Fishing boats carrying decomposed corpses have washed ashore in Japan in recent weeks, leading to speculation they are rickety North Korean vessels that have strayed dangerously far from port under the impoverished nation's push to boost its catch.

There has been no mention from secretive North Korea of any missing boats, but its leader, Kim Jong Un, has put a high priority on fishing as a way of earning foreign currency and providing a sustainable food source that is not reliant on harvests and weather.

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Defectors and experts say fishing boats under the command of the Korean People's Army may have succumbed under pressure from Kim to catch more fish, drifting off course and ill-equipped for rough seas.

"Kim Jong Un has been promoting the fisheries, which could explain why there are more fishing boats going out," said Kim Do-hoon, a professor of fisheries science at Bukyong National University in Busan. "But North Korean boats perform really poorly, with bad engines, risking lives to go far to catch more. Sometimes they drift and fishermen starve to death."

The Japanese coast guard and police reported 12 incidents of wrecked wooden boats, including some that were in pieces, on the country's shores and waters since October, containing 22 dead bodies, including five skulls.

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TV images of some of the boats showed relatively large but otherwise primitive-looking motorized craft and the coast guard said they did not have GPS navigation systems. Those aboard could have died of starvation and exposure to the cold after getting lost.

Although Japan's Meteorological Agency said there was not unusually bad weather in the Sea of Japan this November, the waters are rougher at this time of year due to the onset of cold, northwesterly winds.