The term “fish kill” is used by biologists to describe a sudden die-off of a large number of fish. Fish kills may occur at any time throughout the year and can result from a variety of circumstances. Partial or complete fish kills may be caused by overabundant aquatic plants, pollutants, disease, and natural mortality, among other things. Careful pond and watershed management can reduce the risk but cannot guarantee a pond will never experience a fish kill. Usually there are warning signs that the pond owner can look for and correct before it is too late.
Low dissolved oxygen is the most common cause of fish kills in ponds. Low oxygen conditions usually result from a combination of weather conditions and pond characteristics. The most obvious sign of oxygen depletion is fish gasping at the water’s surface, usually in the early morning. Once the problem has escalated to this point, immediate aeration is the only viable option to prevent a fish kill. To reduce the probability of low oxygen levels becoming a recurring issue, determine which of the scenarios listed below best describes your situation and work to correct it as soon as possible.
Planktonic algae are tiny plants that both produce and consume oxygen. As summer progresses, algal populations multiply rapidly. If excess nutrients are available (run-off from manure covered fields, fertilization, etc.), the algae can become so dense that adequate sunlight cannot penetrate beyond the first few feet of water. The algae (and therefore oxygen) become restricted to this upper, narrow layer of water. The demand for oxygen during night time will often exceed that found in this narrow, upper layer of water and dissolved oxygen levels will decline. Depending on the magnitude of the oxygen decline, a fish kill may occur.
A fish kill can also occur following several days of cloudy weather during the summer or early fall. The reduced sunlight during cloud cover restricts sunlight penetration to the upper layer of water. Those algae in the deeper water that are no longer receiving adequate sunlight will die-off quickly. As bacteria break down the algae, oxygen is used up and fish become stressed or die. Ponds that experience an algal die-off may appear brown in color.
Pond owners can reduce the chance of algal die-off by controlling the amount of nutrients that enter the pond. Care should be taken not to over-fertilize the pond during the summer. Organic fertilizer (hay, manure, etc.), should not be used; inorganic fertilizer is recommended.
Ponds that have recurring oxygen problems may require an aeration system that will increase oxygen levels during critical periods. Aeration systems are available that agitate the pond surface or bubble air from the pond bottom. In an emergency, you can use a water pump and hose to create a fountain which will add more oxygen to the water to reduce fish loss.
During the summer most ponds have adequate oxygen levels in only the upper 6 to 10 feet of water. The colder, deeper water is essentially void of oxygen. The upper, warmer layer of water remains separated from the colder water because of a difference in density between the two. Cold water is heavier than warm water and, therefore, remains associated with the bottom. The thin layer where these two areas meet is called the thermocline.
As fall approaches, the upper water layer cools down and the density difference between the two layers is reduced. A cold rain or strong wind can result in a sudden mixing (turnover), of the two water layers. If the deeper layer is relatively large as compared to the upper layer, then it is possible that a fish kill could result. This problem can be avoided by not building the pond too deep, or by installing an aeration system. Even with precautions, most ponds will eventually experience some level of turnover.
Excessive growth of aquatic vegetation must be controlled gradually. Treating too much of the pond with herbicide at one time creates an overabundance of decaying plant matter. Bacteria that subsist on dead material grow and reproduce rapidly and can easily deplete the oxygen available in the water. To avoid this problem, never treat more than 1/3 of the vegetation in a pond with chemical at one time. If vegetation is excessive and more than one treatment is required, wait at least two weeks between treatments.
Fish kills caused by pollution are actually rare in ponds. The most common pollutants that cause fish kills in ponds are agricultural chemicals. The use of agricultural pesticides or herbicides on the pond watershed (land area that drains into pond), can result in a fish kill as rain water carries the chemicals into the pond or wind drifts chemicals after spraying. Care should be taken not to apply pesticides to the pond watershed when rain or high wind is probable. Use only those herbicides on aquatic vegetation which have been labeled for that purpose.
Most common disease outbreaks occur in the spring when fish are in less than ideal condition following the cold winter months. Spawning season also stresses fish and makes them more susceptible to disease. An extensive disease outbreak could lead to a fish kill that usually occurs over a one to three week period. Disease rarely kills all fish present in a pond. Pay lakes where diseased or injured fish are delivered from some outside source, and lakes and ponds with very dense fish populations are at a higher risk for a disease outbreak. Although fish overcrowding may occur naturally during drought conditions, it’s usually the result of overstocking by man. Follow recommended stocking and harvest rates and you shouldn’t have a problem.
Some fish, especially when over-abundant, will experience die-offs in the spring. This is a result of heavy competition for food during the previous year causing poor body condition and reduced resistance to bacterial and fungal infections. It is most obvious when the water temperature is between 60 and 70 degrees Fahrenheit. These die-offs are common and are usually somewhat beneficial to the overall fish population. Though some are more severe than others, it is nature’s way of thinning an overcrowded situation. Most commonly affected are channel catfish and bluegill, though occasionally redear sunfish and a few small largemouth bass are also seen. Only the fish that were in poor condition, due to their inability to compete for food, will die. The stronger fish that competed well and maintained their good body condition will survive. Spring spawning behavior may also lead to poor body condition and bacterial infections. This is particularly common in bluegill (usually males) during the May spawning period.
Although not very common in Kentucky, fish kills can occur in the winter. If the pond surface freezes and is covered with snow for a long period of time, sunlight cannot reach the algae. This lack of sunlight can cause an algal die-off and subsequent fish kill. Winter kills often go unnoticed until spring when limited fish activity is observed. Pond owners whose pond frequently “winter kills” may consider installation of an aeration system. The mixing action of the aerator not only puts oxygen in the water but also prevents complete ice cover in the vicinity of the aerator.
Some fish will die of natural causes each year. Usually this happens during the spring when fish are in poor condition after the winter or water temperatures increase sharply around spawning time. Natural mortality removes adult fish from the population and makes more resources (food, and space) available for the smaller, younger fish. Routine, yearly harvest will help minimize natural mortality.
Fortunately, disease problems in lakes and ponds rarely reach epidemic levels. Fish are exposed to a wide variety of disease causing agents such as bacteria, viruses, fungi, and parasites on a daily basis. Under good conditions healthy fish are able to protect themselves from most of these organisms. However, under less than optimal conditions, fish can become stressed and susceptible to these pathogens. Stressors that can weaken a fish’s immune system and decrease their ability to fight off infection include poor water quality (low dissolved oxygen and extreme temperature), pollution, over-crowding, improper handling (removing the protective mucus layer), wounds from nest building or aggressive behavior with other fish, and inadequate nutrition. Although fish diseases can occur anywhere, these problems most often occur in ponds where fish are intensively cultured (catfish ponds) for sale and not in a typical recreational pond environment.
Fish diseases can be difficult to diagnose, and treating those diseases in ponds or lakes is generally impractical and costly. Seeing an occasional dead fish is normal. When large numbers of fish begin dying there is usually an underlying stressor that has caused them to become susceptible to infection. Reducing or eliminating stressors is integral to keeping fish healthy and controlling disease. Good pond management practices that can reduce stress and help prevent disease outbreaks include:
• Stocking ponds at recommended rates
• Stocking only healthy, disease free fish from hatcheries
• Never stocking fish caught from other lakes, ponds, or rivers
• Maintaining good quality water
• Preventing animal waste, fertilizers, pesticides, and other pollutants from entering the pond
• Fertilizing the pond and feeding fish at recommended rates
Fish disease and mortality resulting from bacteria and parasites is most prevalent during the spring and early summer. Fish condition is less than optimal following the winter months and at the same time body energy is being utilized for spawning activities. This is the time of year when fish are most susceptible and when bacteria and parasites are most active. Susceptibility also increases in late summer when high water temperatures coupled with low oxygen levels restrict the fish’s ability to fight off infection. While not a comprehensive list of all bacteria, viruses, or parasites that infect freshwater fish, the following are some of the more common fish disease problems encountered in farm ponds.
Bacterial infections can affect a fish’s fins, skin, flesh, mouth, gills, or internal organs. Infections may be externally visible and present as open wounds, red or irritated areas on the skin or mouth, pale and eroded gill filaments, or eroded fins. Some bacteria result in internal infections. Although no external indications appear, these fish will generally be emaciated and act sluggish.
Bacteria that commonly cause disease problems in freshwater fish include species of Aeromonas, Pseudomonas, Flexibacter, and Edwardsiella. Aeromonas and Pseudomonas are two of the most frequently encountered bacteria. Common signs of infection include red streaks at the base of the fins and on the underside of the fish or reddish circular ulcers on the sides of the body. In severe cases the abdomen may be swollen and scales may protrude. Flexibacter infected fish may have light bands across the back (saddleback), eroded areas on the mouth, fins, or gills, and a dirty-yellow material around the mouth. Superficially, Flexibacter infections may often resemble fungal infections. Bacterial infections often occur due to some type of stressor (spawning, low oxygen, parasite infestation). If you are supplementally feeding fish (i.e. catfish) the use of a medicated feed may be a treatment option.
Lymphocystis is probably the most common virus found in ponds, but it rarely causes mortality and is much more unsightly than lethal. Channel catfish virus is much more lethal, but is usually a problem only in intensive culture ponds and rarely encountered in recreational ponds. No treatment options are available for viral infections.
Fungal infections are characterized by a whitish cottony growth on the skin. Fungal infections are generally secondary infections and occur where trauma or disease has created an opportunity for invasion. The most common fungus affecting fish is Saprolengnia. Once a fungal infection starts, it can spread and result in death of the fish. There are presently no fungicides which are feasible for pond use and have EPA or FDA approval. Fungal infections generally do not cause wide-spread mortality and disappear whenever the causative agent of the primary infection ceases.
Parasites are the most common, and diverse, disease causing organisms a pond owner is likely to encounter. Parasites exist in small numbers in most fish populations. In some ponds it is not uncommon to find several fish infected with fluke worms at certain times of the year. However, fluke worms, and many other parasites, do not pose a significant health risk unless fish become stressed or the infestation becomes particularly severe. Parasites may be found externally attached to the skin, gills, or mouth, or internally in the muscle, organs, or body cavity. Parasites range in size from microscopic copepods to round or tape worms which can be several inches in length. Damage to the fish is usually determined by the degree of infestation and may range from minimal to death. There are a few chemicals available to treat some of the parasitic infections; however, these chemicals are fairly harsh and expensive. Treatment of parasitic infections is generally limited to culture situations and rarely used in recreational pond environments. The following are brief descriptions of some of the more common parasites found in farm ponds.
Protozoans usually cause few problems in naturally occurring populations. Most significant losses of fish occur in high-density situations such as channel catfish culture ponds. Fish infected with large numbers of protozoans may exhibit erratic swimming movements, abnormal coloration, loss of appetite, excess mucous production, hemorrhaging, swollen body, or distended eyes. Often, damage to the epithelium (skin) caused by the protozoan infestation provide sites for a secondary bacterial infection.
Red sore disease is one of the most common disease problems encountered in freshwater game fish, including those in farm ponds. Red sore disease is characterized by reddish ulcers or sores on the fish’s body and is generally the result of infection by two different organisms, Aeromonas hydrophila, a bacterium, and Heteropolaria sp. (formerly Epistylis), a protozoan. Ichthyophthirius (or “ich”) is probably the most familiar protozoan infecting fish because of its frequency in aquarium fish. Ich will appear as small white pimples on the skin. When fish are stressed, severe infestations may lead to mortality. Other common protozoans include Trichodina and Costia.
Trematodes (flat worms or fluke worms)
Digenetic trematodes require one or more hosts (usually snails and fish-eating birds) in addition to fish to complete their life cycle. Their larval stages, commonly known as black grubs, yellow grubs, or white grubs, will encyst under the skin over the
entire body of the fish. Severely infected fish may be covered with black or yellow spots. These parasites normally do not kill fish, but may appear unsightly. Proper cleaning and cooking will render the parasite harmless to man. There is no known control of trematodes in ponds. Infestation severity may be reduced by reducing the amount of vegetation in the pond thereby reducing the snail population. Also, redear sunfish actively feed on snails and may be used to help control the snail population.
Nematodes (round worms)
Nematodes, or “round worms,” infect many different species of fish. Depending on the species of round worm and the species of fish, they can be found in almost any part of the fish including the body cavity, internal organs, muscle tissue, and behind the eye. Philometra is one species of round worm that is found mostly in bluegill, but occasionally in largemouth bass. Philometra lodges behind the eye ball causing it to protrude and is responsible for what is known as “popeye” disease.
Tapeworms may be found in the intestine, stomach, or body cavity of fish. Heavy infestations of the tapeworm Corallobothrium in the intestine of catfish may retard growth. The larval form of Proteocephalus (bass tapeworm) may migrate through the body cavity and internal organs causing adhesions which inhibit spawning capability. I the majority of the pond’s fish have a severe infestation the best corrective measure is to renovate the pond and stock parasite-free fish.
Parasitic copepods attach to the gills, body, and fins of fish. Lernaea (anchor worm) and Argulus (fish lice) often penetrate the skin of fish and feed on blood. Infected sites appear as small lesions and often become ulcerated providing access for secondary infections by fungus or bacteria to occur.
Treatment of Fish Diseases
As mentioned previously, treating fish diseases in ponds or lakes is generally impractical and costly. Before any treatment option is attempted it is recommended that you contact the fish disease specialist at Kentucky State University in Frankfort at 502-597-6581. The fish disease expert can accurately diagnose the disease and provide you with the most effective treatment regime. You may also contact your district fishery biologist who may be able to diagnose the disease and provide you with methods of treatment.
The most difficult part about fixing a leaking pond is identifying the location of the leak! Persistent leakage of a farm pond without obvious wet areas below the dam probably indicates that it was built in an area containing shale bedrock or other rock formations. If possible, drain the pond and apply a 12 to 15 inch layer of clay soil. During compaction, the pond bottom should be worked well with a sheepsfoot roller. The clay should be compacted to a thickness of no less than 8 to 10 inches. Prior to this application, all tree stumps or similar structures should be removed and cavities completed filled. Investigations should be made to locate any cracks or crevices in the pond bottom. Again, compaction is important in producing a good seal. The deeper the pond, the thicker the seal should be. In the deeper area of the pond, where the depth exceeds 10 feet, the clay layer should be doubled.
If the exact location of a leak can be identified, the most popular method of sealing involves the use of bentonite. This is a type of clay material that can be applied in the water above the area of the leak. Bentonite will expand when in contact with water, thereby helping to impede flow through the sediment. According to sources, the most effective method is by applying it to the dry pond bottom followed by discing and compacting. Sources for bentonite can be located by contacting farm supply stores, construction companies, or well drillers. Application rates range from 1 to 2 pounds per square foot of pond bottom. There is at least one new product that has bentonite clay affixed to a limestone core that allows more precise placement of material.
If the dam is leaking and bentonite does not solve the problem, the next step is to excavate and re-compact the dam in the affected area. Another fix would be to drain the pond and install a man-made pond liner. Liners are available from commercial dealers and are quite effective if installed correctly. If a liner is used, livestock should not be allowed access to the pond as the liner may be punctured.
Thorough planning and preparation of the future pond site will go a long way in preventing leaks. Remember to avoid rocky or sandy areas when locating a pond site, compact the pond bottom well with soil and clay, and remove all tree stumps and logs from the fill material from the dam. Minor leaks after the pond is first impounded are not unusual and should soon cease. If the leak is persistent, contact your local NRCS office for information on your soil type, or possible leak location and recommended solutions. Be sure to keep the dam clear of woody plants and trees with large root systems. These plants/trees should be removed before they get very large and the root system grows through the dam.
Muddy water can have negative effects other than detracting from the aesthetics of the pond. Muddy water can hinder the feeding ability of largemouth bass, bluegill, and redear sunfish and even reduce their growth. Additionally, phytoplankton growth and abundance is reduced in muddy water. This may compound the problem of poor fish growth in muddy ponds by reducing the amount of food available through the entire food chain.
The appearance of muddy water in farm ponds may be the result of physical, environmental, or biological factors. Any one or all of these causes may be disturbing the pond sediments to the point where they are continually suspended. The best way to determine the cause of your muddy water to fill a glass jar with pond water, cover it with a lid and allow it to sit undisturbed for one week. If the water is still muddy after one week, the cause is physical, likely suspended clay particles. However, if the water is clear and there is a layer of sediment in the bottom of the jar, the cause of the problem is either environmental of biological.
The smaller the clay particles, the longer they remain suspended in the water column. Very small clay particles lack sufficient weight to settle to the bottom and require treatment with one of the options below:
50 pounds per surface acre of agricultural lime;
20 pounds per acre-foot of hydrated lime;
525 pounds per acre-foot of agricultural grade gypsum (hydrated calcium sulfate)
50 pounds per acre-foot of aluminum sulfate (filter alum)
Hay treatment: break open and scatter two square bales per acre in the pond every 14 days. Make no more than 4 or 5 applications per year. An alternative is to place a solid bale every 40 feet in the water along the shoreline.
Environmental factors that may affect pond clarity include wave action due to wind and influx of silt from inflowing water. These are usually temporary and will not produce long term problems. If wave action is chronic, however, planting of tree wind-breaks will reduce disturbance of sediment in shallow areas. Ditches with inflowing water may need to be lined with rock to reduce water velocity and allow sediment to settle prior to entering the pond. Moderate vegetative cover around the pond edges (cattails, rushes, sedges) will protect pond banks and shoreline areas from wave erosion. Maintain good vegetative cover throughout the watershed or establish buffer strips around the pond to prevent excessive sediment from entering the pond.
Often, newly constructed ponds may remain muddy during the first year. This is natural and usually after the sediments have had a chance to settle and vegetation establishes itself around the pond, the problem will go away.
Biological factors appear to be the most common causes of pond turbidity in Kentucky. Most often it is caused by large numbers of catfish, bullheads or common carp that keep soil particles suspended. Chemical eradication of the fish population, or complete draining and refilling of the pond would be necessary to return water clarity. High numbers of crayfish can sometime cause muddiness as well. See the crayfish section below for information on controlling an extensive crayfish population.
Livestock should be fenced away from the pond and should not be pastured in the pond watershed, if possible. Livestock trample and compact pond banks causing them to erode.
Muskrats primarily feed on cattail, bullrush, smartweed, water lily, young willows, and other plants. Muskrats become a nuisance when they begin burrowing into the banks or the dam of farm ponds. They dig their burrows below the surface of the water and extend them at an upward angle until a den chamber is hollowed out above the water line. A rise in the pond water level will force muskrats to dig further and higher into the banks or the dam. Although burrows along the pond bank can lead to problems with sloughing, the greatest damage is created when muskrats tunnel into the dam. This weakens narrow or poorly constructed dams causing them to leak and results in lowering of the water level.
There are a number of methods that can be used to prevent muskrat damage if ponds are still under construction. Damage to the dam can be reduced or eliminated if it is built to the following specifications: the inner face of the dam should have a 3 to 1 slope, with the outer face built at a 2 to 1 slope. The top of the dam should be between 10 to 12 feet wide. The dam should be built high enough so that the top is at least 3 feet above the water level.
Barriers to prevent burrowing can also be incorporated into the construction of new ponds. The placement of stone rip-rap along the inner face of the dam and along the banks will help exclude muskrats. The most effective method is placing sections of chain link fencing, extending 2 feet above and below the water line, along the face of the dam and then placing the rip rap on top.
There are also ways to reduce damage caused by muskrats that already inhabit ponds. Burrows found when the water level is either naturally or intentionally lowered can be plugged with stone rip-rap or other fill material. Removing growths of cattail, willow, or other food sources may force muskrats to vacate the pond. Muskrats can also be hunted or trapped. The most effective times to hunt are either late in the evening or early in the morning. The most effective trap is the Conibear 110 body grip trap. Traps should be placed at the entrance of burrows or in the trails or “runs” along the pond bottom or in the vegetation. Live traps are also available. When live traps are used they should be well camouflaged and baited with attractive baits such as apples or carrots. Animals can then be relocated to a distant site where they will not cause damage.
Prior to hunting or trapping muskrats, pond owners should contact their local Wildlife Biologist or Conservation Officer if this activity is done at a time other than the legal hunting or trapping season.
Turtles are a natural and harmless part of any pond environment and commonly migrate from pond to pond in the spring and on cool rainy nights in the summer. They occasionally become a nuisance to anglers by stealing bait or stealing fish from a stringer. Although turtles feed on sick or diseased fish, they do not harm healthy fish in the pond.
The normal diet of most turtles found in Kentucky farm ponds is about 75 percent vegetable matter, 20 percent animal matter, and about 5 percent fish. If pond owners wish to reduce the turtle population in their ponds, there are several ways to trap them. Devices that are commonly used include baited bank lines, trot lines, or pitfall traps. Pit-fall traps can be constructed in several different forms and allow for turtles to be relocated unharmed. One example is shown below and an internet search will yield other options.
There are many species of crayfish found across the Commonwealth of Kentucky in every type of aquatic and terrestrial habitat. There are several species of burrowing crayfish mostly belonging to two genera: Cambarus and Fallicambarus that are commonly found near and around small ponds. Like their aquatic cousins, burrowing crayfish use gills to extract oxygen from water. Unlike their water-soaked cousins, burrowing crayfish spend most of their lives on land. They dig their burrows down to water so they have a ready source of oxygen. This connection to a high water table explains why most burrowing activity occurs in poorly drained soils near streams or around shallow ditches.
Burrowing crayfish throw soft mud up around their exit holes as they excavate the soil. These chimney-like structures may tower 3-8“ above the soil surface. Both male and female crayfish use the burrows as a refuge to escape from predators and as a resting place during molting and inactive periods. Female crayfish often use these burrows as a nursery for their young, particularly during periods of low water.
Extensive burrowing around ponds and lakes can undermine the structural integrity of dams. Fortunately, several control measures can be used to effectively reduce crayfish densities to levels at which structural damage and water leaks will be minimized. Efforts at total eradication usually are futile because crayfish can migrate considerable distances and will continually reinvade a pond. The most effective control measure is the maintenance of a healthy sportfish population in your pond. Bass, catfish, and large bluegill eat crayfish and can help reduce excessive numbers. Trapping crayfish is also a very effective control method. Commercially made crayfish traps can be purchased or minnow traps may be modified by enlarging the opening to 2 inches in diameter to allow easy access for larger crayfish. Traps baited with any fresh meat, scraps, dog food or other high protein substance overnight will often produce significant catches. When setting traps overnight enclose the bait in a cloth or mesh bag to prevent trapped crayfish from consuming all the bait and reducing trap effectiveness. Crayfish are most active at temperatures between 40 and 75°F. As temperatures rise and fall outside of this range crayfish become inactive and stop feeding.
Chemical control is generally not recommended because burrows usually extend downward to the water table. Pouring toxins into the burrow may kill the crayfish but will also contaminate groundwater resources. The poisoning of large areas is not recommended, however, individuals burrows can be treated to kill the crayfish will minimal risk of contaminating nearby water sources. The US Fish and Wildlife Service recommends treating each burrow with ¼ cup bleach, one teaspoon of chlorine powder, or two or three pellets of lye before tamping the burrow closed. Make sure to treat all burrows as an individual crayfish may have several openings per tunnel system.
With the exception of the European starling and house sparrow, all birds are protected by federal law making it illegal to kill or capture them. There are, however, nonlethal methods that may be used to exclude birds from ponds. These methods include noise-making devices, total or partial exclusion barriers (netting), heron decoys, and an increased human or canine presence.
Landowners having problems with birds should contact their local Wildlife Biologist or Conservation Officer. Additional information on ways to reduce damage caused by birds is available from the State Wildlife Services Office
of the Animal and Plant Health Inspection Service of the U.S. Department of Agriculture.