Trial Manufactures and Experiments of Small-scale Fishways to Ensure Both Upward and Downward Migration of Freshwater Fishes in the Aquatic Area With Paddy Fields

27 Mar Trial Manufactures and Experiments of Small-scale Fishways to Ensure Both Upward and Downward Migration of Freshwater Fishes in the Aquatic Area With Paddy Fields

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Abstract

環境庁が1997年に発表した汽水・淡水魚レッドリストに,絶滅危惧II類としてメダカが掲載されて話題となった.メダカは,河川(恒久的水域)と一時的水域(水田や小水路)の間を移動して,これらの水域を効果的に利用している.また,ドジョウなども同様な生活史を持つ.これらの生息数が減少している理由の一つとして,圃場整備事業があげられる.圃場整備事業は,農業従事者に対して作業時間の短縮や省力化といった様々な恩恵を与えている.一方で冬水の流水停止,水尻や排水路末端の落差形成など水域ネットワークを分断して,魚類の生息環境の悪化を招いている.そこで,工学的な手法による淡水魚類への配慮が求められるようになった.本研究は,水域ネットワークの再構築の一手段として小規模水田魚道の試作と実験を行ったものである. 魚道の試作にあたって,供試魚にドジョウ(底生魚)とメダカ(遊泳魚)を選び,ドジョウについてはあらかじめ遡上行動を観察した.その結果,ドジョウは遡上中に休憩し,遡上の際には引っかかりを利用することが分かった.この結果を参考にして魚道の試作を行い,供試魚を用いた予備実験を行った.そして,カスケードM型魚道と千鳥X型魚道と称する2つのタイプを開発した.また,この2つの魚道について設置勾配や流量を変えた遡上実験を実施し,以下の結果を得た,1)カスケードM型魚道は,ドジョウ(匍匐型,遊泳型)の遡上・降下行動を可能とする.2)千鳥X型魚道は,ドジョウ(遊泳型)とメダカの遡上・降下行動を可能とする.3)ドジヨウの遡上は,夜間に活発化する.4)メダカはドジョウに比べて正の走流性が強い.

English Abstract

In 1999 the Environment Agency, Japan released the red list of brackish and freshwater fishes, in which the number of the Threatened fishes amounted to 110. One of the reasons why many fishes became endangered is implementation of land consolidation project in the paddy field area. This project aimed at reducing labor requirement in farming activities. However, watercourse networks have been disconnected by drop structures in the ca ls and elevation gap between outlets of paddy fields and drai ge ca ls, which resulted in aggravation of the inhabiting environment of fishes. Therefore, it is eagerly required to reconnect watercourse networks by some artificial measures. In this connection this research was carried out to develop small-scale fishways for restoring damaged condition. In this research two kinds of fishes, Misgurnus anguillicaudatus and Oryzias latipes, were selected as sample fishes. Those are usual freshwater fishes in paddy field area of Japan, and Misgurnus anguillicaudatus represents a demersal fish, whereas Oryzias latipes rep-resents a pelagic fish. Two types of small-scale fishways, med Cascade-M type and Chidori-X type, were developed and laboratory experiments were conducted to exam whether the developed small-scale fishways ensure upward and downward migration of sample fishes or not under the different conditions of incli tion and flow rates. The result obtained were as follows; 1) the Cascade-M type fishway e bled upward and downward migration of Misgurnus anguillicaudatus (crawling type and swimming type), 2) Chidori-X type fishway e bled upward and downward migration of Misgurnus anguillicaudatus (swimming type) and Oryzias latipes, 3) Upward migration of Misgurnus anguillicau-datus became active in night time compared to day time, and 4) Oryzias latipes shows stronger rheotaxis compared to Misgurnus anguillicaudatus.