事例研究　CASE STUDY

棗田 孝晴^＊ ・瀬谷 政貴

千葉科学大学危機管理学部　〒288-0025 千葉県銚子市潮見町3

Takaharu NATSUMEDA^* and Masataka SEYA : Effects of weirs on fish fauna of the lowest reaches of two tidal rivers draining into the Tone River estuary. Ecol. Civil Eng. 15(2), 187-195, 2012.

Faculty of Risk and crisis Management, Chiba Institute of Science, 3 Shiomi-cho, Choshi, Chiba 288-0025, Japan

Abstract:To evaluate the effects of weirs on the distribution of diadromous fish species, we surveyed fish fauna below and above the weir located the lowest reaches of two tidal rivers (Takada and Oshi River) draining into the Tone River estuary in summer (July-August) and autumn (October) in 2010. Of 15 fish species captured, 10 (66.7%) were diadromous fish species. One river with lower weir height (50 cm at max) had diadromous fish both below and above the weir. The other river with higher weir height (55 cm + 130 cm), on the other hand, had no diadromous fish above the weir, and similarity index between below and above the weir was quite low. Physical environmental condition of the tributary was almost similar between below and above the weir, suggesting weir-induced substantial differences in fish fauna of the tributary. To solve the problem, mitigation of the height of weirs (small-scale fishways and creating a ramp of cobbles and gravels downstream of weirs) might allow diadromous fish passing such instream obstructions during high water level is proposed as a valid management action in tidal rivers.

Key words:diadromous fishes, high water level, mitigation of the height of weir, tidal river, Tone Rive

摘要

利根川河口堰よりも下流域(千葉県銚子市)で利根川に流入する2つの感潮河川(高田川，忍川) の最下流部に設置された堰の上・下区間において， 2010年夏期(7〜8月)と秋期(10月)に魚類相調査をおこなった. 両河川からは計15種類226個体の魚類が採集され，うち10種 (66.7％) が通し回遊魚であった．堰の落差が比較的小さい (50cm) 高田川では，堰の上・下区間とも通し回遊魚が確認されたのに対して，二段の落差 (55cm+130cm) を持つ忍川の堰の上流区間では通し回遊魚が全く確認されず，堰の上・下区間での魚類相の類似度は著しく低かった. 忍川の堰の上・下区間の物理環境が類似していることから同川では落差の大きい堰がもたらす通し回遊魚の上流への移動阻害が，魚類相に大きな相違をもたらしていることが示された. 感潮河川に設置された堰の改善策として，高水位時に通し回遊魚の堰の通過を可能にする，小規模魚道や石組みなどの設置による落差への軽減措置が，海域と淡水域との連続性を保つうえで有効であると考えられた.

2012年5月2日受付, 2012年8月2日受理
* e-mail: tnatsumeda@cis.ac.jp