The Appalachian brook crayfish is abundant in most higher-elevation streams in south-central Pennsylvania, including many affected by acid precipitation. An apparent paradox noted by other researchers is that this species often attains higher densities in acidic than neutral reaches, despite their lower growth rates in acidified waters. These acidified streams (pH < 5.0) typically do not support fish, though brook trout are present in downstream, less acidic, reaches of the same streams. We hypothesized that the absence of trout might lead to changes in crayfish diets, due to reduced competition and predation. In particular, we expected that these highly omnivorous crayfish would include a greater proportion of invertebrate food in their diets in the absence of trout, increasing their dietary niche width. We tested this hypothesis in two upper elevation streams, Laurel Run (Perry County) and Mountain Creek (Adams and Cumberland Counties), both of which have upstream acidic reaches (pH < 4.7) and downstream more neutral reaches (pH > 5.5) due to remediation with limestone sand. We collected crayfish, potential aquatic food sources, and terrestrial invertebrates at upstream and downstream sites in both streams and analyzed stable isotopes ratios of carbon and nitrogen to estimate the approximate contribution of each component to crayfish diet.

Results/Conclusions Crayfish density was on average higher, but individual size smaller, in the upstream (acidic) reaches. Other benthic invertebrate densities were lower in the acidic reaches, and fish were absent. Behavioral observations indicated at least occasional use of terrestrial invertebrates by crayfish, along with unusual daytime foraging activity, in the more acidic reaches, but not in neutral reaches supporting fish. We expect isotope results to indicate increased dietary niche width of crayfish in the more acidic reaches that lacked fish. Although this study design did not allow us to separate the direct and indirect effects of stream acidification on crayfish diet, it does suggest that the absence of predation and competition from fish may play a role in the paradoxically high densities of Appalachian brook crayfish in these streams.