Population response of triploid grass carp to declining levels of Hydrillain the Santee Cooper reservoirs, South Carolina

Approximately 768,500 triploid grass carp (Ctenopharyngodon idella Valencie nnes) were stocked into the Santee Cooper reservoirs, South Carolina betwee n 1989 and 1996 to control hydrilla (Hydrilla verticillata (L.f.) Royle). H ydrilla coverage was reduced from a high of 17,272 ha during 1994 to a few ha by 1998. During 1997, 1998 and 1999, at least 98 triploid grass carp wer e collected yearly for population monitoring. Estimates of age, growth, and mortality as well as population models, were used in the study to monitor triploid grass carp and predict population trends. Condition declined from that measured during a previous study in 1994. The annual mortality rate wa s estimated at 28% in 1997, 32% in 1998 and 39% in 1999; however, only the 1999 mortality rate was significantly different. Few (2 out of 98) of the t riploid grass carp collected during 1999 were older than age 9. We expect i ncreased mortality due to an aging population and sparse hydrilla coverage. During 1999, we estimated about 63,000 triploid grass carp system wide and project less than 3,000 fish by 2004, assuming no future stocking.