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Fish Mortality In Ponds


By Kenneth Williams

 Natural Mortality

A few fish will die of natural causes throughout the year in all ponds. Most natural mortality goes unnoticed and the dead fish are consumed by carrion consumers such as crawfish and other fish species. Dead fish are most often noticed in late winter and early spring. As water cools over the winter, fish metabolic rate declines and they become less resistant to bacterial and fungal infections. A fish that becomes wounded from predation, territorial fighting or from incidents with anglers is prone to infections at this time.

 Fungus, usually of the genus Saprolegnia spp. appears as grey to brown colored cotton-like patches found on the body of the fish. While bacterial infections may look like bloody spots or areas of skin and scales eaten away to reveal muscle tissue. The edges of the skin around the wound may be bloody. Fish may have multiple infections of bacteria, fungus, virus and parasites. In almost all cases, only a few fish will be found dead.

 Oxygen depletion mortality

For most pond owners, the sight of hundreds of dead fish in their pond is an unpleasant experience and they want to know why the fish have died. If it appears that all of the fish have been killed, pond owners often suspect a poison of some kind was placed in the pond.  However, poison is almost never the cause of a fish kill. In almost all cases, when a pond of fish dies overnight the cause is suffocation from low levels of dissolved oxygen In the water.

 Most warmwater pond fish require an oxygen level of at least 1 mg/l for short – term survival and at about 4 mg/l on average to avoid chronic oxygen stress related diseases. The exact amount of oxygen needed depends on species. Green sunfish and black bullhead catfish can tolerate lower levels of dissolved oxygen than can channel catfish, bluegill or largemouth bass. That is why many small ponds prone to low dissolved oxygen levels may have only green sunfish and or black bullhead catfish.

 Levels lower than about 1 mg/l for even a couple of hours can kill fish. Low dissolved oxygen related fish kills usually occur in the summer when water temperature is high and there is a dense phytoplankton bloom on the pond. The pond may look like “pea soup” from the dense algal growth. Sudden algae die-offs can use up oxygen in the water as it decays causing a fish kill.  Periods of cloudy weather during summer months can also contribute to low dissolved oxygen levels in ponds that may ultimately drop to lethal levels.  Phytoplankton require sunlight for the photosynthetic activity necessary for oxygen production. Clouds blocking the sun over a period of a few days can limit light available for photosynthesis and reduce oxygen production to levels less than the amount consumed in normal respiration. Oxygen levels in the pond may decline for several days before reaching a lethal level.

 Because oxygen is produced by photosynthetic activity, it increases in ponds throughout the day and declines at night as phytoplankton and other aquatic plants use oxygen and release carbondioxide in respiration. Lowest oxygen levels are reached just before sunrise. Fish are most likely to die at or a few hours before this time. Larger fish usually die before smaller ones and in fact, smaller fish may survive to re-populate the pond.

 An oily sheen is often seen on ponds for a few days after an oxygen related fish kill leading observers to believe a chemical of some kind may have entered the water. However, the sheen is due to fats released from the bodies of the decaying fish and will cause no harm to the pond.

 Winter kill

Winter kill also is caused by low dissolved oxygen. Normally oxygen is highest during the winter due to cold water temperatures; the colder the water the higher the saturation point of oxygen. However, oxygen is produced by phytoplankton during photosynthesis or added to the pond by wind mixing. A layer of ice on the pond can prevent wind mixing but will not cause a fish kill because adequate light for photosynthesis is still available. If snow falls on the ice some or all of the light can be blocked. Algae will no long produce oxygen but will continue to use it in respiration. Oxygen levels in the pond will drop until snow is melted or blown away. If snow cover lasts long enough, oxygen concentration may drop to levels lethal to fish even when their oxygen demand is comparatively low because of low metabolic rate due to cold water temperature.

 “Pond turnover”

“Pond turnover” can cause an oxygen depletion fish kill. Ponds stratify in summer. Warm water tends to stay near the surface while cooler water collects near the pond bottom. The two water layers are separated by a narrow band of rapidly cooling water called the thermocline. Most dissolved oxygen in the pond will be found in the upper layer (4-8ft.) of warm water and the thermocline. Oxygen levels in the bottom water may be at or near 0mg/l. Over the summer, dead plants, animals and other organic debris accumulate on the pond bottom. Oxygen is required for this material to decompose. As surface water temperature cools in the fall to near temperatures found on the pond bottom mixing of the entire water column occurs. This is often initiated by winds associated with cold fronts moving through the area.   As the water column mixes accumulated organic matter uses up oxygen in decomposition. A fish kill can result if enough oxygen is used in decomposition to lower pond dissolved oxygen levels below the level lethal to fish.

 Wounds from territorial fighting

In late spring from May through June, a few channel catfish may be found dead. On examination the fish are likely to be male and scarred from bites received defending nesting habitat. These wounds become infected and ultimately lead to the death of the fish. A few male bluegill and largemouth bass may also be found dead of the same causes.

 Toxic algal blooms

Some species of algae release toxic substances into the water when nutrient competition becomes intense. Toxins from these algal species kill other algae resulting in an algal population made up of large numbers of the toxin emitting species. Their numbers may continue to grow and consequently competition for nutrients increases along with release of more toxins. Toxin concentration can reach levels lethal to fish, livestock and other organisms. Toxin release is related to photosynthesis therefore fish will begin to die when photosynthesis is most active, about 9:00AM – 5:00PM. Toxic algae may reach levels sufficient to shade itself out. Shading may cause enough algae to die or “crash” to cause a fish die-off due to more a conventional oxygen depletion problem created by the decaying algae. Common toxic algae include species of Anabaena, Gleotrichia, Dinobryon and Microsystis.

 Hydrogen sulfide

Hydrogen sulfide is a toxic gas created under anoxic conditions often found on pond bottoms covered with large amounts of organic matter, usually an accumulation of tree leaves, sticks and dead aquatic plants. Breakdown of this organic matter creates a thin black mud that is easily stirred and suspended in the water column. Hydrogen sulfide gas is released into the water when the bottom mud becomes agitated or stirred by pond “turnovers”, or the activities of livestock or people. The gas is easily detected by its characteristic “rotten egg” smell.

 Mortality due to bacteria or parasites

Most fish kills caused by bacteria or parasites are triggered by earlier fish stress due to causes such as low dissolved oxygen levels, high temperatures or high ammonia levels. Bacterial infections are difficult to diagnose to species at the pond bank; however, they often appear as bloody rings or streaks on the skin or at the edges of scales. Fish kills caused by bacteria may form a bell curve of mortality, with a few fish dieing at first then increasing numbers that peak then begin to decline. Treatment is possible using antibiotics with fish on feed but is seldom necessary in most ponds other than commercial fish production facilities.

 Mortality due to parasitic infections tends to begin slowly with only one or a few fish. Numbers of dead fish continue to increase over time as parasites multiply. Parasite populations decline as the number of host fish available for infection decreases from mortality. Usually the parasite is always present at sub-lethal levels in the pond. High fatality rates are caused when unusually favorable environmental circumstances occur including a stressed fish population. Parasitic infections can sometime be detected by watching fish activity. The fish will be seen to dive rapidly and rub against rocks or other pond bottom material in an effort to remove parasites. This behavior is called “flashing” because the white belly of most fish will be exposed as they rotate about on the pond bottom. Samples of living or moribund fish can be sent to fish disease diagnostic laboratories for testing. If testing is desired, first contact the laboratory for shipping instructions. It will be necessary to send live fish placed in plastic bags and shipped on ice in a foam container. Do not freeze the fish because freezing destroys tissue. Also do not use dead fish because parasites quickly drop off dead organisms. Parasitic treatments are available but like bacterial infections, treatment is rarely necessary except in commercial facilities.

 Mortality due to unnatural causes

Complete fish kills can occur in ponds due to release of toxic chemicals. Although uncommon, accidental pond contamination can happen. In general, toxic chemicals are more likely to kill young than large fish.


Pesticide run-off has been known to kill fish primarily in agricultural areas, although it also can be a problem in urban ponds. Properly applied pesticide is rarely a cause of fish death. It most often affects ponds when the pesticide has been applied inappropriately or just before a run-off rain event. Rains can wash newly applied pesticides into ponds, lakes and streams where concentrations may reach levels lethal to some or all fish species present. Fish kills can occur when pond owners use pesticides to control ticks and chiggers along pond banks. These chemicals can enter the pond if applied too soon before a rain event. Monitor weather before applying any chemical near a pond.

 Most commonly used herbicides are usually not lethal to fish. However, it may render the fish inedible for human consumption and may prevent livestock from using the water for up to 2-3 weeks.

 If phytoplankton and algae populations are in good condition but zooplankton, insect and fish populations have died; insecticide toxicity may be suspected as a cause of mortality. Also, some species of fish are more susceptible to insecticide poisoning than others. A die-off of only one or a few species may indicate pesticide poisoning. Conversely, dead algae, phytoplankton or other plants found in conjunction with a healthy zooplankton and insect population may suggest a fish kill due to toxic herbicidal contamination. 

  Salt water

Salt water from septic systems, livestock salt licks, old abandoned oil fields or other sources can sometimes enter ponds. This happens most often during times of heavy rains and rising water tables. Salt water may cause the pond to become uncommonly clear if concentrations are sufficient to kill all plant and animal life. Salt water most noticeably kills fish but it also kills many other organisms. Chemical test kits for salt water concentration can be obtained in most pet stores carrying aquarium supplies. Test your water before a fish kill to determine the normal salinity level. Should the pond experience a fish kill, salinity can be measured again. Salinity levels well above normal might suggest fish mortality from a salt water source.


Raw sewage from a broken pipe or poorly maintained septic system can enter ponds and kill fish. The sewage itself is not usually toxic to the fish; however, it uses up available oxygen as it decays and this process can trigger a low dissolved oxygen fish die-off. The sewage is also loaded with nutrients that may cause intense algal blooms. The algae may reach a density sufficient to shade itself out, resulting in a partial or complete die-off of phytoplankton. As the dead phytoplankton decays it may use up enough available oxygen to cause a fish kill.

 Although there are many possible causes of fish mortality, most pond fish kills are usually a result of low dissolved oxygen in the pond and will not result in the complete elimination of fish populations. Many small fish will usually survive. Due to increased food supply available to surviving fish, population numbers and fish size can be back to carrying capacity within 3-4 years. Restocking the pond is seldom necessary except in small ponds that are frequently killed off due to low oxygen levels. These ponds may soon only contain large numbers of stunted green sunfish and or black bullhead catfish. If other sportfish are desired the pond will require renovation and removal of existing fish populations before new stocks are added.






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