Effects of woody debris on the habitat of juvenile masu salmon (<i>Oncorhynchus masou</i>) in northern Japanese streams

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<jats:p>1. The effects of woody debris on stream habitat of juvenile masu salmon (<jats:italic>Oncorhynchus masou</jats:italic>) were examined at two spatial scales, stream reach and channel unit, for first‐ to third‐order tributaries of the Teshio River in northern Hokkaido, Japan. The forty‐eight study reaches were classified into three distinct types: coarse‐substrate step‐pool (CSP), coarse‐substrate pool–riffle (CPR) and fine‐substrate pool–riffle (FPR) reaches. Each reach type included reaches with different riparian settings, broadly classified as forest (relatively undisturbed forest and secondary forest after fires) or grassland (bamboo bushland and pasture).</jats:p><jats:p>2. The reach‐scale analyses showed that neither total pool volume nor pool‐to‐pool spacing was correlated with woody debris abundance in any of the three reach types. Masu salmon density was positively correlated with both woody‐debris cover area and total cover area, but not with total pool volume in the reaches.</jats:p><jats:p>3. Channel‐unit‐scale analyses revealed that woody debris reduced non‐pool velocity, increased pool depth and retained fine sediment in pools in FPR reaches, where the size of woody debris was very large relative to the substrate material size. However, woody debris did not influence any of the hydraulic variables (depth, velocity, substrate) in either non‐pools or pools of CSP and CPR reaches. Habitat use by masu salmon in non‐pools or pools was affected by woody‐debris cover area or total cover area rather than by hydraulic variables in any of the reach types.</jats:p><jats:p>4. The effects of woody debris on habitat at the reach‐ and channel‐unit scales in the study area were less than those indicated by previous work in the Pacific Northwest, North America, owing to the relatively small size of the riparian trees. However, the overall results suggested that woody debris in the study area contributed to masu salmon habitat by providing cover at the smaller, microhabitat scale.</jats:p>

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