Densities of Juvenile Steelhead in Relation to Instream Habitat and Watershed Characteristics

Physical habitat characteristics can affect the abundance and distribution of organisms. However, the spatial scale of the variables used and the resolution of the data can affect both the outcome and applicability of such analyses. We used watershed-level characteristics and instream habitat variables to model the densities of two age classes of juvenile steelhead (Oncorhynchus mykiss) within a watershed in Idaho, USA. Variance partitioning showed that 41 to 50% of the variances in densities were due to unexplained differences between sampling occasions, and the rest resulted from variation at the study site and site-and-year levels, depending on the age class. Instream habitat variables better explained the variation in densities than did models that included watershed-level characteristics. Densities of subyearling steelhead were best explained by stream discharge, showing a negative relationship. Densities of yearling steelhead were best explained by a negative relationship with weekly average temperature; however, this relationship was statistically indistinguishable from zero. Finally, total densities (subyearlings and yearlings combined) were best explained by discharge and average daily temperature. The approach proved useful for understanding limiting habitat factors, but it should be interpreted in concert with detailed knowledge about life history variation in the study system.