Juvenile masu salmon (Oncorhynchus masou) abundance and stream habitat relationships in northern Japan

Relationships between abundance of juvenile masu salmon (Oncorhynchus masou) and stream habitat were analyzed on two spatial scales, stream reach and channel unit, in northern Hokkaido, Japan. Reach-scale analyses revealed that high water temperatures limited the salmon population level and that the salmon abundance increased with cover availability. These two limiting factors were strongly associated with the presence of riparian forest, emphasizing its importance for the salmon population. Channel-unit scale analyses showed greater use of pools by the salmon, with greater depth and abundant cover underlying the patterns of habitat use. However, the relative importance of depth and cover varied among the reach types having different cover availability. In reaches that had abundant cover, cover itself was less important in determining patterns of habitat use within the reaches. In contrast, in reaches with poor cover, patterns of habitat use were strongly affected by cover. The results indicated that habitat characteristics at the reach scale affect not only the salmon abundance of the stream reaches, but also the determinants of local abundance patterns of the salmon within the reaches. It was suggested that larger-scale influences in a hierarchy of habitat scales should be considered for better understanding of fish-habitat relationships.