Here a row of about twenty boathouses sits on the shore, with mountains towering in the background. This scenery has been selected as one of the 100 best fishing village sceneries in Japan.

Why are these boat houses here? In the past, boats were made of wood, and if they were left floating in the sea, they would fall victim to bivalves called shipworms, or Teredo worms. These shipworms would eat away at the wood, damaging the boats. To prevent this, when the boats were not in use, they had to be either pulled ashore or moved into the fresh water at the mouth of a river. In the Oki Islands, winter in particular brings with it strong winds and heavy snowfall. For this reason, covered boat houses like the ones you see here were built to store boats safely.

This type of boat house is found only on the Sea of Japan side of the country. The reason for this is the difference in water level between low tide and high tide. Please take a look at the explanation signboard nearby. On the Sea of Japan side, the difference in water level between low and high tide is only about 50 centimeters. However, on the Pacific Ocean side, the difference can be as much as 2 to 5 meters. A 5-meter difference between tides makes it impossible to easily pull boats ashore out of reach of the water, which is why boat houses like these cannot be built on the Pacific Ocean side of Japan.

But why is the difference between tides so much smaller here on the Sea of Japan? It is said that low and high tides are caused by the gravitational pull of the moon. This is true for the Pacific Ocean, but the Sea of Japan is comparatively much smaller in area, and the moon’s pull does not reach it. If this is the case, how do the tides occur here?

In fact, as the moon’s pull affects the Pacific Ocean and causes high tide, the water level rises and flows into the Sea of Japan, like a dam overflowing. The Tsushima Strait in the south and the Tsugaru Strait in the north play the part of these “dams.” If you look at a map, you can see that these straits are narrow and shallow. Because of this, the flow of sea water is small, and the water level of the Sea of Japan rises only gradually. As the water rises, the Pacific Ocean changes again to low tide, making the Sea of Japan comparatively higher; in turn, the water flows back out into the Pacific Ocean. In this way, while the Pacific Ocean sees low and high tide twice a day, the Sea of Japan only experiences them once a day.

In summary, because the area of the Sea of Japan is small and the straits that act as its entrances are narrow, there is only about a 50-centimeter difference between low and high tide. This characteristic is what allows boat houses like these to be built on the Sea of Japan side.