SYDNEY (AFP) -- Militant environmental activists on Saturday vowed to increase their presence in the Southern Ocean next year in their bid to prevent Japanese whalers from killing the giant mammals.

Paul Watson, captain of the Sea Shepherd Conservation Society vessel the Steve Irwin, said his ship had stopped the Japanese fleet from killing whales for three to four weeks but was now forced to return to port to refuel.
Next year he wants to bring two ships into the Antarctic waters.
""We're aiming to come back next year with two ships which will be staggered, so they'll work as a tag team -- once one ship returns to port to refuel, the other ship can be out chasing the fleet,"" he said.
""The best way to stop their criminal action -- because that's what this senseless slaughter is, criminal -- is to keep on them. ""It's going to be very expensive but it will be worth it.""
Japan, which uses a loophole in a 1986 global whaling moratorium that allows lethal research, aims to slaughter about 1,000 whales this year despite strong opposition from Western countries and environmental groups.
Watson said the Japanese had not killed any whales while being tagged by the Steve Irwin and he was satisfied he would be able to prevent them from killing for seven weeks by the time the hunt was over. ""Our actions are going to have a significant impact on the Japanese quota,"" he told Australian Associated Press.
Sea Shepherd's Steve Irwin, along with the Greenpeace vessel Esperanza, pulled out of the pursuit this week to head back to port to refuel.
The Japanese reportedly began killing whales soon after, prompting Australian Foreign Minister Stephen Smith to voice his ""disappointment"" during a meeting with his counterpart in Tokyo late Thursday.
The Sea Shepherd made international headlines last month when two of its activists boarded one of the Japanese ships and were later removed by an Australian Customs boat -- a tactic Watson said could be repeated.
""The circumstances dictate the tactics we use, but boarding one of their ships again might be a possibility,"" he said. ""By far the most effective method is to keep chasing the fleet. They cannot do anything while we are following them and getting in their way.""