Both Greenpeace and the radical Sea Shepherd Conservation Society will leave the Southern Ocean in days, leaving the Japanese to resume the hunt for almost 1,000 whales.
"If we can get the money for extra fuel we'll go back," Sea Shepherd skipper Paul Watson told Reuters, estimating his protest ship, the Steve Irwin, would be forced to turn for port in Australia as fuel supplies dwindle.
"We're 50-50 right now. We need around $200,000 for another load of fuel," Watson said.
Japan's six-ship whaling fleet has been trying to avoid protest ships in Australia's self-declared whaling sanctuary near Antarctica after protesters stopped whaling operations when two Sea Shepherd activists boarded a harpoon boat.
Greenpeace said on Saturday its anti-whaling ship, the Esperanza, was also returning to port after 14 days chasing the whaling fleet's factory ship, the Nisshin Maru.
But both Sea Shepherd and Greenpeace have been at loggerheads over tactics, with Greenpeace accusing Watson's group of placing lives at risk with past threats to ran the Japanese flagship and this year's high-seas boarding in frigid seas.
Watson in turn accused Greenpeace of raising millions of dollars for its campaign against whaling, but doing little to stop the Japanese other than harassing the fleet with inflatable boats to delay refueling of its whalers.
"We're trying to stop whaling, not use it as a fundraising tool," he said. "They say we are too dangerous. Of course it's dangerous, it's not a game down here and we are trying to stop them killing whales."
Japan plans to hunt almost 1,000 minke and fin whales for research over the Antarctic summer, but has abandoned the cull of 50 humpback whales after international condemnation and a formal diplomatic protest by 31 nations.
Despite a moratorium on whaling, Japan is allowed an annual "scientific" hunt, arguing whaling is a cherished tradition and the hunt is necessary to study whales. Its fleet has killed 7,000 Antarctic minkes over the past 20 years.
Australia's government has sent a fisheries patrol ship and aircraft to shadow the Japanese fleet and gather evidence for an international legal challenge to the whaling program.
Australia's Foreign Minister Stephen Smith will meet his Japanese counterpart within days to discuss the challenge, but said continued whaling would not damage diplomatic relations between the two allies and trading partners.
Watson said the Japanese were frustrating activists with a large ocean-going trawler, the Fukoyoshi Maru No.68, shadowing the Sea Shepherd boat and reporting its position.
"Every move that we make is relayed to the Japanese fleet. We keep them on the run, which is a good thing, but we can never catch them while these guys are on our tails," he said.
Japanese whaling officials have said they plan to try to exhaust the fuel supplies of the anti-whaling ships before they resume whaling.
Watson said if he could not find backing to return south this year, he would go back next year with two ships to chase the whaling fleet in shifts.
"We'll have one vessel out and another one three weeks later. We want to be able to have a ship out here for the whole time, because if we keep these guys running they are not going to kill whales," he said.
Editing by David Fox
© 2008 Reuters