Using Boat Electrofishing to Estimate the Abundance of Invasive Common Carp in Small Midwestern Lakes

The common carp Cyprinus carpio is among the most invasive fish worldwide, but practical methods for estimating its abundance have not yet been developed. Particularly needed are methods that can accurately assess low densities of common carp to enact proactive management strategies before populations reach damaging levels. In this study we tested whether the density of adult common carp in small Minnesota lakes could be accurately predicted from their catch rates using boat electrofishing. We used mark and recapture to estimate the abundance of common carp in eight Midwestern lakes with a wide range of common carp densities (13-400 carp/ha), while also surveying each lake using boat electrofishing. In addition, we reduced common carp abundance by up to 90% in two lakes to test whether this was accompanied by a similar drop in electrofishing catch rates. A regression analysis showed that electrofishing catch rates increased linearly with increasing densities of common carp. A cross-validation procedure showed that boat electrofishing can accurately estimate common carp densities; however, we observed a tendency to overestimate low densities and underestimate high densities. Our results suggest that electrofishing surveys can be routinely employed to estimate common carp densities in small lakes.