Evaluating redband trout habitat in sagebrush desert basins in southwestern Idaho

We estimated abundance quantiles of redband trout Oncorhynchus mykiss gairdneri relative to five site-specific habitat variables (stream shading, bank cover, bank stability, fine sediment in the stream substrate, and cover for adults) and one landscape variable (distance from stream headwaters) on 30 streams in southwestern Idaho during 1993–1998. In addition, the five site-specific habitat variables were used to calculate a habitat suitability rating (HSR) used by the U.S. Bureau of Land Management to determine habitat quality of sagebrush desert streams for redband trout. Variation in abundance increased significantly with increasing HSR; the highest abundances were only found with high HSRs, indicating that the HSR model correctly predicted habitat quality for redband trout. However, a model that consisted of stream shade, distance from stream headwaters, and their interaction best predicted redband trout density, explaining 36% of the variation in adult density in sagebrush desert basin streams; stream shade explained most of the variation in redband trout density. When habitat quality was modeled on shade alone, the precision in predicting adult redband trout density was similar to that of the HSR model, as evaluated with tolerance intervals that contained 80% of future observations of redband trout density with 95% confidence. Increasing stream shade in the uppermost 50 km of a stream would result in the greatest increase in redband trout density. We recommend that land managers primarily evaluate the habitat quality of sagebrush desert streams by quantifying the amount of stream shade provided by riparian shrubs and trees. Use of a multivariable habitat model should be retained for desert streams where shade from riparian plant communities is limited.