原著論文　ORIGINAL PAPER

溜め池周辺水域におけるオオクチバスとブルーギルの分布および個体数変動

宮田　浩・國本　昌宏・井上　幹生^＊

愛媛大学理学部生物学科　〒790-8577　松山市文京町２-５

Hiroshi MIYATA, Masahirio KUNIMOTO and Mikio INOUE*: Spatial and temporal patterns in the density of largemouth bass and bluegill in pond-associated lotic habitats. Ecol. Civil. Eng. 10(2), 117-129, 2007.

Department of Biology, Faculty of Science, Ehime University, Matsuyama 790-8577, Japan

Abstract: Exotic largemouth bass (Micropterus salmoides) and bluegill (Lepomis macrochirus), both of which were introduced from North America, are now widespread throughout Japan, often dominating in irrigation ponds and reservoirs. The rapid expansions of their distributions beyond watersheds are obviously due to human agency; however, their expansions within a watershed may be, more or less, attributable to natural dispersal, which has rarely been investigated. This study presents circumstantial evidence of their dispersal from artificial ponds to irrigation ditches. In the Shigenobu River watershed (Ehime Prefecture, southwestern Japan), 21 study sites were established in river pools and irrigation ditches associated with 6 ponds inhabited by the two species. In each site, population densities of both species were monitored from the spring of 2002 to the autumn of 2003. The monitoring showed that (1) both species attained high densities (maximum density, 8.0 and 12.1 individuals/m2 for largemouth bass and bluegill, respectively) in ditches just downstream of ponds in summer, (2) most of them were small in size (<10 cm in fork length), (3) at every study site, their densities were temporarily variable and zero densities were often recorded (i.e., neither of the two species was found), and (4) the two exotic species comprised considerable percentages of fish assemblages in the ditches (41.9 % in the sum total of estimated number of fishes). These results, in combination with a fact that most study ditches had potential barriers to upstream migration (i.e., falls of 0.8-2m in height), suggest that individuals of both species in the ditches were immigrants from the ponds.

Key words: agricultural pond, exotic fish, invasion, irrigation ditch, Japan

摘要

　オオクチバスとブルーギルは全国各地に分布する外来魚であり，溜め池がそれらの主要な生息場所となっている．本研究では，溜め池につながる農業水路および河川における両種の分布および個体数変動を観測することにより，溜め池から水路を介した両種の自然分散の可能性について検討した．愛媛県松山市近郊の重信川流域において両種が生息する溜め池を６つ選び，それぞれの池につながる農業水路，およびそれらとつながる河川に21箇所の調査地を設定し，各調査地において，2002年の春から2003年の秋までの期間中，定期的に（冬を除いて基本的には毎月１回），両種の個体数密度を調査した．
　その結果，溜め池周辺の農業水路においては，どの調査地においても両種もしくはそのうちのどちらかの生息が確認され，魚類群集全体に占める両外来種の割合は41.9％に達した（調査期間中の，のべ推定個体数に基づく算出）．両種ともに，夏季に溜め池の取水口直下の水路において非常に高密度（最大値，オオクチバス：8.0個体/m2；ブルーギル：12.1個体/m2）に達したが，その一方で，どの調査地においても生息が確認されない時期が多かった．つまり，水路での生息は一時的であった．また，これら水路で採捕された個体の多くは小型個体（尾叉長＜10cm）であった．今回対象とした水路は全て，溜め池から河川に至るまでのほぼ全区間にわたって三面コンクリート水路（幅１〜２m）となっており，かつ，１つを除く全ての水路において魚類の遡上障壁となり得る鉛直落差が存在した．これらのことより，農業水路で確認された両種個体は，溜め池から流出したものと推測された．

2006年３月19日受付，2006年10月30日受理
* e-mail: inom@sci.ehime-u.ac.jp