Grass Carp Bioenergetics and Control of Submerged Vegetation

A biological vegetation control project using grass carp was initiated in 2008 at the Felsenthal National Wildlife Refuge.  We estimated the biomass of aquatic vegetation, before (2008) and after (2010) control.  We estimated annual production of hydrilla.  We calculated consumption based on feed conversion (F/C) ratios from literature.  Consumption to production ratios were used to estimate the proportion of hydrilla production that grass carp might have consumed.  We also calculated consumption using the Wisconsin Bioenergetics Model 3.0.  We performed a sensitivity analysis on activity, egestion, and optimum temperature for consumption.  We compared consumption estimates from the two methods.  For both methods, individual consumption was calculated for maximum, median, and minimum observed growth.  Mean (SD) vegetation biomass was 57.8 (157.2) and 138.1 (346.5) g/m² in 2008 and 2010, respectively.  Hydrilla production ranged from zero to 2,276 g∙m^-2∙year^-1.  The F/C ratio method indicated consumption was 284,726, 125,386, and 50,256 g veg∙individual^-1∙year^-1.  Consumption to production ratios indicated grass carp could control 7390%, 1190%, and <1% of hydrilla production at maximum, median, and minimum observed growth and minimum, median, and maximum production, respectively.  Using the bioenergetic model, consumption was 150,991, 148,651, and 151,913 g veg∙individual^-1∙year^-1.  Adjusting the activity coefficient ±25% changed consumption ±20%.  Adjusting egestion ±25% changed consumption by +30% and -19%, respectively.  Adjusting the optimum temperature for consumption ±24% changed consumption by +75% and -38%, respectively.  Optimum temperature for consumption had the greatest effect on consumption.  All the estimates of consumption from the bioenergetics model method fell between the maximum and median consumption estimates from the F/C ratios method.  Both methods indicated observed growth could account for the observed decline in hydrilla biomass.