Grass Carp (Ctenopharyngodonidella) are commonly stocked in ponds that are overrun with aquatic vegetation. They feed on certain plants and can vastly reduce the amount of vegetation in some ponds. Grass Carp are considered an invasive species, so states generally require three things from people wishing to stock them. The inflow and outflow of the lake or pond must be screened to keep the fish within the waterbody. The fish also needs to be certified as Triploid, which means they have been produced to have an extra set of chromosomes and are rendered sterile. Lastly, states require a permit to be completed and reviewed to ensure the first two requirements are met before the fish can be stocked.
Grass Carp will eat certain plant species preferentially. They generally provide excellent to fair control of American Elodea (Elodea Canadensis), Bushy Pondweed (Najas spp.), Stonewort (Chara / Nitella spp.), Waterstarworts (Callitriche spp.), Waterstargrass (Heterantheradubia), Slender Spikerush (Eleocharis spp.), Salvinia spp., Hydrilla (Hydrilla verticillata), Coontail (Ceratophulummdemersum), Bladderwort (Utricularia spp.), Duckweed (Lemna spp.), Watermeal (Wolffia spp.), Water Milfoil (Myriophyllum spp.) Curly Leaf Pondweed (Potamogetoncrispus) and Floating Leaf Pondweeds (Potamogeton spp.). Typically Grass Carp will not feed on the following plants unless everything else in the waterbody has been consumed or eliminated; Spatterdock / Yellow Pond Lilly (Nupharadvena), Watershield (Braseniaschreberi), Waterlilies (Nymphaea spp.), Water smartweed (Polygonum spp.), Cat-tails (Typha spp.), Pithophora algae (Pithophora spp.), Alligatorweed (Alternanthera philoxeroides), Water Primrose (Ludwigia spp.), Lyngbya algae (Lyngbya spp.), Water hyacinth (Eichhorniacrassipes), Water lotus (Nelumbium / Nymphaea spp.), Pennywort (Centellaasiatica), Parrot’s feather (Myriophyllumaquaticum).
Grass Carp usually grow upwards of 36” and 20lbs in our area. A difference in aquatic weed abundance should be seen in around 6-12 months and the fish should provide continued control for several more years. If over-stocked, a pond or lake can quickly become devoid of aquatic vegetation which negatively impacts water quality and other aquatic organisms. The best way to ensure successful control of aquatic weeds with grass carp is to stock conservatively. After evaluating their effectiveness, the following year, a decision can be made to stock a few more at that time. They are usually re-stocked after a few years have passed replace individuals lost to normal mortality. Low, medium, and high stocking rates are considered 5, 10, and 15 fish per acre, respectively.