Anti-whaling activist group Sea Shepherd said Wednesday it had intercepted the Japanese fleet in its annual Southern Ocean hunt "before a single harpoon has been fired".
Sea Shepherd claims to have saved the lives of 4,000 whales over the past eight whaling seasons with ever-greater campaigns of harassment against the Japanese harpoon fleet.
The militant environmentalist group said the Brigitte Bardot, a former ocean racer, had intercepted the harpoon ship Yushin Maru No. 3 in the Southern Ocean at a relatively northern latitude.
"Given that the large concentrations of whales are found further south, closer to the Antarctic continent where there are high concentrations of krill, this would indicate that they have not yet begun whaling," said Brigitte Bardot captain Jean Yves Terlain.
Former Australian politician Bob Brown, who assumed leadership of the anti-whaling campaign from fugitive Sea Shepherd founder Paul Watson due to legal issues earlier this month, said it was welcome news.
"It is likely that we have intercepted these whale poachers before a single harpoon has been fired," said Brown.
Watson is wanted by Interpol after skipping bail last July in Germany, where he was arrested on Costa Rican charges relating to a high-seas confrontation over shark finning in 2002.
He is on board Sea Shepherd's main ship, Steve Irwin, but has stepped down as skipper and has vowed to abide by a US court ruling in December banning the group from physically confronting any vessel in the Japanese fleet.
The ruling by the US Court of Appeals for the Ninth Circuit requires Sea Shepherd to stay at least 500 yards (metres) from whaling vessels and prohibits "navigating in a manner that is likely to endanger the safe navigation of any such vessel".
The whaling fleet left Japan for the Southern Ocean in late December, planning to catch up to 935 Antarctic minke whales and up to 50 fin whales.
Tokyo claims it catches whales for scientific research -- a loophole in the international ban on whaling -- but makes no secret of the fact that they ultimately end up on dinner plates.
Sea Shepherd's campaign this year is its biggest yet, involving four ships, a helicopter, three drones and more than 100 crew members.