An Assessment of Triploid Grass Carp Stocking Rates in Small Warmwater Impoundments

Various stocking rates for triploid grass carp Ctenopharyngodon idella were evaluated to identify a threshold rate at which macrophytes were Suppressed but not eliminated. Eight southwest Florida urban impoundments (0.8–45.3 ha) containing macrophyte species preferred by grass carp were monitored to document changes in macrophyte biomass, chlorophyll a, and total phosphorus after stocking. Stocking rates were determined from empirical estimates of the macrophyte biomass of the entire lake and ranged from 3 to 10 grass carp/metric ton of vegetation (wet weight). Stocking rates of 4–8.4 grass carphnetric ton resulted in gradual reductions to zero macrophyte biomass in 8–17 months. Macrophyte biomass at two sites stocked with 3 grass carp/ metric ton decreased significantly but did not decline to zero during the 4 or 5 years they were monitored after stocking. Annual mean chlorophyll-a and total phosphorus concentrations remained stable at sites where macrophytes were suppressed but varied greatly at some sites where they were eliminated. Shifts in macrophyte species composition did occur at sites where macrophytes were suppressed or began to regrow after a period of zero macrophyte biomass. However, the number of species present and composing at least 4% by weight of the total macrophyte biomass did not decline.