The Chinese fishermen then sell the products in China labeled as domestic catch. The source said that this has been going on for a long time.

RFA quoted a source in China as saying that "maritime markets" crop up at night on the West Sea, where Chinese trawlers buy fish and other seafood from North Korean vessels.

The North continues to export seafood to China despite a blanket ban under UN sanctions by trading it on the high seas before it is labeled as Chinese, Radio Free Asia reported Thursday.

Another source who travels back and forth by ferry from Incheon to Dandong and Dalian, China, told RFA, "When you stand on the deck of the ferry at night, you can see many fishing boasts that flicker their lights. Those are signals exchanged by North Korean and Chinese vessels to engage in illicit trade on the open seas."

It is apparently an open secret among Chinese fishermen that it is cheaper to pay U.S. dollars for North Korean fish and other seafood than it is to hire Chinese crew.

Chinese fishermen pay for the fuel of North Korean fishing boats and buy their catches on the high seas for dollars. One North Korea expert in Beijing said, "As long as this type of illicit trade is allowed to continue, you can say there is a glaring hole in the latest sanctions against North Korea."

According to the Korea Trade-Investment Promotion Agency, China officially imported $192.5 million worth of North Korean fish and seafood products last year, up 75.9 percent compared to 2015 and making up 7.8 percent of bilateral trade, up from 4.4 percent in 2015.