Quantifying the Habitat Preferences of the Stonefly Pteronarcys Californica in Colorado

The salmonfly (Pteronarcys californica) is a large stonefly species that is ecologically important and recreationally significant to anglers in coldwater river systems throughout the western United States. Salmonflies are sensitive to disturbance and pollution and are considered an indicator of quality coldwater stream habitat. They are declining in range and abundance in some river systems and their extirpation from several western rivers has led to restoration attempts including habitat improvement projects and direct reintroductions. These efforts could be improved with a clear understanding of the causes of these declines, a quantitative description of the species' preferred habitat, and practical management recommendations for the restoration of habitat where it has been lost. The objective of this study was to measure variables that describe the physical habitat of sites supporting varying densities of salmonflies in Colorado. Width to depth ratio, bed slope, D50 sediment size, percent fine sediment, and embeddedness were measured at 18 riffle sites and compared to an index of salmonfly density estimated over 3 years. Correlation analysis and multiple linear regression with model selection were used to identify important variables. Salmonfly density was highest at sites with low amounts of fine sediment, low cobble embeddedness, and large cobble size. Fine sediment (particles < 2 mm) was the single best predictor of salmonfly density and ranged from 0-6% at sites with high salmonfly densities. As habitat characteristics influence the range and density of salmonflies in Colorado and elsewhere, results from this study can inform land use practises, flow management, and river restoration activities to benefit this important indicator species.