The Influence of Bedrock Outcrop on Stream Fish Habitat, and the Assessment of Restoration in a Bedrock Streamchannel

27 Mar The Influence of Bedrock Outcrop on Stream Fish Habitat, and the Assessment of Restoration in a Bedrock Streamchannel

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Abstract

2006年3月に，北海道の真駒内川において，ワイヤーで連結した玉石を複列配置し，岩盤河床を礫河床へと復元する礫床化の現地実験を行った．本研究では，岩盤化が河川性魚類の生息環境，産卵環境に与える影響を明らかにすること，さらに魚類の生息環境，産卵環境に対する実験の効果を検証することを目的とした．実験施設設置後の2007年7～10月に，礫河床区間 (目標区) ，岩盤区間 (対照区) ，実験区間 (復元区) を設け，物理環境，ハナカジカS (小型個体) ，ハナカジカL (大型個体) ，サクラマス幼魚の生息密度，サクラマスの産卵床数を調べた．その結果，岩盤区間では，礫河床区間と異なり礫の堆積が極めて少なかった．また，ハナカジカの生息密度とサクラマスの産卵床数は岩盤区間で低い傾向にあり，河床の岩盤化がこれらに与える負の影響の存在が示唆された．実験区間におけるハナカジカSの生息密度とサクラマスの産卵床数は礫河床区間と同程度であり，実験の効果がみられた．この結果は，小型のハナカジカの生息場ならびにサクラマスの産卵床基質として適した小中礫 (2-64mm) が，実験施設設置後に実験区間において増加したためであると考えられた．一方，実験区間におけるハナカジカLの生息密度は，礫河床区間に比べて有意に低く，岩盤区間と同程度であった．これは，大型のハナカジカにとって有効な生息場となる大巨礫 (＞64mm) が，実験区間においてあまり増加しなかったためであると考えられた．本研究では，岩盤化した河川においても，構造物を設置することにより，局所的な礫河床の復元が可能であることが示された．礫河床はハナカジカやサクラマスだけでなく多くの魚種の様々な生活史段階において必要不可欠である．今後は，礫河床の局所的な修復技術だけでなく，流域における土砂収支バランスを考慮した根本的な河川環境の復元手法を検討する必要がある．

English Abstract

In March 2006, six spur dikes consisting of several sets of the wired boulders, were experimentally placed to restore gravelbed in a 50-m bedrock outcrop reach of the Makoma i River, central Hokkaido, northern Japan. The study purposes were to examine the influence of bedrock outcrop on summer habitats of wrinklehead sculpin (Cottus nozawae) and masu salmon (Oncorhynchus masou), and spawning habitat of masu salmon returning from ocean. The effects of spur dikes on habitat improvement were also examined. We set three sections representing gravelbed (reference), bedrock (control) and experiment (treatment), and investigated physical habitat variables, fish density and the number of masu salmon redd from July to October 2007. In the bedrock section, areal percentage of gravelbed was distinctly lower than that in the gravelbed section. Sculpin abundance and the number of redd were also lower in the bedrock section than in the gravelbed section. In the experiment section, abundance of small-sized sculpin (<90 mm) and the number of redd were similar to those in the gravelbed section, owing to an increase of gravel-pebble (2-64 mm) area in the experiment section facilitated by the spur dikes. Abundance of large-sized sculpin (≥90 mm) in the experiment section was lower than that in the gravelbed section because area of cobble-boulder (>64 mm) suitable for large-sized sculpin habitat did not significantly increase in the experiment section. These results suggested that the rehabilitation measures using the spur dikes foster the accumulation of medium-sized bed materials and accordingly improve fish habitat in bedrock rivers. Gravelbed is essential habitat component not only for a dromous fish but also for benthic fish at various stages of their life histories. We believe that restoration of sediment balance in a catchment is a fundamental measure to preserve gravel bed rivers.