Development of habitat suitability criteria for trout in small streams

We observed 2863 trout in the wild to determine habitat utilization in small streams of the Kings River basin in California's Sierra Nevada mountains. The habitat utilization data were used to develop habitat suitability functions that provide input variables to the instream flow incremental methodology (IFIM) habitat assessment model of the U.S. Fish and Wildlife Service. Observations of habitat utilization of rainbow trout ( Salmo gairdneri ), brown trout ( Salmo trutta ), and brook trout ( Salvelinus fontinalis ) were obtained during the summer months of 1983 and 1984. The observations were made in small streams with discharges ranging from 0.7 m 3 s −1 to 0.03 m 3 s −1 . The streams are at elevations of 1250 to 2530 m. Equal effort was applied to observing undisturbed trout in all habitat types. Snorkeling proved to be the most effective method of observation. Individual trout of all species and life stages were most often observed in the lower half of the water column, utilizing low‐velocity currents of less than 3.0 cm s −1 . From the depth and velocity utilization data, several forms of habitat suitability functions were developed and evaluated: Univariate depth and velocity functions derived from frequency histogram analysis. Univariate depth and univariate velocity exponential polynomial models. Bivariate depth and velocity exponential polynomial models. Univariate exponential polynomial models provided the best fit to the data for each species, based on visual comparisons of response surfaces and contour plots, and comparisons of computed sums of squared errors. Bivariate models fitted to the data resulted in greater sums of squared errors than multiplicative aggragation of univariate models, and frequently predicted utilization at zero depth. The habitat suitability functions derived from the univariate exponential polynomial models provided the best input to the IFIM habitat assessment models.