Ariake Sea

Japanese: 有明海 - ありあけかい

It is an inland sea surrounded by four prefectures, namely, western Kyushu, Nagasaki, Saga, Fukuoka, and Kumamoto, and occupies the inner part of Shimabara Bay. It refers to the sea area roughly north of a line connecting Nagasu Town in Kumamoto Prefecture and Kunimi Town area of Unzen City, Nagasaki Prefecture. On maps from the Meiji period, it is written as Tsukushi Sea, Chikushigata, and Ariake Offshore, and it has also been called Maeumi (Front Sea) because it is a sea deeply connected to coastal people, such as those engaged in fishing activities on the tidal flats. The tidal range reaches about 6 meters at spring tides near the mouth of the Rokkaku River, the largest in Japan. The tidal activity of the Ariake Sea and the sedimentation of Kyushu's largest river, the Chikugo River, as well as other rivers such as the Yabe River, Kase River, Rokkaku River, and Shiota River, have created vast tidal flats along the coast, which are as wide as 7 kilometers at their widest point near the mouth of the Chikugo River. The tidal flats consist of muddy tidal flats made of lagoon soil and sandy tidal flats, with muddy tidal flats especially prevalent in the Saga and Fukuoka prefectures at the inner bay and in Isahaya Bay in the west. The natural land formation caused by the development of these tidal flats, combined with the artificial land formation caused by land reclamation that began around the end of the Middle Ages, has resulted in the plains expanding at a rate of about 1 kilometer per 100 years on the right bank of the downstream Chikugo River. A thick layer of Ariake clay has accumulated on this land, making it a soft ground area.

[Shigeru Kawasaki and Tsutomu Igarashi]

Coastal Industry and Development

The Ariake Sea has long been known as a mysterious sea where shiranui fires burn, but its climate is best represented by its vast tidal flats. The tidal flats are home to many tidal flat creatures such as mudskirs and warabasu, which appear in Kitahara Hakushu's Waga Nama Hitachi. Tidal flat fishing, such as catching mudskirs using a skiff (lagoon ski), is a typical feature of the tidal flats. The tidal flats are also a paradise for wild birds such as snipes and geese. Seaweed farming has grown rapidly since the late 1950s, replacing the farming of oysters, which appear in Kambara Ariake's poems, and shellfish such as agemaki, which appear in Hakushu's famous writings, and has become Japan's largest seaweed production area, surpassing Tokyo Bay.

Additionally, the seabed on the eastern coast of the Ariake Sea contains deposits of coking coal. Its development by Mitsui capital led to the establishment of Japan's leading Miike Coal Mine and a coal chemical complex in the Omuta region. The construction of the Miike Port, using locks, in the late Meiji period was a major undertaking to overcome the obstacles of a shallow bay and large tidal ranges. In order to fully develop the raw coal reserves on the seabed off the coast of Omuta, where cracks are widespread, the world's first artificial islands, including Hatsushima, were constructed (1951). However, coal production has continued to decline since the 1970s, and the closure of the Miike Mine in 1997 led to the decline of the local economy.

The four prefectures bordering the Ariake Sea established the Ariake Sea Regional Comprehensive Development Association in 1952, and drew up development plans such as reclamation plans by closing off the Ariake Sea and developing its underground resources. However, the development of the seaweed farming industry from the 1970s onwards and adjustments to rice production caused these plans to be thwarted.

[Shigeru Kawasaki and Tsutomu Igarashi]

History of reclamation

Reclamation of the tidal flats of the Ariake Sea began towards the end of the Kamakura period after the Mongol invasions, and progressed greatly in the Edo period with the construction of doi (embankments) to develop new rice fields. The area covered was approximately 6,300 hectares in Hizen (Saga Prefecture) and 2,000 hectares in Chikugo (Fukuoka Prefecture). Reclamation in the Hizen-Saga domain was mainly carried out by farmers, and was small-scale village-contracted reclamation known for place names that include karami (entanglement). In contrast, reclamation in the Chikugo Yanagawa domain and Higo domain was often called hiraki (opening), and was characterized by the development of new rice fields run by the domain or contracted out to merchants. After the Meiji period, embankment construction technology improved and the scale of reclamation became larger. The Ariake Reclamation Project, which was completed in 1951 as a national project to stop the tide, had a total area of approximately 1,174 hectares (Ariake construction area), and 318 farming households settled there. In the reclaimed land, people worshiped the sea god, and there were faiths such as the Dragon King and Kaido, who prayed for safety from disasters such as high tides, and faith in the Shiodo Kannon.

However, reclamation without the reorganization of large-scale water-use organizations led to chronic water shortages, and farmland reliant on unstable tidal backflow irrigation (blue water irrigation, where blue water refers to fresh water that flows back due to the tide) that takes advantage of the difference in specific gravity between seawater that flows upstream at high tide and fresh water, and on pumping of groundwater expanded. Furthermore, land subsidence became serious in areas that depended on groundwater, such as the Shiroishi Plain. These reclaimed lands were used for large-scale, highly productive rice cultivation, but in many of the reclamation projects after World War II, plans were halted due to rice production adjustments in the 1970s, and completed reclaimed lands were converted into airport sites. In Isahaya Bay, the tidal barrier that closed off the bay entrance was closed in 1997, and reclaimed land was developed, with farming starting in 2008. However, the causal relationship between the deterioration of the surrounding seaweed environment, such as the failure of the seaweed harvest after the closure of the tide-retaining dike, has become an issue, and the dispute over the opening and closing of the water-tight gate of the dike continues.

The Ariake Sea is a precious wetland in Japan, home to many fish, shellfish and bottom-dwelling organisms unique to tidal flats, and attracts many wild birds such as snipes and geese that feed on them. In Japan, where tidal flats continue to decrease due to development, the conservation of the Ariake Sea tidal flats, where biodiversity is maintained in a delicate balance, is attracting great interest.

[Shigeru Kawasaki and Tsutomu Igarashi]

"The Development Process of Ariake Sea Reclamation" by Masami Senju (1967, Kyushu Regional Agricultural Administration Office)" ▽ "History of Ariake Reclamation" (1969, Kyushu Regional Agricultural Administration Office) ▽ "Akira Hiraoka (ed.), "Kyushu: A Century of Maps" (1997, Kokin Shoin)"

Mudskipper fishing
"Mutsukake" using a ski slope (lagoon skiing). Ariake Sea © Saga Prefecture Tourism Federation ">

Mudskipper fishing

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

Japanese:

九州西部、長崎、佐賀、福岡、熊本の4県に囲まれた内海で、島原湾の湾奥部を占める。ほぼ熊本県長洲(ながす)町と長崎県雲仙(うんぜん)市国見(くにみ)町地区辺とを結んだ線以北の海域をいう。明治時代の地図類などには筑紫海(つくしかい)・筑紫潟・有明沖と記され、干潟(ひがた)での漁労活動など沿岸の人々と深く結びついた海であることから、前海(まえうみ)ともよんできた。干満差は、六角川(ろっかくがわ)河口付近で大潮時には約6メートルにも達し、日本最大である。この有明海の潮汐(ちょうせき)活動と、九州第一の大河、筑後(ちくご)川をはじめ、矢部(やべ)川、嘉瀬(かせ)川、六角川、塩田(しおた)川などの諸河川の堆積(たいせき)作用によって、沿岸には広大な干潟が形成され、筑後川河口付近では最大幅員が約7キロメートルにも及ぶ。干潟は、潟土(がたつち)の泥質干潟と砂の混じる砂質干潟からなり、湾奥部の佐賀・福岡県側と西部の諫早(いさはや)湾などでとくに泥質干潟が発達する。この干潟の発達による自然陸化に、中世末期ごろから始まった干拓による人工陸化が加わって、筑後川下流右岸地域では、100年間に約1キロメートルの割合で平野が伸展してきたという。この陸地には有明粘土層が厚く堆積し、軟弱地盤地帯となっている。

［川崎　茂・五十嵐勉］

沿岸の産業と開発

有明海は古くから不知火(しらぬい)燃ゆる神秘の海として知られたが、その風土はやはり広大な干潟によって代表される。干潟には、北原白秋(はくしゅう)の『わが生ひたち』に登場するムツゴロウやワラスボなどの干潟性生物が多く生息する。はね板（潟スキー）を利用してのムツゴロウとりなどの干潟漁労は、干潟の代表的風物詩である。干潟はまたシギ、ガンなどの野鳥の楽園でもある。蒲原有明(かんばらありあけ)の詩にみるカキや、白秋の名文にあるアゲマキなどの貝類の養殖にかわって、ノリ養殖が1950年代後半以降急激に伸び、東京湾をしのいで日本最大の産地となっている。

　また、有明海の東岸の海底には、粘結性の石炭を埋蔵する。三井資本によるその開発は、大牟田(おおむた)地域に日本屈指の三池炭鉱(みいけたんこう)と、石炭化学コンビナートの成立を導いた。明治後期の閘門(こうもん)式三池築港は、遠浅で干満差の大きい障害を克服するための大工事であった。大牟田沖のノリひびの広がる海面に、海底の原料炭を本格的に開発するため、初島(はつしま)など世界初の人工島が出現した（1951）。しかしながら、1970年代以降は産炭量の減少が続き、1997年（平成9）の三池鉱山の閉山によって地域経済の衰退を招いた。

　有明海に面する4県では、1952年（昭和27）に有明海地域総合開発協会が設立され、有明海の締め切りによる干拓計画や地下資源開発などの開発構想が企図されたが、1970年代以降のノリ養殖業の発達や米の生産調整によって計画は頓挫(とんざ)した。

［川崎　茂・五十嵐勉］

干拓の歴史

有明海の干潟の干拓は、元寇(げんこう)後の鎌倉時代末期ごろから始まり、江戸時代の新田開発で土居(どい)（土手）構築による地先の干拓が大きく進展した。その面積は肥前(ひぜん)（佐賀県）分約6300ヘクタール、筑後(ちくご)（福岡県）分約2000ヘクタールに及んだ。肥前佐賀藩の干拓は農民主体で、搦(からみ)などの名がつく地名で知られる零細な村請け干拓であった。それに対し筑後柳川藩(やながわはん)や肥後藩の干拓は、開(ひらき)の名が多く、とくに藩営や商人請負による新田開発に特色があった。明治以後は築堤技術が進み、干拓規模も大型化した。1951年（昭和26）に国営事業として潮止めを完成した有明干拓は、全面積約1174ヘクタール（有明工区）にも及び、入植農家数318戸を数えた。干拓地では海神を祀(まつ)り、高潮災害などからの安全を祈る竜王、海童信仰や、潮止観音信仰などがみられた。

　しかしながら、大規模な水利組織の再編を伴わない干拓は、慢性的な水不足を招き、満潮時に遡(さかのぼ)る海水と淡水の比重の差を利用した不安定な潮汐逆水灌漑(かんがい)（アオ灌漑。アオとは潮によって逆流する淡水のこと）や地下水の揚水(ようすい)に依存する農地が拡大した。また、白石平野(しろいしへいや)のように地下水に依存する地域では、地盤沈下が深刻化した。これらの干拓地では、大規模な生産性の高い稲作が行われているが、第二次世界大戦後の干拓では、その多くが1970年代のコメの生産調整によって計画が途絶し、完成した干拓地が空港用地に転用されるなどの変化をみた。諫早湾では湾入部を締め切る潮受堤防が1997年（平成9）に締め切られて干拓地の造成が行われ、2008年から営農も開始された。しかし、潮受堤防閉門後のノリ不作など、周辺海域の環境悪化との因果関係が問題となり、堤防水門の開閉をめぐって争いが続いている。

　有明海は、干潟特有の魚介類や底性生物が多く生息し、それを餌(えさ)とするシギやガンなどの野鳥が数多く集まる、国内でも貴重な湿地となっている。開発による干潟の減少が続く日本において、生物の多様性が微妙なバランスで維持されている有明海の干潟の保全が大きな関心をよんでいる。

［川崎　茂・五十嵐勉］

『千手正美著『有明海干拓の展開過程』（1967・九州農政局）』▽『『有明干拓史』（1969・九州農政局）』▽『平岡昭利編『九州　地図で読む百年』（1997・古今書院）』