Sea Shepherd said a tracking device on the tanker Sun Laurel showed it appeared to be making a change of course, heading south back toward whaling grounds in the Southern Ocean, the Australian Broadcasting Company reported Monday.
"I don't know why they're doing this; it doesn't make any economic sense," said Sea Shepherd founder Paul Watson.
"They won't be able to take many whales but we don't want them to take any," he said. "It's a long ways back to the Southern Ocean, [they're] running out of time, they've only got about 10 days left until they can no longer stay down there."
The Sea Shepherd vessel Bob Barker was returning to the Southern Ocean in pursuit of the Sun Laurel. Watson said he believed another Japanese ship, the Nisshin Maru, was also heading back.
Commercial whaling has been banned since 1986. The Sea Shepherd organization has been involved in protests against Japanese whalers, who they say take advantage of an International Whaling Commission rule that allows whales to be killed for scientific research, with the meat then sold commercially.