Re-Examining the Importance of Fish in the Diets of Stream-Dwelling Crayfishes: Implications for Food Web Analyses and Conservation
Taylor, Christopher A., Soucek, David J., The American Midland Naturalist
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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As one of the largest and most abundant invertebrates found in North American aquatic ecosystems, crayfishes often account for a substantial portion of benthic biomass (Momot et al, 1978; Huryn and Wallace, 1987) and play a significant role in the transfer of energy between trophic levels (Lodge et al, 2000). As such, they have long received attention in food web studies (Momot et al, 1978; Creed, 1994; review in Nyström, 2002) . In these studies and others, including recent work (Jones and Bergey, 2007) , crayfishes have been classified as opportunistic omnivores. As pointed out by Momot (1995), this perception of indiscriminant omnivory can in part be traced back to Huxley (1880) who, in his exhaustive treatment of crayfish anatomy and physiology, stated that ' 'few things in the way of food are amiss to the crayfish." This perception was supported by subsequent diet studies of crayfishes that found a wide variety of food items in gut contents (e.g., Prins, 1968; Boyd and Page, 1978; Hobbs III, 1993).
It was not until Momot's (1995) review that the role of crayfishes as indiscriminant omnivores was questioned. Momot (1995) drew attention to studies that demonstrated the high levels of animal matter in crayfish diets compared to other aquatic vertebrate predators. He also suggested that studies using masticated gut contents may exaggerate the importance of plant matter given its slower rate of digestion and, hence, higher relative abundance in guts. Momot hypothesized that crayfishes were primarily carnivorous and became facultative herbivores only after animal protein sources were exhausted (Momot, 1995). A more recent field study by Parkyn et al (2001) examining the feeding ecology of stream dwelling crayfishes using gut contents and stable isotopes supported the role of crayfish as predators.
The study of feeding ecology has been facilitated recently through the use of stable isotopes. Stable isotope ratios of consumers reflect the isotope ratios of all food sources assimilated and have been used by ecologists for nearly 30 y. As rates of excretion between light to heavy isotopes are predictable, the ratios of carbon (C) and nitrogen (N) isotopes in animal tissue can aid in analyzing food web connections and food sources over weeks or months (Peterson and Fry, 1987; Stenroth et al, 2006) as opposed to the short term (2436 h) resolution gained from gut content analyses. Stable isotopes have been employed in previous studies to determine trophic position of crayfishes in lotie (Whitledge and Rabeni, 1997; Parkyn et al, 2001; Bondar et al, 2005; Rudnick and Resh, 2005) and lentie ecosystems (Roth et al, 2006; Stenroth et al, 2006). However, we believe that one potential food resource is noticeably absent from these studies. …