Ponds that contain large numbers of rough fish such as bullheads, carp, perch and suckers must be renovated. These species compete with bass and bluegill for food, cover, and spawning sites. Nuisance fish tend to overpopulate and ruin sportfishing. The best strategy in this situation is to destroy all the fish in the pond and restock with desirable species.
The least expensive method of renovating a pond is to drain it dry, or partially drain it and kill the remaining fish with rotenone. Rotenone comes in liquid or powder formulations. Powdered rotenone is usually available at local feed and seed stores. Liquid formulations are available through mail-order from aquaculture chemical suppliers. Contact your county extension office or DGIF for sources of liquid rotenone. Be aware that you must be a Certified Pesticide Applicator in Virginia to purchase and use rotenone.
Rotenone should be applied when water temperatures are above 65° F, usually in late summer or early fall. A concentration of 2 ppm will cause a complete fish kill. This equates to 5.4 pounds of granular rotenone or 0.65 gallons of liquid rotenone for each acre-foot of water. The volume of your pond measured in acre-feet can be calculated by multiplying the surface area in acres by the average depth in feet. For example: if you have a 2-acre pond with an average depth of 6 feet and you treat with 0.65 gallons of rotenone per acre-foot, you need 7.8 gallons of rotenone (2 acres x 6 feet x 0.65 gallons per acre-foot = 7.8 gallons). Apply the rotenone uniformly over the entire pond using buckets, sprayers, or pumps. Deep areas (over 4 feet deep) should be treated by pumping the rotenone through a hose that is weighted at one end while motoring back and forth across the pond. One of the drawbacks to using rotenone is that thousands of dead fish will be floating on your pond. They must be picked up and discarded.
Rotenone generally stops killing fish within 10 days, depending on water temperature and weather conditions, but it may last as long as 1 month in water temperatures below 60° F. The pond should be restocked soon after the rotenone has stopped killing fish. Refer to the stocking section to determine the species and sizes to restock.
Fertilization
Water fertility determines a pond’s productivity. A more productive pond will support more fish and a larger harvest than a less productive pond. Fertilizer increases pond productivity by stimulating the growth of microscopic plants (phytoplankton and algae). These are eaten by bluegill, which are in turn eaten by bass. Fertilization makes the water turn green, shading the bottom and preventing growth of nuisance aquatic plants (see Aquatic Vegetation section). Adding too much fertilizer can cause unsightly algal blooms with unpleasant odors. Algae die-offs during summer months can cause low oxygen levels, which may result in fish kills (see Fish Kills section).
Most Virginia ponds do not need fertilization. Ponds that are lightly fished, shallow ponds, ponds with high flow-through rates, and ponds with muddy water should not be fertilized. The Department of Game and Inland Fisheries does not recommend fertilization in ponds that drain into waters of the Chesapeake Bay. Contact your local fisheries biologist to help you decide if a fertilization program is appropriate for you.
Once fertilization is started, it should be continued. Fertilization causes the fish populations to expand because of the enhanced food supply. Stopping fertilization drastically reduces the food supply. The result is stunted, slow growing fish that are in poor health.
Fertilizers used in ponds are similar to those used for agricultural crops. However, most agricultural fertilizers do not have the best combination of nutrients for ponds. Fertilizers high in nitrogen such as 21-53-0 or 10-34-0 should be used. Liquid formulations are preferred because they go directly into solution and are more economical. Liquid formulations are applied at the rate of 1 gallon per acre, 4 to 6 times a summer. Applications should begin in spring when the water temperature reaches 60° F, and every 2 weeks until visibility decreases to 18 inches. During the summer months add fertilizer as needed to maintain 18 inches visibility. In the fall, stop fertilizing when water temperature drops below 60° F. Before beginning a fertilization program, have your pond water tested to be sure the lime content is adequate (see Liming section below).
Liming
Ponds with soft, acidic water (typical in much of Virginia) sometimes require the addition of lime to improve fishing. Check the total alkalinity of the pond water to determine if liming is necessary. If alkalinity is below 20 ppm, add agricultural limestone to neutralize the pond bottom. A mud sample (taken from several areas of the pond bottom) should be analyzed to determine the amount of lime needed. Combine the mud samples and spread them out to dry. After the samples dry, take them to your local County Extension Agent for analysis. The agent will recommend the proper liming rate.
Lime should be applied evenly over the entire pond bottom. Common methods of application include shoveling or washing it from a plywood platform mounted on a boat. The best time to apply lime is in late fall or early spring. Typically, ponds require liming every 2 to 4 years. Do not use hydrated (builder’s) lime. It will cause a rapid pH change that may cause a fish kill, and it is hazardous to handle.
Aquatic Vegetation
Aquatic plants supply oxygen, provide cover, and can be food for insects that are eaten by fish. Plants protect shorelines from wave erosion and serve as feeding and nesting habitat for waterfowl (ducks, geese, etc.). Aquatic plants are desirable and beneficial to fish communities but can cause problems with fishing by interfering with angler access. Largemouth bass and bluegill function best when aquatic plants cover 20% to 30% of the pond surface during the summer. Plant densities greater than 30% can cause fish kills (see Fish Kills section). Fish kills can occur when overabundant vegetation dies and decays, causing oxygen depletion. Aquatic plant problems usually do not occur in properly constructed ponds.
Aquatic plants are grouped into four general categories: algae, floating, submergent, and emergent. Pond owners should either obtain a plant identification key or have a qualified person identify the plants in their ponds. In Virginia, the Cooperative Extension Service or your local fisheries biologist can assist with plant identification. If you have more than one kind of plant, make sure the plant you bring in for identification is the problem plant. Aquatic plants can be controlled by manual, chemical, and biological methods. Manual control refers to physically pulling, raking, cutting, digging, shading, or mowing nuisance plants. Manual control of species like cattails is practical when their abundance is low. Manual control should be done in the spring when the plants are first emerging.
Chemically treating a pond with herbicides approved for aquatic use can be effective for many types of vegetation. Chemical treatments can be very effective in controlling vegetation in small areas of a pond such as swimming areas or boat ramps. Herbicides generally provide quick results, but plants often return and several applications may be necessary to maintain control. When using pesticides, protect yourself and others by strictly following all label directions. Certain herbicides can be purchased and used only by Certified Pesticide Applicators, so herbicide treatment may not be possible without paying someone to do it for you.
One of the most effective long term controls for aquatic plants is stocking triploid (sterile) grass carp. Because grass carp have specific food preferences, accurately identifying the plants in your pond is very important. Grass carp consume many vegetation types and have a life span as long as 15 years. They will not reproduce in a pond, will not muddy a pond, and will not disturb the nests of other fish. Triploid grass carp grow rapidly and will control most submerged (underwater) plants. They will consume as much as 40% of their body weight in plants every day during the summer months. A permit (PDF) is required from the Department of Game and Inland Fisheries to stock triploid grass carp into any Virginia waterbody.
The best way to prevent aquatic plant problems is proper pond construction. Shoreline depths of 2 to 3 feet with bank slope ratios of 3:1 are ideal (see Figure 1). In deeper waters, aquatic plants cannot easily establish themselves because sunlight does not penetrate to the bottom.
Muddy or Turbid Water
Muddy water detracts from the pond’s appearance, reduces the pond’s ability to produce fish food (microscopic plants and animals), and restricts sight-feeding fish such as largemouth bass and bluegill from effectively capturing prey. Muddy water also reduces oxygen production by reducing photosynthesis.
Water in new ponds is often muddy because of pond construction, and from erosion of pond banks by wave action. Muddy water in new ponds should clear up as pond banks become vegetated. Rip-rap can be used to stabilize pond banks and reduce muddy waters.
Muddy water in established ponds may be caused by livestock wading along the shoreline, pond sediments that are continually suspended, or nuisance fish such as carp or bullheads. Livestock can be fenced from the pond, as discussed in the Construction section of this handbook, and nuisance fish can be removed with rotenone as discussed in the Renovating Ponds section. Suspended clay particles can be cleared by spreading 50 pounds per acre of agricultural lime. Using lime is a temporary cure and will probably need to be done annually.
Hay can also be used to settle clay particles. Break open and scatter 2 square bales of hay per acre of water. As the hay decays, the clay particles will settle out. If the pond does not clear, additional bales can be added every 2 weeks, not to exceed a total of 10 bales per acre per year.