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群馬県押野川における 落差工の多自然型魚道への改善とヤマメ Oncorhynchus masouの放流実験

多胡治¹⁾・ 織田沢勲²⁾・ 林不二雄³⁾

2) 群馬県土木部沼田土木事務所 〒378-0053 沼田市東原新町1906
3) 群馬県立農林大学校 〒370-3105 群馬郡箕郷町西明屋1005

Osamu TAGO¹⁾, Isao ODAZAWA²⁾, Fujio HAYASHI³⁾: Improvement of the head construction for a nature-oriented fishway and a release experiment using Yamame Oncorhynchus masou in the Oshino River Gunma Prefecture Ecol. Civil. Eng. 3(1), 131-136, 2000.

2) Gunma Prefectural Government, Numata Public Works and Construction Office, 1906 Higashiharashinmachi, Numata 378-0053, Japan
3) Gunma Prefectural College of Agriculture and Forestry, 1005. Nishiakiya, Misato, 370-3105, Gunma, Japan

Abstract:High head constructions become serious obstacles for fish in upstream and/or downstream migration. Traditional fishways have some problems of their function such as difficulty for fish to detect the fishway entrance and of their artificial outlook made of concrete. In this paper, we describe an improved type of the head construction which bears a fishway being harmony with the surrounding landscape by using natural stones, while retaining functional ability to let fish pass over easily. In the process of construction, stones were fixed in the concrete with steel bars, and wooden stakes were inserted in the riverbed to prevent the stones from washing away. In order to make the water flow fit the plan stone arrangement was modified several times during construction. After the improvement, a fish release experiment was conducted using Yamame, Oncorhynchus masou. Ascending rate of the released fish was estimated as high as 52.2% which indicated that the fishway did not prevent Yamame from ascending. This improved type of head construction could be effective as a nature-oriented river work particularly in steep streams where some head construction are required.

Key words: fishway, nature-oriented river work, fish release experiment, head construction, Oncorhynchus masou

摘要

大きな高低差のある落差工は，魚類が遡上できないなどの問題を生じる．今回，魚が遡上出来る機能を確保しつつ，自然石を使用して，落差工を周囲の景観に調和した多自然型魚道に改善した．改善にあたって，自然石の固定方法には旧落差工コンクリート河床部分には鉄筋，普通の流路部（土砂河床）では木杭を用い，自然石の流出防止を図った．魚道の工事中には，構想に合致した水流となるようチェックを行い自然石の配置を修正し，魚類および底生動物などの生息環境が多様になるよう置石などを配置した．
改善後，これらの成果を判定するためヤマメによる遡上実験を行った結果52.2%の遡上が認められ，魚道がヤマメの遡上に妨害とならないと判断された．したがって，今後，川づくりを進める上で多自然型魚道のモデルケースの一つとなると考えられる．

1999年12月13日受付, 2000年5月11日受理
1) e-mail: tago-o@pref.gunma.jp