Map showing where North Korean sailors and deceased suspected North Koreans have drifted to Japan in November 2017. (via AFP/Newscom)

Mysterious “ghost ships”–decrepit wooden boats that are sometimes carrying skeletons–have been washing ashore in Japan in recent weeks.

Last Thursday, the Japanese coast guard recovered one such vessel which was drifting off the western coast of the island of Hokkaido. Ten men were found on the boat and identified themselves as North Koreans. On Monday, another wooden boat containing eight partially decomposed bodies was found on the coast of Japan’s Akita prefecture, which faces North Korea across the Sea of Japan. Previous ghost ships recovered in November also contained crew who claimed to be North Korean fishermen.

According to Jeff Kingston, director of Asian studies at Temple University Japan, about 500 crude vessels thought to be North Korean have washed up on Japan’s coastline since 2011.

“What it signifies to me is the conditions in North Korea are exceptionally grim,” Kingston told ABC News in an interview in Tokyo.

According to Kingston, the increase in suspected North Korean ships arriving on Japan’s shores is attributed to North Korean leader Kim Jong Un’s mandate to expand his country’s fishing industry to counter the nation’s well-documented food shortages. Most of the people and corpses found on these boats are believed to be inexperienced fishermen or refugees attempting to escape North Korea.

Another factor could be North Korea’s reported sale of its fishing rights in its coastal waters to China. “So Chinese fishing trawlers are going through the coastal waters of North Korea and this is pushing North Korean fishermen out beyond their comfort zone into the dangerous Sea of Japan, and some of them aren’t surviving,” Kingston said.