The rickety wooden ships wash up barnacle-encrusted and eerily quiet. Like something out of a nightmare, they’re manned only by skeletons, the passengers and crew killed by an unknown calamity. The boats’ sparse cargo and humble exteriors offer some clues about their origins — fishing nets, signs written in Korean — but do little to explain what tragedy brought them to Japan’s shores.

At least 12 of these “ghost ships” carrying 22 decomposing bodies have appeared along Japan’s northwestern shore since October, the Japanese coast guard told Reuters. Though it’s thought that the “primitive” wooden boats are Korean, the identities of the victims on board are unclear. They could be North Korean defectors who set out to sea in search of asylum. Or they might be fishermen who, in desperate hope of increasing their catch, strayed dangerously far from their home ports.

According to the South China Morning Post, Japanese authorities are working under the latter assumption.

“We know that the regime in North Korea is pushing its farmers and fishermen to produce greater amounts of food,” Robert Dujarric, director of the Institute of Contemporary Asian Studies at Temple University’s Japanese campus, told the Chinese newspaper. “To my mind, the most likely explanation is that these were simply fishermen who were trying to fulfill large quotas and simply ran out of fuel too far out at sea to get home.”

The “primitive looking” motorized boats, each about 10 to 12 meters long, were not equipped with GPS navigation, the Japan Times reported. Adrift and ill-equipped for an extended time at sea, their crews likely died of exposure or hunger.

In most cases, the bodies were in such bad shape it was impossible to determine the cause of death. One boat that came ashore late last month held six skulls, assorted other remains and only one full body with a head attached. It was described as “nearly intact.”

Though ghastly, the appearance of the “ghost ships” is sadly common in Japan — the numbers this year haven’t even been particularly high. According to the Japan Times, 34 of the boats have drifted over since the beginning of the year, compared with 65 in 2014 and 80 the year before that.

The numbers are especially high during the fall and winter because of cold prevailing winds from the northwest, coast guard spokesman Yoshiaki Hiroto said.

The season is also the prime period for squid, sandfish and king crab — valuable export products, according to Reuters.

Since taking office in 2011, North Korean leader Kim Jong Un has pushed to boost food production in his impoverished country, where roughly 41 percent of the population is undernourished, according to the United Nations. Analysts and defectors also say that Kim has instituted incentives for fishermen to bring in a bigger haul, which can be exported to China as a way to get foreign currency.

“There is a possibility this incentive system led people to take more risks,” Lee Jong-won, an international relations professor at Japan’s Waseda University, told Reuters.

Also speaking to the news agency, Lee So-yeon, a North Korean army defector who arrived in Seoul in 2008, noted that the country’s more than 1 million-member military is heavily involved in food production and that the army sometimes hires civilian fishermen to make money.

One ship that washed ashore late last month bore the label “Korean People’s Army No. 325” in red Hangul characters on its hull, according to the South China Morning Post. (Hangul is the Korean alphabet).

But analyst John Nilsson-Wright told CNN he thinks it more likely that the boats carried defectors attempting to flee poverty or political oppression in North Korea.

Most people looking to leave the country flee overland into China or, less often, by sea to South Korea. But Nilsson-Wright suggested that defectors may be opting for the riskier route across the Sea of Japan because the traditional routes are being more closely monitored.

“What we do know is that for those people living outside of Pyongyang [the North Korean capital] … life remains extraordinarily hard,” said Nilsson-Wright, the head of the Asia program at the British think tank Chatham House. “It may be an economic necessity as much as a desire for political freedom encouraging some people in the North to try and leave the country.”

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