Density-Dependent Changes In Grass Carp Growth And Mortality In Long-Term Aquatic Plant Management

The Grass Carp Ctenopharyngodon idella is an herbivorous fish that has been stocked throughout the United States for aquatic plant control, commonly to manage hydrilla Hydrilla verticillata. Triploid Grass Carp were purported to have a mean life span of about 11 years based on otolith-assigned ages and mortality estimates for fish associated with large stocking efforts through which aquatic plants were eliminated. Recent studies in aquatic systems where preferred foods were not eliminated have found that Grass Carp may live up to 30 years or more, but information about growth and mortality is lacking. About 130,000 Grass Carp have been stocked since 1995 to manage hydrilla in combination with fluridone application in Lake Gaston, Virginia-North Carolina, USA. Hydrilla coverage peaked at about 1,300 ha in 2003 and was reduced to about 130 ha by 2014. Since 2006, a total of 419 Grass Carp have been collected for age and growth analysis. A Bayesian approach was used to estimate von Bertalanffy growth function parameters that incorporated the effects of a linear trend in year, hydrilla abundance, or Grass Carp density and to assess population changes. Fish that were stocked in 1995 persisted through 2017. Mean maximum length (L-infinity) of Grass Carp decreased by about 200 mm with an increasing number of Grass Carp per hectare of hydrilla, and fish reached maximum size earlier in life. Indirect mortality estimates derived from growth parameters increased with Grass Carp density. Because preferred food availability affects mortality rates, stocking numbers should vary accordingly to maximize the effectiveness of hydrilla control programs.