Growth and Population Size of Grass Carp Incrementally Stocked for Hydrilla Control

In weed control plans that use Grass Carp Ctenopharyngodon idella for intermediate control of hydrilla Hydrilla verticillata, the knowledge of population dynamics improves efficacy of management. Our objective was to characterize growth, mortality, and associated population metrics of long-lived (up to 16years) triploid Grass Carp that were incrementally stocked into Lake Gaston, VirginiaNorth Carolina, starting in 1995. Grass Carp (ages 116) were collected by bowfishers during 20062010. Growth of Grass Carp was described by the von Bertalanffy growth model as L-t = 1,297[1 e(0.1352(t +1.52))], where L-t is TL at age t. We used three methods to estimate Grass Carp mortality, and annual abundance and biomass of Grass Carp were estimated from each mortality estimate. Estimated annual mortality ranged from 0.20 to 0.25 depending on the method used. The use of constant mortality rates versus age-specific mortality rates produced divergent models of Grass Carp biomass and represented a different approach for tracking the progress of weed control. Grass Carp biomass (but not abundance) was related to hydrilla coverage in Lake Gaston based on several scenarios that described time lags between Grass Carp stocking in year i and decreases in hydrilla coverage (in years i, i + 1, ..., i + 5). Regardless of the mortality estimate used to derive Grass Carp biomass, the strongest biomasshydrilla coverage relationship was observed for a time lag of 4years. Fish older than age 10 constituted nearly 50% of the total Grass Carp biomass in Lake Gaston during some years, and the relationship between Grass Carp biomass and hydrilla coverage was strongest when fish up to age 16 were included in models. These results indicate that Grass Carp up to at least age 16 are important for weed control, thus highlighting the need for stocking models and bioenergetics models that include contributions of older fish when assessing long-lived Grass Carp populations. Received January 23, 2012; accepted October 3, 2012