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Fish Kills in Ponds

Date Published: August 4, 2000


Dan Catchings
District II Fisheries Supervisor
Eastaboga, Alabama

An occasional problem that confronts pond owners is the loss of part or all of the fish population in their ponds because of a fish kill. Fish kills can eliminate or severely reduce the sport fishing provided by ponds. A basic knowledge of the causes and symptoms of fish kills and possible remedial measures will be advantageous to the pond owner.

One of the most common types of fish kills in ponds is caused by depletion of dissolved oxygen in the water. Oxygen must be present in the water at or above certain minimum concentrations for fish to survive. The symptoms of an oxygen depletion fish kill are as follows--the fish will be swimming near the surface, attempting to gulp air; large fish will be killed first, followed by smaller fish; the kill will occur at night or in the early morning hours; the majority of fish will be killed within a few hours time; the water in the pond will be brown, gray or black; and a foul odor is detectable when the fish kill starts.

Oxygen depletion fish kills are most commonly caused by a die-off of the microscopic green plants (phytoplankton) in the pond, or overturns in which oxygen deficient water from the deeper levels of the pond mixes with water in the upper levels. Phytoplankton imparts a green color to the water in a properly fertilized pond. It is essential as part of the food chain and as a supplier of oxygen. A sudden die-off of these plants may occur, especially during periods of cloudy, overcast weather, when the plants use more oxygen than they supply to the pond. As the phytoplankton die and decompose, the remaining oxygen in the pond is consumed and within a short time, a fish kill occurs due to oxygen depletion.

To prevent oxygen shortages due to phytoplankton die-off, proper fertilization should be practiced. However, ponds should not be fertilized to the point where they become a "pea soup green," with visibility of only two to three inches instead of the ideal 12 to 18 inches of visibility. A herbicide can be used to thin out phytoplankton growth when it becomes too dense. Caution should be used to prevent a fish kill. If oxygen depletion occurs, apply either triple superphosphate fertilizer at the rate of 17 pounds per acre, or liquid fish pond fertilizer at one gallon per acre to re-establish the phytoplankton.

Overturns can lead to oxygen depletion fish kills. During summer, water in a pond will become stratified (form layers), with warmer water in the upper level and the colder water near the pond bottom. This stratification is particularly severe in ponds with dense growths of blue-green or "scum" algae on the surface. The colder water may become oxygen-deficient due to interaction with certain elements in the pond bottom and lack of photosynthesis by phytoplankton at greater depths.

An overturning or mixing of this colder, oxygen-deficient water may occur after a heavy rainfall. If the rainfall is of sufficient quantity, it produces a mixing of the pond water as the cold rainwater sinks to the pond bottom. It displaces the oxygen-poor water, which mixes with the water in the upper levels of the pond. The result is a fish kill due to insufficient oxygen in the water.

Remedial measures for oxygen depletions in ponds can be any method that adds sufficient oxygen to the pond water to prevent or minimize a fish kill. A water pump can be used to pump fresh water into the pond, or the water in the pond can be sprayed into the air and allowed to fall back into the pond. This will oxygenate the water, however, it is important that water that is sprayed is water taken from near the surface and not stagnant bottom water. Any device that agitates and aerates the water sufficiently can be of value. Do no use well water as a source for pumping fresh water into the pond, as it can be very low in oxygen.

Pesticides cause a number of fish kills in ponds throughout Alabama each year. Convulsive, erratic swimming and lethargy are symptomatic of pesticide toxicity. If an organo-phosphate insecticide is the cause, the pectoral fins of the fish will be reversed. The pectoral fins of a healthy fish point toward the tails, but the pectoral fins of a fish poisoned by insecticide curl up and point toward the head. Generally, the small fish will be killed first and eventually all sizes of fish may die. 

Fish ponds should not be built in areas adjacent to crops that are likely to be treated with pesticides toxic to fish. Diversion ditches should be dug around the pond to prevent toxic runoff water from entering it.

If a pesticide-caused fish kill is suspected, a one-gallon water sample should be taken as soon as possible in a clean glass container. A piece of aluminum foil should be placed over the mouth of the container under the lid, and the container should be labeled with the collector's name and address. The name of any herbicide or pesticide recently used in the watershed should also be included. The sample should be packed in a styrofoam container to prevent breakage, iced down and shipped to: The Alabama Pesticide Residue Laboratory, 1081 S. Donahue Drive, Auburn, Alabama 36832. Shipment by bus is preferred for such a large heavy package. The label should contain the notation "Urgent--Please phone 844-4705 on arrival." The laboratory will test the sample and send the results to the pond owner.

Other major causes of fish kills in ponds are parasites and disease. Fish killed or weakened by parasites and/or disease may show visible external signs, such as lesions, hemorrhages or changes in pigmentation. The fish may be sluggish or swimming erratically.

A fish kill caused by parasites or disease will extend over a longer period of time, compared to a kill caused by oxygen depletion, when large numbers of fish are killed within a few hours. The fish kill may occur over a period of several days or even several weeks, with a few fish dying each day. The number of dead fish gradually increases until it peaks, and then decreases as the parasite or disease runs its course.

Diagnosis of a suspected parasite or disease kill can be made by the Parasite and Disease Laboratory at Auburn University's Department of Fisheries and Allied Aquaculture. Contact the lab at 334-844-4786 for information concerning this service. There is a charge for this service. The decision about using this service should also include questions of how the diagnosis will be used. The treatments for many diseases are not worth the effort and expense of the pond owner. The pond owner often decides to let the disease run its course.

Swarming of the winged form of the fire ant in later winter or early spring has caused many fish kills. As the swarming ants fall into a pond, bluegills gorge themselves on the ants. If enough ants are eaten, the fish will die, but the majority of fish feeding on the ants will become ill and recover without lasting effects. Fire ant-caused fish kills do not present a serious problem by significantly reducing the fish population. If a fire ant kill is suspected, examination of the fish's stomach will confirm the problem.

If a fish kill occurs in your pond, contact your District Office for advice and possible assistance.

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