The crews of the so-called "ghost ships" that have turned up in the waters off Japan's western coast in recent weeks may have been driven to starvation by pressure from North Korean leader Kim Jong Un to increase their catch, a fisheries expert has said.

At least 13 boats with more than 20 decomposing bodies on board have been found off Japan's western coast over the past two months, baffling investigators who have yet to fully explain their origin and what had actually killed their crews.

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But Kim Do Hoon, a professor of fisheries science at South Korea's Bukyong National University, recently told the press that the fishermen may have starved to death due to pressure from the North Korean government to increase their catch.

"Kim Jong Un has been promoting the fisheries, which could explain why there are more fishing boats going out," the professor said. "But North Korean boats perform very poorly, with bad engines, risking lives to go far to catch more. Sometimes they drift, and the fishermen starve to death."

The Japanese coast guard has been finding the dilapidated boats -- some of them capsized -- along the stretch of sea extending from the Fukui prefecture to the southern tip of Hokkaido. They have been called ghost ships because they have been found either completely empty, or only with corpses on board.

Most observers seem to agree that the mystery vessels are North Korean fishing boats, many of which are out trawling for king crab, squid and sandfish this time of year. Korean Hangul markings on at least one of the boats reportedly spells out "Korean People's Army", according to CNN.

The bodies retrieved from the boats have all been found in an advanced state of decay, with body parts beginning to fall away. This indicates that they had been dead for a long time, reports say. Investigators admit that the state of decomposition of the bodies makes it difficult to ascertain exactly when and how they died.

Japanese coast guard officials have said that the boats are nothing new, and that they are typically found along Japan's coastline or on the coast of Russia's far east. One official told the BBC that 65 such boats had drifted into Japan's waters last year alone.

What has the media and the Japanese public buzzing, however, is that the latest arrivals appear to be coming at a higher rate than usual. Japan's Internet forums are rife with rumors about the boats, and some in the press have suggested that the people on board may have been North Korean defectors.

However, there are those who say that Kim has been pushing North Korea's fisheries sector to increase its catch in the past months, suggesting that this could be a key to unravelling the mystery. Recently, the state-run Korean Central News Agency (KCNA) ran a story about the North Korean leader's visit to Fishery Station No.15 under Korean People's Army Unit (KPA) 549.

Kim was reportedly delighted by the volume of the catch he saw there, and the KCNA report describes how the North Korean leader exhorted the fishermen in the station to catch even more fish.

"He called on the station to set a high goal for remarkably increasing the yearly output of fish in a short span of time on the basis of this year's achievement," CNN quotes the report as saying.

Tagsghost ships, North Korea