Sea Shepherd Conservation Society ships have been chasing the Japanese whaling fleet for weeks in the icy seas off Antarctica, trying to block Japan's annual whale hunt, planned for up to 945 whales.
Japan has halted the hunt since Feb. 10 after persistent "violent" disruptions by the anti-whaling protesters, said fisheries agency official Tatsuya Nakaoku.
So far, the attacks have not caused any injuries or major damage to the vessels, he said, but the protesters are throwing rancid butter in bottles and once the protesters got a rope entangled in the propeller on a harpoon vessel, causing it to slow down.
"We have temporarily suspended our research whaling to ensure safety," he said. The fleet plans to resume hunting when conditions are deemed safe, he added, but declined to say how long the suspension is planned for.
The whale hunts, which Japan says are for scientific purposes, are allowed by the International Whaling Commission as an exception to the 1986 ban, but opponents say they are a cover for commercial whaling because whale meat not used for study is sold for consumption in Japan.
The Sea Shepherd group has been shadowing Japan's whaling fleet for several years, and its campaign has drawn high-profile donor support in the United States and elsewhere and spawned the popular Animal Planet series "Whale Wars."
Japan's fisheries agency has called Sea Shepherd a terrorist group for its militant actions.
Grant Pereira, an advisory board member for the group, welcomed the halt, saying Japan "should have suspended (the hunts) 10, 15 years ago," he said. "It's morally and legally wrong to kill whales."
While whale meat isn't widely eaten in Japan, many people here believe it is hypocritical for Westerners to condemn whaling while they kill cows, pigs, fish and lambs for food. It's a cultural difference, they argue.
Last year, one of the Sea Shepherd's boats sank after colliding with a Japanese vessel. The boat's captain, New Zealander Peter Bethune, was later arrested when he boarded a whaling ship from a Jet Ski, and brought back to Japan for trial. He was convicted of assault, vandalism and three other charges and given a suspended prison term. Bethune has since returned to New Zealand.
Nakaoku said the anti-whaling ships have been chasing the Japanese fleet right behind the lead whaling ship, Nisshin Maru.
"It's extremely regrettable that our research activity has been obstructed by the acts of sabotage, which could lead to serious injuries or damage," he said. "We hope to return to normal operation as soon as possible."
The Japanese fleet left Japan late last year, and its ongoing expedition is planned for several more weeks.
Japan has failed to fulfill its catch quota in recent years due to escalating protests by the anti-whaling campaigners. Last year, Japanese whalers ended up catching only about half of their target.