A battered wooden vessel was found bobbing parallel to the waves rolling onto a quiet stretch of coastline in Japan's northern Akita Prefecture in the early hours on Sunday. Local police soon made a gruesome discovery, recovering the remains of eight people from the boat.

Read more: 'North Korean' men wash up on Japan coast

The bodies had begun to decompose, the police said in a statement, and several were partly skeletonized, hinting at the amount of time that their boat had been adrift in the Sea of Japan.

About 20 meters long but missing a superstructure, a rudder and masts, the boat held some clues as to the identities of the occupants. An eight-figure number in fading red paint was faintly discernible on the bow and the hulk contained life jackets with Korean characters. The police also found boxes of North Korean cigarettes.

And while the authorities have declined to make a firm declaration on where the boat began its journey, it appears to be just the latest incident of a North Korean "ghost ship" washing up on a Japanese beach.

Not an isolated discovery

A couple of days earlier, the remains of another wooden boat and two bodies were found in the surf on Sado Island, also off the northern coast of Japan. 

And Japanese Coast Guard rescue vessels earlier this month recovered three survivors from a North Korean ship that sank around 300 kilometers (180 miles) from shore. The remainder of the 15-man crew was lost.

And on November 23, residents of the Akita Prefecture town of Yurihonjo reported to police that unfamiliar men were standing around the marina late at night. Officials later discovered they were from North Korea and their 20-meter wooden squid fishing boat had fortuitously drifted into the port after its engine broke down. The men were reportedly in good health, spoke Korean and wished to be returned home. 

Read more:Japan to launch an investigation into mysterious 'ghost boats'

"The people of Akita Prefecture are certainly worried," Takeshi Suma, head of the prefectural government's International Affairs division, told DW.  "We have heard that they are from North Korea, but nothing has been confirmed. All we hear is that the police are investigating," he said, adding that coastal residents are on alert for suspicious sightings.

On Tuesday, Chief Cabinet Secretary Yoshihide Suga echoed that call during a press conference.

"The coast guard and police have to cooperate to step up sea patrols around Japan," he said. "The government intends to improve this to ensure we can guard against suspicious boats or people arriving in Japan."

Drifting from North Korea?

And it is likely that more fragile North Korean fishing boats will wash ashore in the months ahead.

"North Korea has a fairly desperate food situation at the moment and it has been reported that the leadership has told the fishing fleet to bring in bigger catches," Jeff Kingston, director of Asian Studies at the Japan campus of Temple University, told DW.

"But the evidence is that these wooden dinghies are just not suitable for the rough waters of the Sea of Japan in autumn," he said.

The poor state of repair of the vessels and their equipment is the most likely cause of their demise, with the latest discovery off Akita missing its rudder and its engines inoperable. Once the ship loses power, it is at the mercy of the currents and the winds. It can take weeks for a boat to come ashore, which explains why their crews are often found partly decomposed.

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Forty incidents this year

"There are a number of other factors behind these cases - which are not isolated as there have been 40 similar incidents along the northern Japan coast so far this year and as many as 500 in the last five years or so," said Kingston.

"Fishermen are being pushed out further from shore because their coastal waters have been overfished, while the government has also leased out areas to Chinese fishing fleets," Kingston said.

"It is also likely that many of these crews are not experienced further offshore and they are going out in boats that are clearly unstable and ill-equipped for the conditions that they face," he added. "It is clear that a lot of lives are being put at risk.

"And it must be pointed out that we only hear about the ones that are found when they wash ashore. I would suspect that there are an awful lot more that simply do not make it back to safety."