Habitat structure along channel-unit sequences for juvenile salmon: a subunit-based analysis of in-stream landscapes

1. Habitat structure and habitat use by juvenile masu salmon, Oncorhynchus masou Brevoort, in small streams in northern Hokkaido, Japan, were examined by considering 'subunits' (patches within channel units) as structural elements of stream reaches. 2. Whole wetted channel surfaces of three study reaches were divided into 0.5 x 0.5 m quadrats, which were grouped into eight subunit types according to water depth and velocity, and substratum conditions by a cluster analysis. The subunit distribution showed a regular mosaic pattern corresponding to the channel-unit sequence in each of the three reaches. 3. Juvenile masu salmon exhibited a strong preference for a subunit type characterized by greater depth and moderate current velocity (deep-moderate subunit; mean depth = 0.29 m; mean velocity = 0.19 m s(--1)). This subunit type usually occurred downstream of stretches with fast current. The preference of masu salmon for the deep-moderate subunit could be because of its usual spatial position in relation to other subunit types as well as to the characteristics of the subunit itself. 4. The results suggest that the value of a habitat is determined not only by the characteristics of the habitat itself, but also by those of adjacent habitats. Therefore, habitat use by stream fish should be studied in the context of the whole 'in-stream landscapes'.