Yatsushiro Sea

An inland sea stretching from central to southwestern Kumamoto Prefecture. It is also called the Shiranui Sea. It is about 10 kilometers east to west and 75 kilometers north to south. It is surrounded by the Uto Peninsula, Uki City, Hikawa Town in Yatsushiro County, Yatsushiro City, Ashikita Town in Ashikita County, Tsunagi Town, Minamata City, Izumi City in Kagoshima Prefecture, Akune City, Nagashima Island, Amakusa Shimo Island, Amakusa Kami Island, and Oyano Island, and has a water area of ​​about 1,200 square kilometers. It is divided into the northern half of the sea, which is shallow and more like an inland bay, and the southern half, which becomes more open ocean as the sea depth increases, and there is a stark contrast not only in the bottom material and fishing conditions but also in the shape of the coastline. In the northern half of the basin, where the Ono River, Hi River, Kuma River, and many other rivers flow, the supply of large amounts of sediment and nutrients has created shallow waters with well-developed tidal flats that are ideal for spawning and raising young fish, as well as creating good fishing grounds for clams, shrimp, and seaweed. The coastline on the eastern edge has been transformed into a linear artificial coastline through reclamation of the tidal flats. In contrast, in the southern half of the basin, there are fewer rivers flowing into it, the bottom sediment has changed to sand and gravel, and the sea depth is almost always more than 20 meters, so the main fish species are sea bream, hairtail, conger eel, and squid. Fishing methods have also changed from gill nets used in the northern half of the basin to longlines, single-line fishing, and boat seine nets. The coastline is a ria coast created by the submersion of the southern Kyushu mountain range and the low mountain ranges of Uto and Amakusa, so it is rich in coves and presents beautiful scenery. Furthermore, many of the deep sea inlets serve as bases for aquaculture, and you can see ponds and rafts everywhere cultivating sea bream, yellowtail, pearl oysters, etc. As industrialization along the coastline has brought about environmental pollution, and in particular the Minamata disease outbreak caused by wastewater from the Chisso Minamata factory began in 1953 (Showa 28), attracting nationwide attention as a major pollution incident.

The appearance of the shiranui around the first day of the eighth month of the lunar calendar is well known, and is believed to be caused by the abnormal refraction of fishing lights. It also appears in the Keikō chapter of the Nihon Shoki.

[Yamaguchi Morito]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

熊本県中部から熊本県南西部にかけての内海。不知火海(しらぬいかい)ともいう。東西約10キロメートル、南北約75キロメートル。宇土(うと)半島、宇城(うき)市、八代郡氷川(ひかわ)町、八代市、葦北(あしきた)郡芦北町、津奈木(つなぎ)町、水俣(みなまた)市、鹿児島県出水(いずみ)市、阿久根(あくね)市、長島、天草下(あまくさしも)島、天草上(かみ)島、大矢野島に囲まれ、水域面積約1200平方キロメートル。浅海でより内湾性の強い北半水域と、海深の増大に伴って外洋性を増す南半水域とに分けられ、底質、漁相だけでなく海岸線の形状に至るまで際だった対照をなしている。大野川、氷(ひ)川、球磨(くま)川ほか多くの河川の流入する北半水域では、多量の土砂、栄養塩類の供給が、干潟の発達した多種多様な魚族の産卵、稚魚育成にかなった浅海域を形成しているほか、アサリ、エビ、ノリの好漁場もつくりだしている。また東縁の海岸線は干潟の干拓によって、直線的な人工海岸に変化している。これに対し、南半水域では流入河川も少なく、底質も砂礫(されき)にかわるとともに海深もほとんど20メートルを超えるので、タイ、タチウオ、ハモ、イカなどの魚族が中心となっている。漁法も北半水域の刺網から延縄(はえなわ)、一本釣り、船引網などに変化している。海岸線は九州山地南部ならびに宇土・天草低山地の没入によって生じたリアス式海岸のため、入り江に富み、風光明媚(めいび)な景観を呈している。また海深に恵まれた入り江は、養殖漁業の拠点となっている所が多く、タイ、ハマチ、真珠貝などの養殖池や筏(いかだ)が随所に見受けられる。なお沿岸の工業化に伴い、環境汚染が生じ、とくにチッソ水俣工場の排水により、1953年（昭和28）ころより水俣病が発生、巨大公害事件として全国的に注目された。

　旧暦8月1日前後に出現する不知火は有名で、漁火の異状屈折現象によるものとされ、『日本書紀』景行(けいこう)紀にもみえる。

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