Invasive Exotic Fish Control by Application of Encircling Net and Water Level Drawdown Operation

27 Mar Invasive Exotic Fish Control by Application of Encircling Net and Water Level Drawdown Operation

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Abstract

福島県阿武隈川水系三春ダムの蛇沢川前貯水池において，岸に平行して定置網を設置し，人為的に水位低下させることで魚類を網内に集結させて魚類を捕獲した．フィールドとした三春ダムは制限水位方式のダムであり，非洪水期 (10 月 11 日 ～ 6 月 10 日) から洪水期 (6 月 11 日 ～ 10 月 10 日) にかけて，貯水位を 8 m 低下させる運用をしている．捕獲試験は，2007 年 ～ 2011 年に実施した．捕獲した魚類のうち，外来魚のオオクチバスとブルーギルは回収し，それ以外の在来魚等は再放流した．オオクチバスの大型個体は，前貯水池と本貯水池の連結部 (幅 5 m) で多く捕獲でき，これは繁殖のために遊泳している個体と考えられた．一方，ブルーギルはこの時期は遊泳せず，浅い場所に集まっていた．水位低下を利用した定置網での捕獲の結果，オオクチバスと 2 歳魚以上のブルーギルは年々減少し，ギンブナをはじめとする在来魚等は増加傾向であった．在来魚等の若い個体が継続的に生産・維持されることが水域の魚類群集のバランスを保つ上で重要である．蛇沢川前貯水池では，2010 年以降，貯水池内で繁殖したギンブナ，コイの若い個体が顕著に増加しており，捕獲による駆除の効果と考えられる．

English Abstract

In Japan, some of introduced exotic fish such as largemouth bass (Micropteraus salmoides) and bluegill (Lepomis macrochirus) are highly competitive over indigenous fish species, causing loss of biodiversity and degradation of aquatic ecosystem in a site. As this issue became apparent, many strategies have been implemented to control the exotic species. In a breading season of largemouth bass and bluegill, from May to June, they flock to shallow areas for spawning. In a front reservoir of Miharu dam, encircling nets were placed parallel to shores in order to lock the exotic fish in the target sites. As the reservoir level was lowered by a drawdown operation, fish in the areas were inescapably driven into the nets. This drawdown capturing method has been carried out from 2007 to 2011. Among the fish captured, the invasive exotic species, largemouth bass and bluegill, were elimi ted and the others were released. Large sized individuals of largemouth bass were collected more in a joint area (about 5m width) between the front and main reservoir. Bluegill does not migrate around the reservoir, but they gather in shallow areas in spawning season. As a result of the consecutive practice, abundance of the largemouth bass and bluegill (2nd year and older) have been decreased. Whereas, the number of indigenous juveniles seen, such as Carassius auratus langsdorfii, has increased and increase of the indigenous populations has been affirmed.