Wild boar population growing on remote Japanese islands as they master swimming

HIMEJI, Hyogo -- Islets in Japan's Seto Inland Sea sandwiched between the Honshu and Shikoku islands are facing a plague of wild boars believed to have migrated by swimming across the sea. This Mainichi Shimbun reporter recently visited one remote island in Hyogo Prefecture to learn about the boar infestation.

In the winter of 2020, Hironobu Sakamoto, 74, spotted a black object floating off the Ieshima Islands in Himeji, Hyogo Prefecture. Having spent over half a century as a professional fisherman, Sakamoto thought to himself, "I don't remember a buoy being here." He approached in his fishing boat, and it turned out what he thought was a buoy was a wild boar, swimming.

"It was paddling its legs like a dog. It was moving slowly, and I thought it would drown," Sakamoto recalled.

The Ieshima Islands are made up of over 40 islets about 18 kilometers southwest of Himeji. Four of the islets (with a total area of 18 square kilometers) are inhabited by some 4,000 people. Blessed with an abundance of seafood, the fishery is the main industry, bringing in sea bream and gazami crab.

There used to be no wild boars on these islets. Starting in 2020, the University of Hyogo and Hyogo Prefecture's Wildlife Management Research Center did a joint study using 60 automatic cameras. They estimated that there were about 640 of the animals on the islets. According to islanders and other witnesses, the animals started showing up in settlements around 2010.

The island group is roughly 10 kilometers from Japan's main Honshu island and 15 kilometers from Kagawa Prefecture's Shodo Island, off the Shikoku region. Akito Okada, 57, who lives on Boze Island, one of the Ieshimas, told the Mainichi Shimbun that he saw an adult boar swimming with a piglet. Wild boars have been spotted paddling between islets, but there is no clear answer as to where they originated.

Wild boars have caused damage on the Ieshima Islands in recent years. They dig up household farms, and according to Junjiro Ueda, 64, who responds to boar problems at the local residents' association, they get into accidents with motorcycles and there have been reports of people getting bitten and injured after encountering the animals.

A staff member at the Hyogo prefectural Ieshima nature center, an accommodation facility, expressed concerns over wild boars' effect on the ecosystem, saying, "I get the sense that crab and snake populations are declining. I suspect they're getting eaten by the boars."

Islanders have been trying to tackle the situation. The Ieshima Islands had no residents with a hunting license until 2018, but since 2019, 13 people have acquired licenses to trap boars, to try to wipe them out. In fiscal 2020, islanders and others captured 66 boars, and 105 the following year.

The Ieshima Islands are not the only islets troubled by the wild boar infestation. Takeo Kuriyama, an associate professor at the University of Hyogo, said in 1978 wild boars were confirmed on only 30 or so remote islands in Japan, but by 2013 that had increased to 220.

There have been numerous reports of wild boars spotted in the Seto Inland Sea, where there are many islands. In November 2018, the Imabari Coast Guard patrol vessel Setogiri saw a wild boar about 3 kilometers off the Ehime Prefecture city of Shikokuchuo. A crew member captured the animal on video, swimming smoothly and approaching the vessel while sticking its nose into the air. The coast guard office released the footage to the media.

Shunjo Takahashi, a professor emeritus at Nara University who specializes in biogeography and wrote a book about wild boars swimming to get to surrounding islands, says boars may be capable of swimming over 20 kilometers. He explained, "Sometimes they drown while trying. They risk their lives to cross the sea."

Takahashi said the wild boar habitat started expanding around 1970 due to factors including warmer winters and increasing abandoned farmland. From around 1980, the animals started being spotted swimming in the sea across Japan. He believes that boars began swimming in the open water as they sought new habitats due to growing population pressure, and after they were driven out to the sea when humans hunted them down and exterminated them on the shore.

Omnivorous wild boars cause agricultural losses of some 4.6 billion yen (roughly $35.5 million) nationwide per year. Takahashi pointed out, "We need to understand that boars can swim, and approach the problem with broad-ranging measures in (Japan's) maritime areas."

(Japanese original by Kenji Tatsumi, Kobe Bureau; Video provided by Akito Okada and the Imabari Coast Guard Office)