Grass carp are freshwater fish that feed primarily on aquatic plants. In the 1970s, they were introduced to a number of lakes and ponds in Florida as part of an experimental effort to control aquatic weeds, particularly the exotic plant Hydrilla (Hydrilla verticulata). The goal was to use these herbivorous fish to "graze down" nuisance plants.
Fortunately, the fish had a healthy appetite for Hydrilla and other types of nuisance aquatic plants. There was one drawback however; when over-stocked, grass carp consumed virtually every plant in a lake or water resource. Now, after several years of trial and error, it has been shown that, when stocked in appropriate numbers, grass carp can provide a low-cost, long-term, herbicide-free way of controlling nuisance plants.
IMPORTANT: There are potential problems associated with using grass carp as a form of aquatic plant control in a lake. Be sure to read the "Caveats and Limitations" section below for more information. Also, only triploid grass carp may be released into Florida waters and a permit must ALWAYS be obtained from the Florida Fish and Wildlife Conservation Commission (FWC) before doing so, even in private waters: http://myfwc.com/wildlifehabitats/habitat/invasive-plants/grass-carp/
For every stocking request received by the FWC and/or FDEP, a permit application must be filled out and a review process must be completed by fisheries and/or aquatic plant biologists from the area. Information recorded on the application (e.g., lake surface area, volume, possible escape routes and aquatic plant abundance, fish stocking rate, etc.) is then saved and compiled into the Triploid Grass Carp Database. The database is maintained by the Florida Fish and Wildlife Conservation Commission (FWC).
Grass carp permitting applications provide the FWC with a constant source of information regarding the effectiveness of stocking triploid grass carp, and associated problems. For example, they now have a much better idea of how many grass carp should be used, based on the size and volume of the lake and the abundance of aquatic plants.
As a general rule, FWC recommends a stocking rate of two to ten carp per acre. However, a complete assessment of the lake must be done before grass carp stocking will be considered (see "Caveats and Limitations").
Triploid grass carp are hatchery-raised fish that have been sterilized to eliminate any possibility of them reproducing in Florida waters. Sterilization is an essential measure considering their long life span (more than 10 years) and their ability to out-smart virtually any type of harvest technique (e.g., nets, hook and line, even poison). Sterilization is required so that this exotic fish does not reproduce and become a problem species, as have so many other exotic plants and animals in Florida. To produce sterile individuals, fish breeders subject fertilized roe to high pressure and/or temperature for several minutes.
Information is also now available concerning the effectiveness of grass carp versus the use of herbicides. In some instances, a combination of methods has been found to be most effective.
FWC Nonnative Species Profile: http://www.myfwc.com/wildlifehabitats/profiles/fish/nonnative-fish/grass-carp/