Population response of triploid grass carp to declining levels of hydrilla in the Santee Cooper Reservoirs, South Carolina

Abstract

Approximately 768,500 triploid grass carp (CtenopharyngodonidellaValenciennes) were stocked into the Santee Cooperreservoirs, South Carolina between 1989 and 1996 tocontrol hydrilla (Hydrilla verticillata(L.f.) Royle). Hydrillacoverage was reduced from a high of 17,272 ha during 1994to a few ha by 1998. During 1997, 1998 and 1999, at least 98triploid grass carp were collected yearly for population monitoring.Estimates of age, growth, and mortality, as well aspopulation models, were used in the study to monitor triploidgrass carp and predict population trends. Condition declinedfrom that measured during a previous study in 1994.The annual mortality rate was estimated at 28% in 1997, 32%in 1998 and 39% in 1999; however, only the 1999 mortalityrate was significantly different. Few (2 out of 98) of the triploidgrass carp collected during 1999 were older than age 9.We expect increased mortality due to an aging populationand sparse hydrilla coverage. During 1999, we estimated about63,000 triploid grass carp system wide and project less than3,000 fish by 2004, assuming no future stocking.management, population sizeCtenopharyngodon idella, Hydrilla