Paul Watson, militant anti-whaling group Sea Shepherd's director-turned-observer, said this year would likely see the lowest haul by the Japanese whalers in history, with "no more than 75" of the mammals killed due to the group's campaign of harassment.
Sea Shepherd said Japan's entire harpoon fleet had left the Antarctic whale sanctuary on Saturday and was heading north, suggesting that their annual hunt may have come to an end.
That compares with a catch of 267 last year - 266 minke whales and one fin whale - and is dramatically below the target of 935 minke whales and up to 50 fin whales set for this season by Japan's Institute for Cetacean Research.
"The entire Japanese whaling fleet is now north of sixty degrees and out of the Southern Ocean Whale Sanctuary," Watson said.
"Is whaling over for the season? We are not positive but we are 80% sure that it may be over."
"This campaign will see the lowest take by the Japanese whaling fleet in the entire history of their Antarctic whale hunts."
Watson said the Korean-owned, Panamanian-flagged supply tanker the Sun Laurel was currently 48 hours from the factory ship Nisshin Maru, with a four-day return trip to the whaling sanctuary looking increasingly unlikely.
"This would leave about a week to kill whales and with the weather quickly deteriorating it would hardly be worth the effort."
Watson said Sea Shepherd had seen the Japanese kill just two minke whales, and they had only had two days of unobstructed hunting in the whole season, which began in late December.
"My conservative estimate of the number of whales killed this year is no more than 75. It could be much lower but certainly not higher," he said.
Watson described the campaign, in which each side accused the other of ramming attacks, as "enormously successful" and said Sea Shepherd would "continue to follow the whaling fleet north to ensure that they do not return to kill whales".
The Sea Shepherd Conservation Society has chased the Japanese fleet hunting whales off Antarctica for several years in a bid to stop the animals being slaughtered.
Japan says it conducts vital scientific research using a loophole in an international ban on whaling, but makes no secret of the fact that the mammals ultimately end up as food.