Analysis of Grass Carp Dynamics to Optimize Hydrilla Control in an Appalachian Reservoir

The primary objectives of this study were: 1) to evaluate the movement patterns, habitat use, and survival of triploid grass carp Ctenopharyngodon idella stocked to control hydrilla Hydrilla verticillata in a riverine reservoir (Claytor Lake, Virginia), 2) to examine grass carp population dynamics and hydrilla growth dynamics in Claytor Lake to guide long-term management efforts, and 3) to describe the aquatic plant community in the New River upstream of Claytor Lake to assess the potential for alterations due to potential grass carp herbivory. Only 3% of radio-tagged grass migrated out of Claytor Lake during the 2-year study. Grass carp movement patterns were significantly correlated with temperature-, weather-, and habitat-related variables. Grass carp selected specific cove, shoal and tributary habitats colonized by hydrilla. First-year survival of grass carp was 44% in 2011, and 25% in 2012. Grass carp growth rates were rapid in 2011, but declined in 2012 concurrent with significant reductions in hydrilla abundance. Based on grass carp population dynamics observed in Claytor Lake, our stocking model predicted that hydrilla could be controlled through 2030 by a grass carp standing stock of 5-6 metric tons. We documented 12 plant species in the New River upstream of Claytor Lake, 9 of which are preferred plants for grass carp suggesting that the plant community could be altered if migration rates increase. Grass carp can be effective for managing hydrilla in riverine reservoirs; however, continued monitoring of grass carp population dynamics, migration rates, and vegetation abundance could facilitate greater precision in management efforts.