Background/Question/Methods            Invasive species often have significant impacts on native species populations, but effects of native species on invasives are frequently overlooked.  The round goby (Apollonia melanostomus), a European invader, has exhibited explosive population growth throughout the Great Lakes.  Since this invasion, the Lake Erie watersnake (Nerodia sipedon insularum) has shifted from a diet of native fish to > 90% round gobies.  We investigated the effects this shift may have on round goby populations by determining the rate of round goby consumption by Lake Erie watersnakes.  Digestive rate (time until an average size round goby was 90% digested) was determined from 57 digestive rate trials on adult watersnakes. Digestive trials lasted 4-16 hours and were conducted at 25° and 30°C.  Maximum voluntary prey consumption was measured by providing thirteen adult watersnakes with free access to round gobies for eight hours per day on five successive days. Feeding frequency of free-ranging watersnakes was estimated by palpation of wild-caught snakes throughout the active season.

Results/Conclusions

Quadratic regression analysis demonstrated that adult watersnakes are capable of digesting 90% of an average size round goby in 16.4 hours at 30°C and 20.1 hours at 25°C.  During maximum prey consumption trials, males consumed 39.2% and females consumed 30.0% of their body mass in a 5-day period.  These laboratory trials indicate that watersnakes are capable of consuming and digesting more than one average sized goby per day.  Among free-ranging snakes, females were found with prey more often than males and feeding frequency varied over the active season.  On average, 12% of adult females and 7% of adult males contained prey. Using results from digestive rate trials, maximum prey consumption trials, estimated feeding frequency, and a population size of 12,000 adult watersnakes, projected annual consumption ranged from 147,630 – 2,253,233  (2,970 – 40,051 kg) round gobies per year.  A projection from estimates of field metabolic rate in a related snake species yielded 56,177 kg of round gobies consumed per year.  Even the highest projections fall far short of the estimated round goby population size in the western basin of Lake Erie (nearly 10 billion); at most, watersnake predation likely has only local effects on round gobies.