原著論文 ORIGINAL PAPER

倒流木の除去が河川地形および魚類生息場所におよぼす影響

阿部 俊夫¹⁾²⁾・ 中村太士¹⁾

1) 北海道大学大学院農学研究科森林管理保全学講座〒060-8589 札幌市北区北9西9
2) 現所属: 森林総合研究所森林環境部 〒 305-8687 茨城県稲敷郡茎崎町松の里1

Toshio ABE¹⁾²⁾, Futoshi NAKAMURA¹⁾: Effects of experimental removal of woody debris on channel morphology and fish habitats. Ecol. Civil. Eng. 2(2), 179-190, 1999.

1) Department of Forest Science, Graduate School of Agriculture, Hokkaido University, N9, W9, Sapporo 060-8589, Japan
2) Present address: Forest Environment Division, Forestry and Forest Products Research Institute, P.O. Box 16, Tsukuba Norin, Ibaraki 305-8687, Japan

Abstract:Experimental removal of coarse woody debris (CWD) was conducted in a low-gradient, second-order stream covered by deciduous riparian forest in northern Hokkaido, Japan. We examined channel morphology, habitat variables and fish densities in 2 study reaches (cleaned and control, 100 m length) during autumn (Sept. - Oct.) of 1993, spring (May - Jun.) and autumn (Sept. - Oct.) of 1994. Immediately after the first survey, CWD and other small organic materials within the cleaned reach were removed by hand or hand-winch. Aggraded areas in the cross-sections of the cleaned reach were larger than those of the control reach during the autumn of 1993 - spring 1994. Cross-sectional changes tended to correlate with channel variables, such as curvature and width, in the cleaned reach, and with CWD amounts in the control reach. After CWD removal, small pools associated with CWD disappeared and pool number decreased, but pool volume did not vary. Cover area decreased, but depth, velocity, and substrate did not change in the cleaned reach. Density of juvenile masu salmon (Oncorhynchus masou) declined rapidly, but that of stone loach (Noemacheilus barbatulus toni), a benthic fish, did not show any significant changes. Deposition of sediment after CWD removal was associated with development of gravel bars at channel bends or wider sections. Pools were recreated by lateral scouring at the opposite sides of banks where gravel bars developed. We concluded that CWD pieces play an important role in forming pools and controlling pool number, although they are not effective in increasing pool volume in low-gradient meandering streams. Thus, a decline in masu salmon population could be attributed to a decrease in cover and pool number in this stream.

Key words: coarse woody debris, channel morphology, fish habitat, Iow-gradient stream, northern Hokkaido

摘要

倒流木を実験的に除去し,河川地形と魚類生息への倒流木の影響を検証した.北海道北部の緩勾配山地河川に除去区と対照区を設け,1993年秋に地形や生息場所持性,魚類生息密度を調査した後,除去区の倒流木を除去し,翌春と翌秋に再調査を行った.
倒流木の除去後,土砂の堆積重は;・時的に増加した. 地形変化は,対照区では倒流水に,除去区では流路平面形状に現制される傾向にあった.淵の体積は減少せず, 個数のみが減少した.カバー量は減少した.魚類生息密度は,サクラマス幼魚では明瞭に低下したが,底生性のフクドショウでは変化は小さかった. 緩勾配河川での倒流木除去は,砂礫堆発達と側方洗堀による淵再生をもたらし,淵体積の減少は生じにくいと考えられた.倒流木は,カバーと淵個数を増加させることで,サクラマス生息へ灯影響を写えていると結論できた.今後の河川管理では,倒流木の河川生態学的価値も考慮する必要がある.

1999年3月8日受付, 1999年6月28日受理
2) e-mail: toshioa@ffpri.affrc.go.jp