Story highlights Japan confirms that it took 333 minke whales

Four vessels set off for Antarctic waters in December

(CNN) Japan's whaling fleet has returned with more than 300 whales harvested from Antarctic waters, according to the country's Fisheries Agency.

A four-ship fleet from Japan's Institute of Cetacean Research traveled to the Antarctic Ocean and killed 333 minke whales. Some 230 were female; about 90% of these were pregnant, according to the report.

The research was conducted as part of an effort to understand the minke whale populations in the Antarctic Ocean, the Japanese Ministry of Fisheries said in a statement on its website. The purpose was to study the best methods for managing minke populations, the ministry said. It said there were no incidents with anti-whaling activists.

In the past, opponents, including New Zealand and Australia, have raised concerns about the legitimacy of the scientific research contention. In 2014, the United Nation's International Court of Justice ordered Japan to halt its whaling program, over concerns of its whaling activities in the Antarctic region.

Photos: U.N. to Japan: Stop whale hunting Photos: U.N. to Japan: Stop whale hunting U.N. to Japan: Stop whale hunting – A crew member mans the harpoon on a Japanese whaling vessel in the Antarctic in 1993. Japan has gotten around the international ban on whaling by saying it's conducting "scientific research," but the U.N. International Court of Justice banned the practice, ruling that the "scientific research" rationale is a smokescreen for commercial whaling. Hide Caption 1 of 6 Photos: U.N. to Japan: Stop whale hunting U.N. to Japan: Stop whale hunting – A dwarf minke whale swims in the Great Barrier Reef, Australia. Japan has killed 3,600 minke whales since 2005. The U.N. court noted Japan had produced only two peer-reviewed papers on minke whales since 2005 and thus "the scientific output to date appears limited."

Hide Caption 2 of 6 Photos: U.N. to Japan: Stop whale hunting U.N. to Japan: Stop whale hunting – The Japanese whaling ship Yushin Maru No. 1 cuts through the ice flows in the Southern Ocean in Antarctica on January 25, 2011. Japan has hunted for whales extensively in the Southern Ocean, which includes a whale sanctuary. Hide Caption 3 of 6 Photos: U.N. to Japan: Stop whale hunting U.N. to Japan: Stop whale hunting – A blue whale comes up for air while feeding in Monterey Bay near Monterey, California, on July 1, 2012. Hide Caption 4 of 6 Photos: U.N. to Japan: Stop whale hunting U.N. to Japan: Stop whale hunting – A minke whale is unloaded at a port in Kushiro, in the northernmost main island of Hokkaido, Japan, on September 2013. Hide Caption 5 of 6 Photos: U.N. to Japan: Stop whale hunting U.N. to Japan: Stop whale hunting – Three dead minke whales lie on the deck of the Japanese whaling vessel Nisshin Maru in the Southern Ocean. Hide Caption 6 of 6

On social media, Greenpeace, a longtime opponent of Japan's whaling program, stated in a tweet : "The Japanese whaling fleet defies the UN and kills 333 whales, including 200 pregnant mothers."

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