It is sometimes necessary to remove some or all of the fish from a pond as part of a renovation plan or other fish management strategy. In recreational fish ponds that are to be stocked with bream and bass, it is essential that no other fish of any species be present (see “Other Fish Species” section in HGIC 1712, Stocking & Harvesting Recreational Fish Ponds). This is necessary in order to have a proper balance between bass and bream to ensure good fishing for an extended period of time.
Fish may be killed by using rotenone, a chemical compound extracted from the roots of certain species of the bean family. Rotenone, also known as derris or cube, is used as an insecticide as well as a piscicide (fish poison). Rotenone, used as a piscicide, is a restricted use chemical, and a license is required for its purchase and application. If the rotenone is to be used only in a property owner’s own pond, then a Private Pesticide Applicator’s License is all that is required. This license may be obtained through the local Clemson Extension Service office. There is a small fee for the license, and an examination is required. If rotenone is to be applied on someone else’s property or for a fee, a Commercial Category 5 Pesticide Applicator’s License is necessary. Information on requirements for a commercial applicator’s license may be obtained from the Clemson University Department of Fertilizer and Pesticide Control (864-646-2150).
Fish killed by rotenone may not be eaten according to regulations of the United States Food and Drug Administration. Treated water is not harmful to animals, which may drink from the pond if the rotenone is applied in accordance with instructions on the label. In some instances, swine may be sensitive to rotenone, and caution should be exercised before allowing swine to drink from treated ponds. Rotenone should only be applied to waters under control of the applicator. Rotenone may not be applied to flowing or public waters except by authorized personnel.
Rotenone is most commonly available in two forms, 5 percent water-wettable powder and a 5 percent liquid. Other concentrations are manufactured, so the label should be read carefully to determine amounts of rotenone required. Rotenone in powder form may cause respiratory irritation if inhaled. A proper respirator mask should be used during mixing of rotenone powder. Liquid rotenone spray should not be allowed to drift. As with all pesticides, read and follow the label directions completely.
If powdered rotenone is used, it must first be mixed with water to form slurry. Additional water can be added to the slurry to form a liquid that can be applied to the pond at the proper rate. Liquid rotenone can be mixed with twice its volume of water before application, or it can be poured in the concentrated form directly into the water, if the container spout is plugged to allow control of the flow to ensure proper coverage of the entire pond. The liquid or slurry should be poured into the prop wash of a boat engine as the boat moves across the entire pond. More rotenone should be applied over deeper water than in the shallows. All potholes and standing water within the pond site must be treated, regardless of size.
Lowering the water level of the pond by draining or pumping is recommended because it will reduce the volume of water to be treated and the amount of rotenone that must be purchased. Drain outlets in the pond must be closed prior to the application of rotenone. Any rotenone that escapes downstream is toxic to fish, and the applicator is responsible for any fish mortalities.
The amount of rotenone required to treat a pond effectively is determined by the volume of water expressed in acre-feet and the species of fish in the pond. Rotenone will lose its toxicity at temperatures of 65 to 75 °F in five to 21 days after application. Potassium permanganate can be used to detoxify rotenone, but a fisheries biologist should be consulted for application rates. The best time to apply rotenone for pond renovation is the first week in September to reduce the chance of introduction of undesirable species prior to stocking bream in November.
|Rotenone Application Table|
|Treatment||Quantity Rotenone Required|
|5% Formulation (ppm)||Active Rotenone (ppm)||5% Liquid (gal/ac ft)||5% Powder (lb/ac ft)|
|Normal Pond*||0.5 – 10||0.025 – 0.050||0.17 – 0.33||1.4 – 2.7|
|Bullheads or Carp Present||1.0 – 2.0||0.050 – 0.100||0.33 – 0.66||2.7 – 5.4|
Carp – Organic Pond
|2.0 – 4.0||0.100 -0.200||0.66 – 1.3||5.4 – 10.8|
|Selective Treatment||0.10 – 0.13||0.005 – 0.007||0.03 – 0.05||0.27 – 0.3|
|*Unless it is certain that bullhead, catfish, carp or other rough fish are not present in the pond, it may be prudent to apply rotenone at the higher application rate.|
How to Calculate Rotenone Application
To calculate the amount of rotenone required to treat a pond, it is necessary to first determine the volume of the pond in acre feet (ac ft) and multiply this figure by the amount of liquid or powder called for in the table above.
- Determine the surface area of the pond in acres. If this is not known, or it is impractical to measure directly, the local USDA Natural Resources Conservation office can determine area estimates from aerial photographs.
- Determine the average depth (feet) by measuring the depth in several places, taking into consideration the shallow edges and the deepest points. Take the mathematical average of these depths.
- Multiply the surface area (acres) times the average depth (feet) to get the volume in acre-feet. Example: A 1-acre pond 4 feet deep would have a volume of 4 acre feet.
After applying rotenone, fish in a treated pond will begin to surface within five minutes to one hour, and dead fish may continue to be found for several days. Dead fish may be either collected from ponds and disposed of properly or may be left in the pond for natural environmental decay. The length of time rotenone will remain active will depend on the temperature of the water. The warmer the water, the more quickly the rotenone will detoxify.
Before the rotenone-treated water is stocked with fish, the water should be tested to make certain that it is no longer toxic. While there is a chemical test for presence of rotenone, a simple test for toxicity may be made by placing a few fish in minnow baskets in a few locations in the pond. Since there is a danger of test fish escaping, only species intended for stocking (such as bass, bream or channel catfish) should be used. Deeper, cooler water is likely to remain toxic for a longer period of time. Generally one to three weeks is required to detoxify rotenone before restocking a pond. Pond managers should plan ahead and determine when fish will be delivered to restock the pond and be sure an ample time period for rotenone detoxification is scheduled.