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Triploid grass carp evolve as an aquatic-plant management tool

Fish Busters' Bulletin

Saturday, October 01, 2011

Media contact: Bob Wattendorf

Florida's outstanding freshwater fisheries and many species of wildlife are dependent on natural aquatic vegetation. Rooted aquatic plants stabilize shorelines, prevent erosion, reduce turbidity (muddy water), provide cover for fish to hide from predators, serve as food for insects and waterfowl, help reduce algal blooms, provide shade and cover for fish, and serve as a visible feature to help anglers locate sport fish.

Invasive plant species, however, can be harmful and have few natural checks. The spread of water lettuce, hyacinth and hydrilla are prime examples of nonnative plants that require management.

Proposals in the late 1960s and early 1970s to stock open water bodies with diploid (fertile) grass carp (Ctenopharygodon idella) to feed on nonnative plants quickly became controversial. These Asian carp spawn in similar habitats to striped bass, and naturally reproducing populations could have gotten out of control and wreaked havoc on native plant and wildlife communities.

Florida helped lead the way with grass carp research, determining their food habits and helping create triploid (functionally sterile) grass carp. Florida biologists also developed the certification program using Coulter Counters, which the U.S. Fish and Wildlife Service and many state agencies use to ensure only triploid grass carp are stocked. However, even with sterile fish, if too many grass carp are stocked and plants are eliminated, the problem may last as long as the fish are alive or longer - and that can be more than 15 years.

Thus far, in Florida, thanks in part to the diligence of management agencies, there are no documented cases of grass carp spawning in the wild. Recently, however, Fish and Wildlife Conservation Commission (FWC) biologists discovered a few diploid escapees in the Suwannee River, having come from flooding in bordering states. This caused renewed concern and the need to repeat the message about the importance of stocking only triploid grass carp.

So legitimate concerns revolve around use of grass carp and a full gamut of options for aquatic plant management are considered, depending on circumstances.In 2008, the FWC participated in "A Risk Analysis Pertaining to Use of Triploid Grass Carp for Biological Control of Aquatic Plants." The review concluded that in public waters, agencies should develop management plans for each stocking. The safest approach was determined to be an initial herbicide treatment followed by low-level stocking of triploid grass carp.

The FWC's Invasive Plant Management Section also recently concluded a public review process to create a new "Agency Position on Hydrilla Management" (; search "Hydrilla Position"). The position recognizes that native aquatic plant communities provide ecological functions to support diverse fish and wildlife populations. Hydrilla, as an invasive, nonnative plant, requires management. However, in water bodies where hydrilla is established, the FWC will manage it in light of the primary use of the water body. Plans will incorporate public input, be adaptive and reflect local conditions.

Chemical control is expensive (up to $750 per acre per year). Mechanical control is even more so - about twice as costly as chemical control. Biological controls using insects or diseases have not proved suitable for managing hydrilla, leaving triploid grass carp as the most effective biocontrol.

These fish can control certain aquatic plants in moderate-sized lakes at a cost of $20 to $250 per acre. In private ponds, golf course ponds, irrigation ditches and similar locations, where sport fishing is not the primary activity, certified triploid grass carp provide an environmentally sound, cost-effective way of controlling aquatic plants. Such stockings require a permit from the FWC. The Invasive Plant Management Section issues nearly 1,500 such permits annually, saving users money and reducing herbicide use (; select Aquatic Plants).

FWC personnel also plan and stock triploid grass carp in public waters. Currently, staff is monitoring about 100 locations. However, in situations where sport fisheries and waterfowl hunting are important, the fish may eat too many plants, destroy important habitat, adversely affect recreation and negatively affect the local economy.

In summary, FWC uses a permit program to allow citizens to purchase and stock triploid grass carp as a cost-effective means of controlling plants in self-contained private waters. The FWC also saves state money by using fish prudently in public water bodies to reduce the need for expensive chemicals, but it draws the line at stocking triploid grass carp in large, open systems where triploid grass carp are unpredictable and could negatively affect the state's immensely valuable sport fisheries and the delicate balance of our natural ecosystems.

 - Dave Eggeman contributed to this column. He is a biologist with the FWC's Invasive Aquatic Plants Section.

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