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.
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," CNN reported
Japan has continued to reject international orders to stop its program, alleging that its whaling activities are vital to a larger body of research, as opposed to commercial purposes.
Scientific research gets exemption from the 1986 international ban on commercial whaling. But the International Court of Justice rejected Japan's scientific claims and ordered an end to its JARPA II research, which claims to study the maintenance and improvement of the minke whale population and the effects of environmental changes on the whale's food supply, according to its website.
Japan launched a new research program after the court ruling in 2014 that says 333 whales could be killed annually, according to Japan's Fisheries Agency and Ministry of Foreign Affairs.
The four vessels left the port of Shimonoseki, southwest of Tokyo, in December 2015 and returned Thursday. The expedition was part of a 12-year program that will kill 4,000 minke whales.