Aquaculture for all
The Fish Site presents: The Vienna Sessions - Conversations about aquaculture. 9 video interviews with aquaculture thought leaders. Watch here.

Pond Management Guide - Chapter 4

title2_r2_c1.gif View Printable Version (PDF) Watch the Pond Management Video

Chapter 4 Solving Problems

Fish Kills
Fish die from a variety of natural causes. Observing a few dead fish in a pond is not uncommon and is no reason for concern unless it continues for several days. When fish die in large numbers, however, there is reason for concern.

A common cause of fish kills is oxygen depletion. This condition usually occurs during summer in very fertile ponds as a result of pond turnover or the die-off of an algal bloom. During hot weather most ponds have a layer of water near the bottom that contains little or no dissolved oxygen. When high winds or cold rain cause this water to mix with the upper pond water, oxygen levels often drop low enough to kill fish. Oxygen depletion also occurs when dead algae or other plants decay in the pond after herbicides have been applied to control weeds.

Preventing oxygen depletion is difficult, but the following suggestions may help:

  • Follow the prescribed fertilization guidelines and do not overfertilize!

  • Do not allow livestock to wade in the pond or animal waste to enter the pond.

  • Do not treat aquatic weeds with herbicides during the summer months without consulting a weed control specialist, fisheries biologist or Extension agent. If a herbicide application is necessary, treat no more than one-fourth to one-third of the pond at a time to prevent oxygen depletion and a resulting fish kill. Aerating the pond (see below) can also help reduce the chances of an oxygen-depletion fish kill.

  • During extremely hot weather, check your pond regularly at sunrise for signs of stressed fish. If fish are observed at the pond's surface gulping for air, stop feeding the fish and aerate the pond as soon as possible. Oxygen can be added to the pond by circulating the water with an irrigation pump or by running an outboard motor around in the pond. Commercial aerators do an excellent job of aeration. The paddlewheel type is especially effective, as it moves a large volume of water.

Although fish kills caused by pesticides, herbicides, or other chemicals are not as common as those caused by oxygen depletion, some do occur. If you suspect that your fish were killed by a pesticide or herbicide, try to determine what chemical was involved and call the NCDA Pesticide Section in Raleigh, North Carolina (919-733-3556). For fish kills caused by other chemicals or animal waste spills, call the DENR Division of Water Quality in Raleigh, North Carolina (919-733-5291).

Be very careful when spraying herbicides or other pesticides, as many are highly toxic to fish.
Always read and follow label instructions!

Fish kills resulting from low pH (acidic water) are even less common than chemical kills. Usually pH kills occur when heavy rains wash tannin (an acidic substance found in leaves) from wooded areas. Low pH can be increased easily by applying agricultural limestone. The amount of lime required can be determined by sending samples of the mud from the pond bottom to the NCDA Soil Analysis Laboratory for analysis. (See the earlier section on liming procedures.) Contact your county Agricultural Extension Service office for assistance in sending soil samples.

Fish kills caused by diseases usually occur when fish are already stressed by poor water quality or overcrowding. In most situations little can be done once a disease strikes except wait for it to run its course and see what is left. Here again, prevention is the key: fish the pond properly, maintain good water quality, and watch for signs of problems such as poor fish growth, thin fish, and excessive numbers of small fish.

Fish Parasites

Wild fish normally carry a variety of parasites, and usually show no negative effects unless the infection is extremely heavy. Largemouth bass are commonly infected with the bass tapeworm, which lives in the intestine of bass; this species does not infect humans. Two other parasites frequently observed by anglers are the yellow grub, and the black grub or black spot parasite. The yellow grub is the larval stage of a trematode worm which forms small whitish or yellowish cysts in the flesh and near or just beneath the skin. Black grub parasites are also the encysted larvae of trematode worms, and appear as a small black spot about the size of a small pinhead, in or just beneath the skin.

Both of these parasites have a complex life cycle involving snails, fish, and fish-eating birds such as herons or kingfishers. The adult worms live in the mouth and throat of fish-eating birds, and shed their eggs into the water as the bird feeds. The eggs hatch and the free-swimming larvae infect snails. Later, advanced larvae emerge from the snail and penetrate the skin of a fish. When the fish is eaten by a bird, the cycle is completed.

While all these parasites are aesthetically unappealing, they will not infect humans and are killed by thorough cooking. There are no chemical treatments available to eliminate these parasites in pond situations. However, snails are a preferred food of redear sunfish, so establishing a good population of these fish in the pond may help disrupt the life cycle of parasitic trematodes.

Poor Fishing

Most complaints about poor fishing stem from crowded or stunted bream (bluegill) or bass populations. The best way to prevent these problems is to fish the pond properly. Correcting an unbalanced fish population is a lot more trouble than keeping it in balance from the start. If a fish population becomes unbalanced with too many small bream it may be possible to correct the problem by removing excess bluegill, or by stocking 25 to 50 largemouth bass (8 to 12 inches long) per acre. This solution may be prohibitively expensive, however, as bass in this size range are costly. Stunted bass populations can be corrected by removing excess bass (see earlier section).

Sometimes poor fishing can result from competition of gamefish with undesirable fish such as wild sunfish, shiners, bullheads, and crappies. These fish may enter the pond via feeder streams or be purposely stocked by anglers with good intentions. Again, prevention is much easier than the cure. When building a new impoundment, make sure that all wild fish are eliminated before stocking the pond with hatchery fish. Also, do not place wild fish in the pond or allow minnows to be used as bait.

Aquatic Weeds

Aquatic weeds often cause serious problems in ponds, interfering with fishing, boating, swimming, and irrigation. In addition, when vegetation is dense, bream often become overcrowded and stunted because the weeds prevent bass from adequately reducing their numbers. Extremely dense growths of filamentous algae and submerged weeds may also cause fish kills as a result of nighttime oxygen depletion. It is generally better to keep your pond clear of aquatic weeds. For assistance identifying aquatic vegetation and appropriate control alternatives, contact you county Cooperative Extension Center. More information is available in Extension publication AG-437, Weed Management in Small Ponds (PDF), AG-438, Weed Control in Irrigation Water Supplies (PDF), and Ag-449, Hydrilla: A Rapidly Spreading Aquatic Weed in North Carolina, also available from your County Cooperative Extension Center.

Weeds that root to the bottom or begin forming on the bottom are usually a problem only in ponds that are shallow or have shallow areas (water less than 24 inches deep). Anytime sunlight can penetrate to the pond bottom, rooted aquatic weeds and filamentous algae may become established. Once established, many weeds have the ability to spread to deeper water.

Problems with planktonic algae and floating weeds, such as duckweed, usually develop in very fertile ponds. Ponds that receive runoff from livestock operations or other nutrient-rich areas are prime candidates for duckweed and algal problems.

The following methods have proven effective in North Carolina for controlling aquatic vegetation.

Winter Drawdown.

Reducing the surface area of a pond by one-third to one-half from mid-November to the first of March helps control many submerged rooted aquatic plants by exposing them to drying and freezing. A side benefit in bass-bluegill ponds is improved fish population balance resulting from increased predation of bass on bluegills forced out of cover. Unfortunately, some weed species, such as hydrilla, cannot be controlled by winter drawdown because they produce tubers or other overwintering reproductive structures. Winter drawdown is not recommended in ponds of less than 1 acre.

Manual Weed Removal.

Removing the plants by methods such as pulling, raking, or chaining works best on small patches of plants that are rooted in shallow water. Manual removal is most effective if performed in late spring or early summer before the plants form seeds. Be sure to dispose of the vegetation properly, especially alligator weed, which may root and grow on dry land.

Chemical Treatment.

Weeds can be killed by treating the pond with one of the herbicides labeled for aquatic use. To determine which herbicide to use, ask an Agricultural Extension agent to identify the weeds. Unfortunately, herbicides only treat the symptoms and do very little to cure the problem. Weeds frequently return soon after treatment if no action is taken to deepen the pond or eliminate the nutrient source. Be sure to follow restrictions on consumption of fish following any chemical treatment.

Triploid Grass Carp.

These vigorous, fast growing, reproductively sterile, herbivorous fish can be used to control unwanted aquatic vegetation under certain conditions. They are an effective biological control agent for submersed weeds such as hydrilla, chara, elodea, widgeongrass, bladderwort, fanwort, coontail, pondweed (Potamogeton), and naiads. Grass carp occasionally provide partial control of duckweed, eurasian watermilfoil, variable leaf milfoil and some types of algae (but not usually recommended for these species), and they generally are not very effective in controlling eelgrass, smartweed, American lotus, yellow waterlily, fragrant waterlily maidencane, dollarweed, alligatorweed, torpedograss, and cattails. Grass carp grow large and provide effective control for 5 to 8 years.

Recommended grass carp stocking rates are generally 10-15 fish per acre in small ponds and 10-20 fish per vegetated acre in larger impoundments. Stock large fish (8-10 inches long) to reduces losses from predation by largemouth bass and wading birds. Given the opportunity, grass carp will try to move upstream or downstream, so to protect your investment make sure that grass carp cannot readily escape from your pond.

In their diploid (reproductively fertile) form, grass carp may damage valuable native vegetation and displace native fishes. Consequently, only grass carp that have been genetically manipulated to make them triploid (and therefore sterile) are allowed. If your pond is 10 acres or larger in size, or you intend to stock more than 150 grass carp, a permit must be obtained from the Wildlife Resources Commission. You can obtain an application for a permit to stock triploid grass carp by calling or writing to:

N.C. Wildlife Resources Commission
Division of Inland Fisheries
512 N. Salisbury St., Room 442
Raleigh, NC 27604-1188
Telephone (919) 733-3633, ext. 278

More information is available in Extension publication AG-456, Using Grass Carp for Aquatic Weed Management, available from your County Cooperative Extension Center. Be sure to purchase your grass carp from a licensed grass carp supplier; the list of licensed suppliers is available from your County Extension Center.

Pond Deepening

Deepening all areas of the pond to a minimum of 24 inches will reduce weed infestations. Most pond owners use this method as a last resort, but for shallow ponds it is often the only lasting solution.

Pond Dyes

Adding a non-toxic pond dye to your pond is another way to help prevent weed problems. Pond dyes turn the water a bluish-green color and help control weeds by shading the bottom so plants can't get established. They should not be used in ponds that are fertilized, because they will interfere with plankton bloom development.

Other Pond Problems - Muskrats

These burrowing animals often cause pond banks to collapse and dams to leak. Keeping the pond edge mowed and controlling emergent vegetation will discourage muskrats from taking up residence. Once established, however, these rodents are best controlled by trapping. If you do not want to trap them yourself, it is usually easy to find a local fur trapper to do the job. Muskrats should be trapped during the regular trapping season by a licensed trapper unless a depredation permit has been obtained from a Wildlife Enforcement Officer.


Occasionally, beavers take up residence in ponds. When they do, they usually cause considerable damage. They often block drain pipes and dam spillways, and they dig dens in the pond banks and dams. As with muskrats, trapping is the best way to remove these animals.


These slow-moving creatures are primarily scavengers and do not harm fish populations. They may, however, eat fish off a stringer, or in the case of snapping turtles, eat a few ducks. Snapping turtles can be caught on large set hooks baited with scrap meat or fish, or they can be baited into wire baskets. Other types of turtles that like to bask in the sun can be caught in sink box traps (Figure 10).

Figure 10: Traps for removing turtles from ponds.


Most snakes seen in and around ponds are nonpoisonous water snakes. The only problem they present is that they may scare pond owners and anglers. The best way to control snakes is to keep the pond banks mowed, thus eliminating their hiding places.


Resident waterfowl such as domestic geese and ducks may cause problems if they become too abundant, especially in small ponds. They can cause turbidity and algal problems, damage shoreline vegetation, and leave unsightly droppings on pond banks and piers. They may also become aggressive during nesting season. If it becomes necessary to remove waterfowl, your county Cooperative Extension Center may be able to recommend a local animal control company to assist you.

Muddy Water

The first step in clearing a muddy pond is to eliminate the source of the turbidity. Common causes of muddy water are runoff from nonvegetated acreage in the watershed, livestock wading in the pond, or some undesirable fish species (such as common carp or bullheads) stirring up the bottom of the pond.

After the source of the turbidity has been eliminated, the water will usually clear naturally, but this may take from several weeks to several months, depending on the soil type in the watershed. Some ponds may not clear naturally because their water chemistry keeps the clay particles from settling out.

The following treatments have proven successful in clearing muddy ponds:

  • Apply 300 to 500 pounds of gypsum (land plaster) per surface acre. The gypsum should be finely ground and spread over the ponds surface.

  • Spread 7 to 10 bales of hay and 40 pounds of superphosphate per acre over the surface of the pond. Do not use this treatment during the summer months because of the danger of depleting the oxygen.

  • Apply 100 pounds of cottonseed meal and 40 pounds of super phosphate per surface acre. Do not use this treatment during the summer months because of the danger of depleting the oxygen.

In mild cases liming or a standard fertilization program may effectively clear the pond.