Tokyo , Japan ( CNN ) -- Japanese authorities arrested an activist from New Zealand on Friday for illegal trespassing after he boarded a whaling ship last month in waters off the Antarctic . Peter James Bethune , 44 , is accused of jumping from a Jet Ski onto the Shonan Maru 2 , the security ship of a Japanese whaling fleet , after the Ady Gil boat , of which Bethune was captain , sank . Bethune belongs to the activist group Sea Shepherd Conservation Society . The group said the New Zealander was attempting to make a citizen 's arrest of the Shonan Maru 2 skipper for the collision that sank the Ady Gil , a futuristic vessel used to intercept and block harpoon ships , in January . Hirotaka Akamatsu , Japan 's minister of agriculture , forestry and fisheries , said Japan would take a firm stance against Bethune . He said Sea Shepherd 's acts were not acceptable . `` Their violent acts are escalating , '' Akamatsu said . Bethune was taken into custody a month ago aboard the Shonan Maru 2 , which arrived from Antarctic waters -- where Japan conducts its annual whale hunt -- back to Japan on Friday . He was formally arrested then . Bethune has legal representation in Japan , said Sea Shepherd spokeswoman Traci Walter . A spokesperson for the New Zealand Ministry of Foreign Affairs and Trade said the government was providing consular assistance to Bethune on his arrival in Japan . The Bethune case highlights an ongoing feud between Japanese whaling fleets and conservation agencies , especially the hardline Sea Shepherd . Activists have used butyric acid -- found in rancid butter and vomit -- and fired paint guns at Japanese whaling ships , which in turn have repelled protesters with water cannons . Japan 's Institute of Cetacean Research , a branch of the fisheries ministry that deals with whaling , accuses Sea Shepherd of jeopardizing the safety of fleets that are conducting research legally . Sea Shepherd uses its boats to interfere with whaling and fishing boats , and its efforts have included ramming a Portuguese whaler in 1979 . Two years ago , Sea Shepherd activists boarded a Japanese ship and handcuffed themselves to the vessel with plastic ties . Japanese authorities had called the Shonan Maru 2 incident the latest `` publicity stunt '' by Sea Shepherd activists . Sea Shepherd has accused the Shonan Maru 2 of destroying the Ady Gil during the skirmish in the Southern Ocean -- a term used to describe parts of the Indian , Pacific and Atlantic oceans that surround Antarctica . However , Japanese authorities deny their ship intentionally hit the high-tech catamaran and have said the activists were harassing their whaling fleet . In the early 1980s , the International Whaling Commission determined that there should be a moratorium on commercial whale hunting . But whaling is allowed under international law when done for scientific reasons , which Japan cites as the legal basis for its hunts . The country 's annual hunt kills up to 1,000 whales a year .
Bethune belongs to Sea Shepherd Conservation Society . Group : Bethune was trying to make citizen 's arrest of Shonan Maru 2 skipper . Skipper was involved in collision that sank Ady Gil , which targets harpoon ships . Sea Shepherd : Bethune has legal representation in Japan .