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

—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. Invertebrates form the vast majority of food items consumed by stream-dwelling trout (Fleener 1951; Griffith 1974; Allan 1978; Hubert and Rhodes 1989; Dunham et al. 2000). Because trout species often show similar feeding behaviors in the same stream (Griffith 1974), there may be a large amount of diet overlap between sympatric species. Although diet overlap should not be an issue when supply exceeds consumption, the potential for competition exists, should food availability become limiting. The presence of seasonal food limitations in some trout populations in winter (Cunjak et al. 1987) and summer (Cada et al. 1987; Ensign et al. 1990) indicates that food can * Corresponding author: [email protected] 1 Present address: Appalachian Laboratory, University of Maryland Center for Environmental Science, 301 Braddock Road, Frostburg, Maryland 21532-2307, USA. Received November 27, 2002; accepted March 10, 2003 be limiting. When food limitations arise, species having a large amount of diet overlap may compete for food, and the better competitor may eventually replace the inferior species. Replacement may occur over the entire stream or locally in areas where diet overlap is greatest, in cases where habitat may mediate diet overlap. Circumstantial evidence suggests that nonnative brook trout Salvelinus fontinalis can replace native cutthroat trout Oncorhynchus clarki in western U.S. streams (Griffith 1988; Varley and Gresswell 1988). Brook trout are native to the eastern USA but have successfully invaded streams inhabited by cutthroat trout in the Intermountain West (Fuller et al. 1999). Brook trout have diets and a feeding strategy similar to those of cutthroat trout (Griffith 1974), which suggests that interspecific competition for food may be a factor in displacement of cutthroat trout. However, little information exists on diets when these species are sympatric, and the results are equivocal. Griffith (1974) re34 HILDERBRAND AND KERSHNER ported high similarity in diets of age-0 brook trout and cutthroat trout; however, similarity decreased in older fish. In contrast, Dunham et al. (2000) found relatively high diet overlap in adults of the two species in Nevada streams. Additional information on feeding behaviors of brook trout and cutthroat trout would be valuable in assessing the potential for interspecific food competition. In streams where brook trout and cutthroat trout occur together, brook trout tend to predominate in the lower-gradient reaches and beaver ponds, whereas cutthroat trout predominate in higher-gradient reaches (MacPhee 1966; Griffith 1972). If food competition does structure trout distributions, we predict that diet overlap between brook trout and cutthroat trout will be high in beaver pond and low-gradient reaches and low in high-gradient reaches. In this paper we present information on food availability, consumption, and food selectivity of nonnative brook trout and native Bonneville cutthroat trout O. c. utah and assess resource limitation and overlap in resource use, two prerequisites for interspecific food competition. Our first objective is to describe the composition and quantity of the invertebrate drift in Beaver Creek, Idaho. Our second objective is to provide information on diets, consumption levels, and food limitations for nonnative brook trout and native Bonneville cutthroat trout. Our final objective is to use the food availability and consumption information to assess the potential for competition between brook and cutthroat trout.