ADELAIDE, Australia — An anti-whaling activist from New Zealand is in custody on a Japanese vessel and will be taken to Japan to face charges after secretly boarding the ship as part of a protest, officials said today.

Peter Bethune, a member of the U.S.-based Sea Shepherd activist group, jumped aboard the Shonan Maru 2 from a Jet Ski on Monday with the stated goal of making a citizen’s arrest of the ship’s captain and presenting him with a $3 million bill for the destruction of a protest ship last month.

The Japanese government has decided to bring Bethune to Japan for questioning, Fisheries Agency official Osamu Ishikawa said. He will be charged with trespassing and assault and tried under Japanese law, Ishikawa said.

He said officials were working out the details of how to transport Bethune to Japan — whether to keep him on the vessel, which will be at sea for a few more weeks, or to drop him off in a port call and fly him back.

The brazen boarding was the latest escalation of a campaign by Sea Shepherd to hamper Japanese whaling activities.

Japan’s Institute of Cetacean Research, which sponsors the whale hunt, said Bethune used a knife to cut the vessel’s protective net to enable his boarding and that he told whalers he then threw the knife into the sea. The crew treated him for a cut on his thumb he received while boarding, the institute said.

Under Japanese law, intruding on a Japanese vessel without legitimate reasons can bring a prison term of up to three years and a fine up to 100,000 yen (US$1,100).

Bethune was being held in a room by himself with guards posted outside, Fisheries Ministry official Toshinori Uoya said.

New Zealand Foreign Minister Murray McCully said it seemed Bethune’s intention was to be detained aboard the whaling ship, but his country nevertheless had an obligation to try to help him and was seeking cooperation from Japanese diplomats.

McCully met Japan’s ambassador today, and New Zealand’s top diplomat in Japan met senior officials there Monday.

Sea Shepherd said Bethune demanded the cost of replacing the Ady Gil, an activist ship he captained that was destroyed in a collision with the Shonan Maru 2 last month, and the surrender of the whaling ship’s captain on attempted murder charges.

The Ady Gil sank after the collision, though there were only minor injuries.

Japan has six whaling ships in Antarctic waters under its scientific whaling program, an allowed exception to the International Whaling Commission’s 1986 ban on commercial whaling. It hunts hundreds of mostly minke whales, which are not an endangered species. Whale meat not used for study is sold for consumption in Japan, which critics say is the real reason for the hunts.

The Sea Shepherd sends vessels to confront the fleet each year, trying to block the whalers from firing harpoons and dangling ropes in the water to try to snarl the Japanese ships’ propellers. The whalers have responded by firing water cannons and sonar devices meant to disorient the activists.