The Use of Grass Carp in Alabama Ponds
Joseph B. Jernigan
District IV Fisheries Biologist
Many ponds in Alabama suffer from aquatic plant weed infestations. Heavy weed infestations can negatively affect fish populations, cause poor esthetics, limit access for fishing and in severe cases, cause fish kills. Physical, chemical, and biological methods can be used to control weeds, but the most often used method is the use of a fish known as grass carp or white amur.
The physical removal of weeds (raking out) is one method of weed reduction. This method is cumbersome and tiring, with regrowth from seeds and fragmentation allowing weeds to return quickly.
The use of approved aquatic herbicides to control problem weeds is very efficient and can be used to control specific weed problems while not harming nontarget weeds. However, herbicides are expensive and if used improperly, may cause a fish kill.
Biological weed control methods include stocking insects or fish that will eat the weeds in the pond. The most successful of the biological control agents is grass carp. Grass carp are very economical, costing an average of $5 per fish. Grass carp provide effective weed control on a wide variety of plants for several years.
Native to rivers in parts of Russia and China, grass carp were introduced to the United States in 1963 for weed control and as a food source. Although many states only allow sterile fish and require a permit prior to stocking. Alabama requires a permit to stock grass carp in public waters, but there are no restrictions on stocking grass carp in private Alabama ponds. Grass carp can be purchased from commercial fish producers throughout the state.
Grass carp feed almost continuously when water temperatures are between 70 and 80 degrees, and under ideal conditions can eat 2-3 times their body weight in a day. Grass carp can grow to over 10 pounds in the first year, and fish over 40 pounds are not uncommon in ponds with a heavy weed infestation. Contrary to popular opinion, adult grass carp do not eat sport fish or compete with sport fish for food. Once a grass carp reaches six inches in length, it will feed almost exclusively on plant matter.
Although grass carp are best used to control filamentous algae (moss) and many submerged plants, they will control floating plants when stocked at higher rates. Therefore, stocking rates vary depending on the species of plant to be controlled and the severity of the infestation. Normal stocking rates are 5 to 20 fish per acre. However, even with the high stocking rate, weed control is slow compared to other methods, and it may take an entire growing season before satisfactory control is achieved.
In ponds with an established population of adult bass, the grass carp stocked should be at least 8 to 10 inches in length to prevent loss due to predation. However, fingerlings are suitable for newly stocked ponds.
Grass carp do not reproduce in ponds so periodic restocking is required to maintain good weed control. They are a riverine fish that prefer flowing water, and can easily leave a pond, going over the spillway, during heavy rains. Therefore, ponds that have even occasional overflow should be equipped with a special fish barrier to prevent fish from escaping when water flows over the spillway. In most pond situations, effective control is obtained for several years with a single stocking, but you should restock at the first sign of unwanted weeds returning.
If you have questions about grass carp, spillway barriers, or other weed control methods consult with a fisheries biologist from the Division of Wildlife and Freshwater Fisheries or your local county extension agent. Look us up on the web at www.outdooralabama.com or write us at Fisheries Section, Division of Wildlife and Freshwater Fisheries, 64 North Union Street, Montgomery, Alabama 36130.
Stocked grass carp can also be caught on hook and line,
providing tremendous sport and a tasty meal.
Note: In Alabama, it is illegal to move or stock a grass carp or other aquatic organism from one public water to another without a permit.
Written in November of 1999.