Using Triploid Grass Carp to Control Aquatic Vegetation

Invasive aquatic weeds can create major impacts on freshwater ecosystems. One of the primary reasons invasives are able to thrive, spread rapidly, and outcompete native species is that the environmental checks and predators that control these species in their natural settings are lacking in the ecosystems and habitat in which they become introduced.

The subsequent damages they cause occur on many ecological levels including competition for food or habitat (feeding, refuge, and/or spawning), direct predation and consumption of native species, introduction of disease or parasites, and other forms of disruption that lead to the replacement of the native species with the invasive species. As a result, invasives often cause serious harm to the environment, the economy, and even human health.

Some of the more commonly occurring non-native aquatic plant species that impact East Coast lakes, ponds, and reservoirs include curly-leaf pondweed, eurasian watermilfoil, hydrilla, and water chestnut.

The introduction of triploid grass carp to freshwater lakes and ponds can be an effective solution and natural alternative to managing and mitigating aquatic weed growth. When stocked at a proper rate, at correct sizes, targeting proper plant species, and the right time, triploid grass carp can reduce or eliminate the need for chemical treatment of the water to control aquatic vegetation.

Originally from Asia, grass carp have been imported to the United States since the 1960s to intentionally release into controlled freshwater environments for aquatic plant control. Grass carp, which rely almost entirely on aquatic plants for their diet, prefer to eat many of the non-native aquatic plant species that negatively impact freshwater environments, including the aforementioned pondweed species and watermilfoil.

Triploid Grass Carp in Woodridge Lake

Woodridge Lake is a beautiful 385-acre freshwater lake tucked away in the hills of Litchfield County, Connecticut. The lake, which is fed by the Marshepaug River, is a man-made resource, with a dam at one end that allows the level of the lake to be controlled.

Woodridge Lake Property Owners’ Association (WLPOA) closely monitors the lake, conducting water sample testing on a weekly basis. As with all waterbodies, the lake experiences aquatic weed growth, some years worse than others due to a variety of factors including climate change.

As a method to naturally mitigate aquatic weed growth, WLPOA plans to introduce triploid grass carp to the waterbody. A study by the Connecticut Agricultural Experiment Station states that grass carp is “the only biological control used successfully in Connecticut.”

Since the grass carp are an introduced species, only triploid grass carp, which are sterile, can be used. This eliminates the possibility that the stocked fish can reproduce and overpopulate the lake, or if any were to escape the lake they could not affect other waterbodies. As an additional measure of protection, to ensure that the carp remain in the lake, a screen, or emigration control device, is required. Princeton Hydro, in partnership with WLPOA, Rowledge Pond Aquaculture, and CTDEEP recently completed the installation of a carp screen.

The screen, which was custom designed by Princeton Hydro, is located in the outlet structure of the Woodridge Lake Dam, downstream of the spillway crest and within the concrete stilling basin of the spillway structure. Subsequently, the installation and operation of the carp screen will have no impact on spillway capacity or water surface elevations at the spillway crest. In addition, there will be no impact on the flow capacity or the water surface elevations of the Marshepuag River downstream of the dam outlet structure.

The emigration control device is a modular, vertical-bar screen composed of eight sections. A modular screen design was chosen to facilitate off-site fabrication and easier installation, as well as repair of an individual section, if necessary. Installed, all eight sections transect the entire 40-foot width of the spillway structure.

WOODRIDGE LAKE CARP EXCLUSION DEVICE DESIGN by Princeton Hydro

The carp screen was specifically designed to be easy to operate and maintain, minimizing clogging and facilitating easy cleaning from the downstream side of the screen during a range of flows. The operation and maintenance plan also consists of inspections every three months and precipitation-based inspections conducted by the WLPOA staff.

Overall, the use of grass carp will help Woodridge Lake manage aquatic weed growth in a natural way and maintain a healthy and vibrant lake environment for years to come.

To learn more about Rowledge Pond Aquaculture, the oldest private fish hatchery in Connecticut, go here: rowledgepond.com. For more information about Princeton Hydro’s lake management services, go here: bit.ly/pondlake.