Densities of wild masu salmon, Oncorhynchus masou Brevoort, were investigated before (late‐September) and after (mid‐November) autumnal habitat shifts in a small river in Hokkaido, northern Japan. Abundance of instream cover habitat formed by coarse woody debris, submerged vegetation and undercut banks was quantified. Density of 1+ and older masu salmon was significantly correlated with cover abundance in autumn, although cover was not significant for density of 0+ fish, indicating that clear‐cut reaches were utilized by 0+ masu salmon in autumn. In early winter, densities of 0+ and 1+ and older masu salmon were both correlated with cover availability. These results suggest that the amount of cover habitat is an important regulator for densities of juvenile masu salmon during winter.
SUMMARY: Freshwater environments in Hokkaido have been significantly altered by human activities. Restoration work in a channelized reach of the Shakotan River was carried out in 1996. Habitat complexity was increased by the addition of structures to the restored section which replaced a simple stream structure with pools, riffles and runs. More spawners and juveniles were found in the restored reach than in the unimproved control reach. Restoration with log dams and deflectors improved habitat for masu salmon, and is a useful habitat enhancement technique in natural streams. We encourage more programs such as ours where biologists and engineers work together to improve habitat conditions in streams to benefit salmon.
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