Japanese fishing authorities said their whaling mother ship in the antarctic was "harassed" by Sea Shepherd members.
A vessel operated by the group started trailing Japanese whaling ships when they entered the area, London newspaper The Guardian reports. Ships carrying whaling harpoons can't harvest their catch without the mother ship, which carries processing facilities.
A Japanese fleet in the region has processed fewer than 100 whales since it arrived in the region last year. Japan can kill as many as 1,000 whales per year for "scientific research" under a 1986 agreement with the International Whaling Commission.
Whaling officials said the fleet decided to halt "scientific whaling for now" because of the pressure from the conservation group, The Guardian adds.
Japan is one of three countries that hunt whales.
Environmental activist group Greenpeace in a December survey said that most people told interviewers that they didn't approve of Japan's whaling practices.