The anti-whaling group Sea Shepherd said Sunday three members from the group Forest Rescue boarded the Japanese vessel Shonan Maru 2 under cover of darkness the previous night off Bunbury, Western Australia, the Australian Broadcasting Corp. reported.
Forest Rescue spokesman Michael Montgomery said the group hopes the Japanese boat will have to give up its shadowing of Sea Shepherd's flagship vessel, the Steve Irwin, to return the three men to land.
The Steve Irwin was towing another of the organization's boats, the Brigitte Bardot, to Fremantle.
An anti-whaling activist who boarded the Shonan Maru 2 last year received a suspended sentence and was deported from Tokyo after spending five months in prison, the ABC said.
Notable deaths of 2014 [PHOTOS]
Syrian Al Qaida group executes Lebanese soldier