Peter Bethune, a protester with Sea Shepherd, was detained Monday aboard the Shonan Maru 2, which last month collided with and sank the Ady Gill, a Sea Shepherd boat that Bethune was aboard when it went down.
Bethune boarded the Shonan Maru 2 Monday while it was under way in the Southern Ocean so he could make a citizen's arrest of its captain and present the crew with a $3 million bill for the loss of the Ady Gill, the Australian Broadcasting Corp. reported.
Glenn Inwood, a New Zealand-based spokesman for the whalers, accused Bethune of piracy.
"It doesn't matter what his flight of fantasy was, whether he thinks he was there to make an arrest, whether he thinks he was there to deliver a bill, it's illegal," Inwood said.
Bethune cut his hand boarding the ship and was being treated by a doctor on the ship as Japanese authorities considered their next move, Inwood said.