Removing Unwanted Fish

Fish populations in neglected and poorly managed lakes rapdily become out of balance and ponds may become overpopulated with unwanted fish species. Small lakes and ponds often become crowded with small or stunted bass or bluegill populations or become populated with green sunfish, bullhead catfish, shiners, or other unwanted fish species. The best lake management option in these situations may be to destroy all fish in the pond and simply start over.

Removing or killing the fish population usually is much easier and less expensive if the pond can be drained dry or partially drained and the fish concentrated. This also drastically lowers the volume of water that must be treated and makes this lake management technique much easier. Fish will survive in very small pools or puddles away from the main body of water, so in order to get a complete kill, you must treat all puddles around the lake and in the watershed, regardless of size!

Rotenone is a registered aquatic chemical that is used to kill fish. In most areas, rotenone for pond and lake renovation can be purchased from most farm supply stores. As with most chemicals, you must have a private applicator license to purchase and use this chemical or hire someone that does. Rotenone comes in liquid or powder formulations, at a concentration of 5% active ingredient.

Rotenone should be applied at a rate of 10 pounds per acre-foot. The volume of water in the lake (in acre-feet), or that remaining after drawdown, must be estimated so this concentration of rotenone can be calculated. One gallon of the liquid rotenone formulation (5%) is sufficient to treat approximately 1 acre-foot. The acre-feet in a particular lake can be calculated by multiplying the surface area in acres times the average depth in feet. For example, a 4-acre lake with an average depth of 8 feet would have 32 acre-feet, and would require 32 gallons of the liquid 5% formulation to treat.

Powdered rotenone should be mixed with water (about 2 gallons per pound of powder). Liquid rotenone also should be diluted with water at a rate of about 10 gallons of water to 1 gallon of rotenone. Apply rotenone evenly over the lake using buckets, sprayers or pumps. If the lake is more than 4 feet deep, use a hose to pump rotenone into deep sections of the lake. Rotenone applied properly and at recommended rates will not harm most livestock, such as goats and cows, even if they drink the water. Pigs, however, might be affected by the rotenone formulation, and ducks and geese may suffer if they gorge themselves on dead or dying fish. In addition, make sure no water containing rotenone runs off your property to kill fish downstream!

Rotenone is usually applied in the summer or fall when water temperature is above 70 degrees F. Contact a fisheries biologist or trained professional for additional information on purchasing and applying rotenone. Rotenone will typically dissipate within 3 to 10 days, depending on weather conditions. Generally it is safe to restock 2 to 3 weeks after applying rotenone. To check for the presence of rotenone, place a few small bluegill in a minnow bucket and float it in the pond. If the fish are still alive after 24 hours it is safe to restock.