Long‐Term Population Response of Triploid Grass Carp Stocked in Piedmont and Coastal Plain Reservoirs to Control Hydrilla

Abstract

AbstractThe triploid Grass Carp Ctenopharyngodon idella has been used to control hydrilla Hydrilla verticillata infestations in southern U.S. reservoirs for several decades. After eliminating hydrilla in the water column, Grass Carp must be maintained in sufficient densities to control hydrilla regrowth from the tuber banks in the hydrosoil. We monitored the long‐term response of triploid Grass Carp populations that had eliminated hydrilla within the water column in two Piedmont (Lake Norman and Mountain Island Lake, North Carolina) and two Coastal Plain reservoirs (the Santee Cooper system comprising Lakes Marion and Moultrie, as well as the connecting canal in South Carolina). Triploid Grass Carp stocked in Lake Norman and Mountain Island Lake exhibited both slow growth and erratic, but potentially high, mortality. Due to erratic survival in the two Piedmont reservoirs, we could not estimate mortality using a catch curve. Fish stocked into the Santee Cooper system not only grew larger and faster, they also persisted (i.e., significant numbers of age‐16–21 fish were collected during sampling in 2011). We hypothesize that Piedmont reservoirs without hydrilla in the water column and with little naturally occurring aquatic vegetation have a very low carrying capacity for triploid Grass Carp. Consistent, long‐term survival of triploid Grass Carp in the Santee Cooper system may be due to available food provided by hydrilla regrowth in the water column, floating vegetation, and less‐palatable, native, submersed vegetation. Hydrilla management in systems with residual plant food could involve estimating an average mortality rate and maintaining enough fish (i.e., about one fish per four hectares of surface area) to control hydrilla regrowth. In Piedmont reservoirs, possible management alternatives could include maintenance stockings based upon (1) yearling stocking rates that were successful in the past, (2) stockings determined from indirect measures of mortality such as from von Bertalanffy growth equation parameters, or (3) stockings derived from measures or indices of abundance such as counts conducted at night by bowfishers.Received January 28, 2014; accepted April 20, 2014