd had described the three activists -- Geoffrey Owen Tuxworth, 47, Simon Peterffy, 44, and Glen Pendlebury, 27 -- as "prisoners." The organization said the men boarded the vessel to force Japan's fleet to stop hunting whales.
This is the third time activists have boarded ships involved in the Japanese whaling fleet.
In 2008, two Sea Shepherd activists sneaked aboard a Japanese vessel. They were released back to the anti-whaling group.
In 2010, Pete Bethune, a New Zealander, also boarded the Shonan Maru #2.
Bethune was arrested, taken back to Japan, and tried in a Tokyo court. He spent five months in prison and was released on a two-year suspended sentence.