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Fish Kills

Fish Kills

When a number of dead fish are found in one place, the incident is referred to as a fish kill, and there is significant reason to suspect pollution. The three main causes of fish kills are poisoning, disease, and suffocation.


Fish may be poisoned by a wide range of polluting substances, including pesticides, acids, ammonia, phenols, cresols, compounds of metals, detergents, or cyanides. Many of these substances are used in industrial processes or in agriculture and are released through drains or are accidentally spilled into waterways. Acid rain, derived from industrial pollutants in the atmosphere, causes rivers to become toxic for various kinds of fish. Some types of toxic algal blooms kill fish. During the 1990s the dinoflagellate Pfeisteria piscicida caused fish kills, ranging from a few hundred to a million fish at one time, in estuaries of the southeastern United States.


In natural environments, disease alone does not usually result in mass mortality, but under the artificial conditions of a hatchery or an aquaculture operation, disease can spread rapidly and cause a fish kill. The disease may be caused by viral infections, bacteria, fungi, or internal or external parasites.

In these same natural environments, it is more common for fish to be weakened by disease and then killed en masse by some stressful environmental situation, such as low-oxygen concentration, temperature extremes, or pollution. When fish move from cold water into much warmer water such as a heated effluent from a generating station, bubbles may form in their tissues and they die from gas bubble disease.


Suffocation occurs when the oxygen concentration in the water falls below the level at which fish can survive. A common cause is eutrophication, which is the artificial stimulation of plant growth by pollution with fertilizers, sewage, or atmospheric fallout. When the excess plant growth decays, it lowers the oxygen concentration. The discharge of dead organic matter into a watercourse from a sewer or from an industrial operation has the same effect. The accidental spilling of a herbicide into a lake or stream may kill large quantities of aquatic vegetation, causing low-oxygen conditions.

Nuisance algal blooms may also cause suffocation. In 1994 in St. Helena Bay, South Africa, a large bloom of toxic and nontoxic algae formed in an estuary and extended into the open sea more than thirty kilometers out from the shore. The bloom sank and decomposed, forming an area with almost no oxygen and with lethal levels of hydrogen sulfide. Approximately fifteen hundred tons of dead fish and sixty tons of dead rock lobsters were washed ashore.

Many fish kills could be prevented by reducing the amount of pollution, especially nitrogen and phosphorus, entering waterways. Applications of fertilizers should be matched to the needs of the crop, sewage effluent should receive advanced treatment, and atmospheric emissions from industry and transport should be carefully controlled at source.

see also Acid Rain; Agriculture; Hypoxia; Oxygen Demand, Biochemical; Phosphates; Thermal Pollution; Water Pollution; Water Pollution: Marine.


burkholder, j.m. (1999). "the lurking perils of pfeisteria." scientific american 282:4249.

meyer, fred p., and barclay, lee a., eds. (1990). field manual for the investigation of fish kills. resource publication 177. washington, dc: u.s. fish and wildlife service.

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"fish kills offer challenge to deq." available from

Kenneth H. Mann

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Fish Kills

Fish kills

Fishing has long been a major provider of food and livelihood to people throughout the world. In the United States, 50 million people enjoy fishing as an outdoor recreation38 million in fresh water and 12 million in salt water. Combined, they spend over $315 million annually on this sport. It is no surprise, then, that public attitude towards factors that influence fishing is strong.

The Environmental Protection Agency (EPA) is charged with overseeing the quality of the nation's waterways. In 1977 they received information on 503 separate incidents in which 16.5 million fish were killed. In 1974, a record 47 million fish were killed in the Black River near Essex, Maryland, by a discharge from a sewage plant.

Fish kills can result from natural as well as human causes. Natural causes include sudden changes in temperature, oxygen depletion, toxic gases, epidemics of viruses and bacteria, infestations of parasites , toxic algal blooms, lightning, fungi , and other similar factors. Human influences that lead to fish kills include acid rain , sewage effluent , and toxic spills.

In a 10-year study of the causes of 409 documented fish kills totaling 3.6 million fish in the state of Missouri, S. M. Czarnezki determined the percentage contributions as: 26% municipal-related (sewage effluent), 17% from agricultural activities, 11% from industrial operations, 8% by transportation accidents, 7% each by oxygen depletions, nonindustrial operations, and mining, 4% by disease, 3% by "other" factors, and 10% as undetermined.

Fish kills may occur quite rapidly, even within minutes of a major toxic spill. Usually, however, the process takes days or even months, especially in natural causes. Experienced fishery biologists usually need a wide variety of physical, chemical, and biological tests of the habitat and fish to determine the exact causative agent or agents. The investigative procedure is often complex and may require a lot of time.

Species of fish vary in their susceptibility to the different factors that contribute to die-offs. Some species are sensitive to almost any disturbance, while other fish are tolerant of changes. As discussed below, predatory fish at the top of the food chain/web are typically the first fish affected by toxic substances that accumulate slowly in the water.

The most common contributor to fish kills by natural causes is oxygen depletion, which occurs when the amount of oxygen utilized by respiration , decomposition , and other processes exceeds oxygen input from the atmosphere and photosynthesis . Oxygen is more soluble in cold than warm water. Summer fish kills occur when lakes are thermally stratified. If the lake is eutrophic (highly productive), dead plant and animal matter that settles to the bottom undergoes decomposition, utilizing oxygen. Under windless conditions, more oxygen will be used than is gained, and animals like fish and zooplankton often die from suffocation.

Winter fish kills can also occur. Algae can photosynthesize even when the lake is covered with ice because sunlight can penetrate through the ice. However, if heavy snowfall accumulates on top of the ice, light may not reach the underlying water, and the phytoplankton die and sink to the bottom. Decomposers and respiring organisms again use up the remaining oxygen and the animals eventually die. When the ice melts in the spring, dead fish are found floating on the surface. This is a fairly common occurrence in many lakes in Michigan, Wisconsin, Minnesota, and surrounding states. For example, dead alewives (Alosa pseudoharengus ) often wash up on the southwestern shore of Lake Michigan near Chicago during spring thaws following harsh winters.

In summer and winter, artificial aeration can help prevent fish kills. The addition of oxygen through aeration and mixing is one of the easiest and cheapest methods of dealing with low oxygen levels. In intensive aquaculture ponds, massive fish deaths from oxygen depletion are a constant threat. Oxygen sensors are often installed to detect low oxygen levels and trigger the release of pure oxygen gas from nearby cylinders.

Natural fish kills can also result from the release of toxic gases. In 1986, 1,700 villagers living on the shore of Lake Nyos, Cameroon, mysteriously died. A group of scientists sent to investigate determined that they died of asphyxiation. Evidently a landslide caused the trapped carbon dioxide-rich bottom waters to rapidly rise to the surface much like a popped champagne bottle. The poisonous gas killed everyone in its downwind path. Fish in the upper oxygenated waters of the lake were also killed as the carbon dioxide passed through.

Hydrogen sulfide (H2S), a foul-smelling gas naturally produced in the oxygen-deficient sediments of eutrophic lakes, can also cause fish deaths. Even in oxygenated waters, high H2S levels can cause a condition in fish called "brown blood." The brown color of the blood is caused by the formation of sulfhemoglobin, which inhibits the blood's oxygen-carrying capacity. Some fish survive, but sensitive fish such as trout usually die.

Fish kills can also result from toxic algal blooms. Some bluegreen algae in lakes and dinoflagellates in the ocean release toxins that can kill fish and other vertebrates, including humans. For example, dense blooms of bluegreen algae such as Anabaena, Aphanizomenon, and Microcystis have caused fish kills in many farm ponds during the summer. Fish die not only from the toxins but also from asphyxiation resulting from decomposition of the mass of algae that also die due to lack of sunlight in the densely-populated lake water. In marine waters, toxic dinoflagellate blooms called red tides are notorious for causing massive fish kills. For example, blooms of Gymnodinium or Gonyaulax periodically kill fish along the East and Gulf Coasts of the United States. Die-offs of salmon in aquaculture pens along the southwestern shoreline of Norway have been blamed on these organisms. Millions of dollars can be lost if the fish are not moved to clear waters. Saxitoxin, the toxic chemical produced by Gonyaulax,is50 times more lethal than strychnine or curare.

Pathogens and parasites can also contribute to fish kills. Usually the effect is more secondary than direct. Fish weakened by parasites or infections of bacteria or viruses usually are unable to adapt to and survive changes in water temperature and chemistry. Under stressful conditions of over-crowding and malnourishment, gizzard shad often die from minor infestations of the normally harmless bacterium Aeromonas hydrophila. In the same way, fungal infections such as Ichthyophonus hoferia can contribute to fish kills. Most fresh water aquarium keepers are familiar with the threat of "ick" for their fish. The telltale white spots under the epithelium of the fins, body, and gills are caused by the protozoan parasite Ichthyophthirius multifiliis.

Changes in pH of lakes resulting from acid rain are a modern example of how humans can cause fish kills. Atmospheric pollutants such as nitrogen dioxide and sulfur dioxide released from automobiles and industries mix with water vapor and cause the rainwater to be more acid than normal (>pH 6.5). Nonprotected lakes downwind that receive this rainfall increase in acidity, and sensitive fish eventually die. Most of the once-productive trout streams and lakes in the southern half of Norway are now devoid of these prized fish. Sweden has combatted this problem by adding enormous quantities of lime to their affected lakes in the hope of neutralizing the acid's effects.

Sewage treatment plants add varying amounts of treated effluent to streams and lakes. Sometimes during heavy rainfall raw sewage escapes the treatment process and pollutes the aquatic environment . The greater the organic matter that comprises the effluent, the more decomposition occurs, resulting in oxygen usage. Scientists call this the biological or biochemical oxygen demand (BOD), the quantity of oxygen required by bacteria to oxidize the organic waste aerobically to carbon dioxide and water. It is measured by placing a sample of the wastewater in a glass-stoppered bottle for five days at 71 degrees Fahrenheit (20 degrees Celsius) and determining the amount of oxygen consumed during this time. Domestic sewage typically has a BOD of about 200 milligrams per liter, or 200 parts per million (ppm); rates for industrial waste may reach several thousand milligrams per liter. Reports of fish kills in industrialized countries have greatly increased in recent years. Sewage effluent not only kills fish; it can also create a barrier to fish migrating upstream because of the low oxygen levels. For example, coho salmon will not pass through water with oxygen levels below 5 ppm. Oxygen depletion is often more detrimental to fish than thermal shock.

Toxic chemical spills , whether via sewage treatment plants or other sources, are the major cause of fish kills. Sudden discharges of large quantities of highly toxic substances usually cause massive death of most aquatic life. If they enter the ecosystem at sublethal levels over a long time, the effects are more subtle. Large predatory or omnivorous fish are typically the first ones affected. This is because toxic chemicals like methyl mercury , DDT, PCBs, and other organic pollutants have an affinity for fatty tissue and progressively accumulate in organisms up the food chain. This is called the principle of biomagnification . Unfortunately for human consumers, these fish do not usually die right away, so people who eat a lot of tainted fish become sick and possibly die. Such is the case for Minamata disease , named for the first documented connection between the death of fishermen and methyl mercury contamination.

[John Korstad ]



Czarnezki, J. M. A Summary of Fish Kill Investigations in Missouri, 19701979. Columbia, MO: Missouri Dept. of Conservation, 1983.

Ehrlich, P. R., A. H. Ehrlich, and J. P. Holdren. Ecoscience: Population, Resources, Environment. San Francisco: W. H. Freeman, 1977.

Goldman, C. R., and A. J. Horne. Limnology. New York: McGraw-Hill, 1983.

Hill, D. M. "Fish Kill Investigation Procedures." In Fisheries Techniques, edited by L. A. Nielson and D. L. Johnson. Bethesda, MD: American Fisheries Society, 1983.

Meyer, F. P., and L. A. Barclay, eds. Field Manual for the Investigation of Fish Kills. Washington, DC: U.S. Fish and Wildlife Service, 1990.

Moyle, P. B., and J. J. Cech Jr. Fishes: An Introduction to Ichthyology. 2nd ed. New York: Prentice-Hall, 1988.


Keup, L. E. "How to 'Read' A Fish Kill." Water and Sewage Works 12 (1974): 4851.

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