We examined the diets of two species of stream-dwelling North American crayfishes (Orconectes propinquus and O. rusticus) at eight sites in the Midwestern United States both by measuring natural abundances of stable isotopes of carbon and nitrogen and by quantifying gut contents. Our goals were to test the hypothesis proposed by others that crayfish are primarily carnivorous, and in particular, to examine the frequency of fish consumption by crayfishes. Stable isotope profiles indicated that both species of crayfish were omnivorous and had a trophic position between those of other invertebrate consumers and fishes. Isotope profiles demonstrated that lower quality foods such as leaf litter, periphyton and fine particulate organic matter made up a larger proportion of the diets of both species than did animal material and that fish comprised approximately 12% of the diet of both crayfishes; this percentage was less variable than that of other food types. Gut content analysis corroborated stable isotope results in finding a similar percent occurrence of fish matter in both species. Our results suggest that the consumption of fish by crayfishes is often underestimated or ignored.
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Vol. 163 • No. 2