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

Abstract

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 16 years) triploid Grass Carp that were incrementally stocked into Lake Gaston, Virginia–North Carolina, starting in 1995. Grass Carp (ages 1–16) were collected by bowfishers during 2006–2010. 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 biomass–hydrilla coverage relationship was observed for a time lag of 4 years. 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

ACKNOWLEDGMENTS

We thank the volunteer bowfishers for Grass Carp collection, and we are grateful to Cory Kovacs and Catherine Lim (Virginia Department of Game and Inland Fisheries) for otolith preparation and age determination during 2006–2008. Phil Kirk, two anonymous reviewers, and the editors provided comments that greatly improved this manuscript. Funding for the project was provided by the LGWCC and the Acorn Alcinda Foundation.