Influence of Habitat Type on Food Supply, Selectivity, and Diet Overlap of Bonneville Cutthroat Trout and Nonnative Brook Trout in Beaver Creek, Idaho

Abstract

We collected invertebrate drift samples and stomach contents of native Bonneville cutthroat trout Oncorhynchus clarki utah and nonnative brook trout Salvelinus fontinalis in Beaver Creek, Idaho, during August 1995 to assess food availability and the potential for competition. Regardless of whether samples came from beaver pond or from high-gradient or low-gradient reaches, aquatic Diptera numerically dominated drifting invertebrates by at least fivefold over all other categories captured in 1-h drift samples. Abundances and drift densities of drifting invertebrates were high in the three reach habitat types sampled: beaver pond (3,152 individuals; 18.9 invertebrates/m³), high-gradient reach (5,216 individuals; 26.5 invertebrates/m³), and low-gradient reach (4,908 individuals; 17.2 invertebrates/m³). Cutthroat trout consumed significantly more invertebrates per individual than did brook trout. However, there was no relationship between fish length and consumption. Diets of both brook and cutthroat trout were dominated by Diptera in beaver ponds and terrestrial invertebrates in the high-gradient reach. In the low-gradient reach, Diptera dominated brook trout diets, whereas both Diptera and terrestrial invertebrates dominated diets of cutthroat trout. Both trout species consistently selected terrestrial invertebrates and Trichoptera in all reach types. Diet overlap between brook and cutthroat trout was 92% in beaver ponds, 75% in the high-gradient reach, and 65% in the low-gradient reach. The high degree of diet overlap suggests the possibility of competition between nonnative brook trout and cutthroat trout if food should become limiting, but we found little evidence that food was limiting during late summer in Beaver Creek.