The report’s authors used various satellite technologies to analyze marine traffic in northeast Asia in 2017 and 2018 and found that hundreds of Chinese fishing vessels were sailing in waters off North Korea. The Chinese ships appeared to be fishing there illegally, pushing North Korea’s own fleet, which is poorly equipped to travel long distances, further away from the North Korean coast and into Russian and Japanese waters.

But that does not appear to have deterred some 900 Chinese ships in 2017 and 700 the following year, according to Global Fishing Watch’s report.

The nonprofit said these Chinese ships likely caught more than 160,000 metric tons of Pacific flying squid, one of the region’s most valuable seafood products, in 2017 and 2018 — more than South Korea and Japan combined during the same period. The estimated catch was worth more than $440 million.

Jaeyoon Park, a senior data scientist at Global Fishing Watch and co-lead author of the study, said that the vessels spotted comprised “about one-third the size of China’s entire distant water fishing fleet.”

“It is the largest known case of illegal fishing perpetrated by vessels originating from one country operating in another nation’s waters,” he said.

With so many ships near the North Korean coast, the country’s own fishing fleet was then pushed out, forced to sail further away from shore to find their catch, and the consequences were deadly, according to Jungsam Lee, another one of the study’s co-authors.

“It is too dangerous for them to work in the same waters as the Chinese trawlers,” Lee said. “That’s why they’re pushed to work in Russian and Japanese waters and that explains why some of North Korea’s damaged vessels showed up on the beaches of Japan.”

CNN has reached out to China’s Ministry of Foreign Affairs for comment.

A sustainability issue

Northeast Asia’s waters are some of the most opaque and contested seas and fisheries anywhere in the world. China, Russia, Japan and the two Koreas are all engaged in some sort of territorial dispute with one another.

Fish stocks there have been declining dramatically in recent years, another major problem that the parties have failed to work out. Pacific flying squid stocks have dropped by 80% in South Korean waters and 82% Japanese waters since 2003, according to Global Fishing Watch.

Experts like Park believe that though Japan and South Korea have worked independently worked to make squid fishing more sustainable, “the absence of multilateral cooperation and information-sharing between all the countries involved in this transboundary fishery means it is impossible to get sound science and a regional management plan in place for the stock.”

South Korea’s Ministry of Oceans and fisheries said it was reviewing the findings, while Masanori Miyahara, the president of the Japan Fisheries Research and Education Agency, said in a statement accompanying the Global Fishing Watch study that the lack of shared data is “is a major challenge considering the critical importance of squid in the region.”

“Illegal fishing in these waters is a very serious matter in Japan,” Miyahara said.

Park said his team’s research has highlighted a “fundamental failure in properly and transparently managing a shared resource” and that there is an “urgent need for cooperation between the countries involved in this fishery.”