Naruto Strait

A strait between Shikoku and Awaji Island. It is about 1,350 meters wide between Magozaki on Oge Island in Tokushima Prefecture and Tosaki on the southwestern tip of Awaji Island in Hyogo Prefecture. Its deepest point is about 90 meters. It is known for its rapid currents and is one of the centers of Setonaikai National Park. 300 meters west of Tosaki lies a shallow reef called Nakase, which divides the strait in two. The area to the east of Nakase is called Ko-o and the area to the west is called O-o-o-shi, and it is in the O-o-shi, about 900 meters wide, that the so-called "Naruto whirlpools" are formed.

The rapid tides in the Naruto Strait are caused by the large difference in water levels between the inland Harima Nada Sea and the open sea Kii Channel, separated by a narrow strait. The tide that flows from the Harima Nada Sea to the Kii Channel at low tide in the open sea is called ochishio (forward tide), and the reverse at high tide is called sakashio. The tides alternate about every six hours, and when the water levels in both sea areas are roughly average, it is called yaeshio. The rapid tides and large whirlpools can be seen at low tide, and the whirlpools are larger at spring tides every month, especially at the spring tide around March 3 of the lunar calendar, when the difference in water levels is about two meters and the tidal currents reach 18 kilometers per hour. Several whirlpools with a diameter of about 10 meters are formed and continue for several tens of seconds as they move south. To watch the tides, visitors can take sightseeing boats that depart from Fukura Port on Awaji Island and Kameura Port in Naruto City, but they can also be seen from the observation deck of Naruto Park in the northern part of Ogeshima Island.

In 1976, construction of the Onaruto Bridge began as part of the Kobe-Naruto route of the Honshu-Shikoku Bridge. It is a 1,629-meter-long suspension bridge that was completed in 1985. The Naruto Strait is referred to as "Awanomito" in the "Nihon Shoki" (Chronicles of Japan), and is described as "the tide is already running very fast." It is also written about in many ancient poems, such as the "Manyoshu," and in Ki no Tsurayuki's "Tosa Nikki" (Diary of a Man'yoshu), it is written that after calling at Tosa-domari on Ogeshima Island, "they left their boats at midnight and crossed the Awa sluice gate" to avoid attacks by pirates. In addition, the strait has been used for line-fishing for sea bream and harvesting of Naruto wakame seaweed since ancient times, and is known for its delicious taste.

[Takagi Hideki]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

四国と淡路(あわじ)島間にある海峡。徳島県大毛(おおげ)島の孫(まご)崎と兵庫県淡路島南西端の門崎(とさき)間で、幅約1350メートル。最深部約90メートル。急潮で知られ、瀬戸内海国立公園の一中心となっている。門崎西方300メートルに中瀬(なかせ)とよばれる浅い岩礁があり、海峡を二分している。中瀬東方を小落(こお)とし、西方を大落としと称し、いわゆる「鳴門の渦潮」が生じるのは、幅約900メートルの大落としの部分である。

　鳴門海峡の急潮は、狭い海峡を挟んで内海側の播磨灘(はりまなだ)と外洋側の紀伊水道の干満による水面の差が大きいことから生じる。外洋の干潮時に播磨灘から紀伊水道に流れる潮を落潮(おちしお)（順潮）といい、満潮時にその逆に流れるのを逆潮(さかしお)という。落潮と逆潮は約6時間で交替し、両方の海域の水位がほぼ平均したときを八重潮(やえしお)とよぶ。急な潮流と大きな渦がみられるのは落潮のときで、毎月大潮のときは渦流が大きく、とくに旧暦3月3日ごろの大潮のときは水位差も約2メートルになり、潮の流れは時速18キロメートルに達する。渦は直径約10メートルのものが数個生じ、南へ移動しながら数十秒間続く。観潮には淡路島の福良(ふくら)港、鳴門市亀浦港から出る観潮船によるが、大毛島北部の鳴門公園の展望台から望むこともできる。

　1976年（昭和51）、本州四国連絡橋の神戸・鳴門ルートの一環として大鳴門橋の建設が始まった。全長1629メートルの吊橋(つりばし)で、1985年に完成。鳴門海峡は『日本書紀』神代紀に「粟門(あわのみと)」とあり、「潮すでにはなはだ急(はや)し」と記される。『万葉集』など古歌にも多く詠まれ、紀貫之(きのつらゆき)の『土佐日記』には、大毛島の土佐泊(どまり)に寄港し、海賊の来襲を避けて「夜なかばかりに船を出(い)だして、阿波の水門(あわのみと)を渡る」と書かれている。なお、海峡では古来タイの一本釣りや鳴門ワカメの採取が行われ、味のよいことで知られる。

［高木秀樹］

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