Variation in Juvenile Steelhead Density in Relation to Instream Habitat and Watershed Characteristics

Physical habitat characteristics can affect the abundance and distribution of organisms, and are frequently used to predict the standing crop of stream fish for purposes of understanding their ecology and better direct management. However, the spatial scale of the investigation and the resolution of the data can affect the outcome of such analyses. In this study we coupled watershed-level characteristics with instream habitat variables to model the density of two age-classes of juvenile steelhead Oncorhynchus mykiss in a watershed in the Clearwater River basin, Idaho. Density varied considerably across time and space. Variance partitioning showed that 41-50% of the variances in density were due to unexplained differences between sampling occasions (residual variance), and the rest resulted from variation at the site and site-and-year levels, depending on the age-class. Instream habitat variables better explained the variation in density than did models that included watershed-level characteristics. The density of subyearling steelhead was best explained by stream discharge, with a negative relationship. The density of yearling steelhead was best explained by a negative relationship with average weekly temperature; however, this relationship was statistically indistinguishable from zero. Finally, total density (subyearlings and yearlings combined) was best explained by discharge and average daily temperature. We believe that our approach is useful for identifying the physical factors associated with the density of stream salmonids, but we stress that findings from correlative studies should be interpreted in concert with detailed knowledge about life history variation in the study system.