原著論文 ORIGINAL PAPER

渡良瀬貯水池における水位低下・干し上げと鳥類個体数の動向

岩木　晃三¹⁾・ 佐藤　宏明²⁾

1)財団法人日本生態系協会　〒171-0021　東京都豊島区西池袋２-30-20音羽ビル
2) 財団法人河川情報センター　〒102-8474　東京都千代田区麹町１-３ニッセイ半蔵門ビル

Kouzou IWAKI¹⁾ and Hiroaki SATOU²⁾: Short-term responses of bird populations to lowering of water levels and drying-out treatments in the Watarase Reservoir.. Ecol. Civil. Eng. 9(2), 115-127, 2006.

1) Ecosystem Conservation Society-Japan. Otowa Bldg. 2-30-20 Nishiikebukuro Toshima-ku, Tokyo 171-0021, Japan
2) Foundation of River & Basin Integrated Communications, Japan. Nissei Hanzoumon Bldg. 1-3 Kojimachi Chiyoda-ku, Tokyo 102-8474, Japan

Abstract:Water levels in the Watarase Reservoir, central Japan, were altered as part of a water quality improvement program. Specifically, the water level was lowered to near drying-out of the reservoir. This level exceeds the normal lowest watermark, and is hereafter referred to as “drying-out". Since such treatments may affect the birds in the reservoir, a population survey was conducted in and around the reservoir from January 26th to May 15th, 2004. The survey was conducted in the three main ponds (Watarase Reservoir, Tatara-pond, and Joho-pond) throughout five water-level phases. Based upon levels in the Watarase Reservoir, these were defined as falling, maintaining the lowest level, drying-out, rising, and full. At each survey point, the number of birds present changed in relation to even slight changes in water level. However, covariance analysis indicated these population changes to be non-significant between each point and water level phase. No significant differences were detected between the bird populations at Watarase Reservoir and Tatara-pond over the duration of the study. Although a slight difference was detected between population decreases at Watarase Reservoir and Joho-pond, the decrease at the latter site where no water level lowering took place, was more conspicuous than at the former site. The decline in the population of birds detected in this study reflected the natural cycle of seasonal migration, and was not an effect of the drying-out of the reservoir.

Key words: birds, drying-out, fall of the water level, population change, Watarase Reservoir

摘要

本研究では，渡良瀬貯水池での水質改善を目的とした水位低下・干し上げによる鳥類個体数への影響の有無を，人為的な水位変動のない自然湖沼との比較によって明らかにすることを目的とした．
　調査は，2004年1月26日から5月15日までに渡良瀬貯水池，多々良沼，城沼など9ヶ所において水鳥類を対象に85回の個体数カウントを行い，渡良瀬貯水池と比較しうる結果が得られた多々良沼と城沼の個体数の増減をを分析した．分析対象はオナガガモを除くカモ科およびカイツブリ科鳥類とした．併せて，種数と出現種の変化にも注目した．
　各期別すべてで3池沼の個体数変化率に有意差がなかった．また全調査期間では，渡良瀬貯水池と多々良沼に有意差がなく，渡良瀬貯水池と城沼に有意差があったが，城沼の傾斜がより強かった．
　渡良瀬貯水池で水位低下期から干し上げ期に記録されなくなった種があり，多々良沼と城沼でも記録されなくなった種があった．渡良瀬貯水池では，最低水位維持期と干し上げ期にサギ科，チドリ科，シギ科，カモメ科などの渉禽類および魚食猛禽類のミサゴで新たな記録や個体数の増加がみられた．多々良沼と城沼では，同じ時期に個体数が顕著に増加した種はなかった．
　以上のことから，水位低下期から満水期においてみられた個体数の減少に関しては，水位低下・干し上げの影響は少なく，自然条件下における季節的移動（渡去）が主原因であったと考えられる．

2005年7月12日受付，2006年8月1日受理
1) e-mail: k_iwaki@ecosys.or.jp