User:Jlikes2Fish

Dead fishes pile up on the western shores of Salton Sea, California

Fish kill is an event in which dead or dying fish are observed in a body of water. Fish kill can represent small numbers of fish or hundreds of thousands of fish. Different causes can lead to fish kill. The most common cause is reduced oxygen in the water, which in turn may be due to factors such as drought, algae bloom, overpopulation, or a sustained increase in water temperature. Toxicity is a real but far less common cause of fish kill. It is very difficult to predict when a fish kill is going to come about. Even when conditions that contribute to fish kill are known to exist, prevention is hard because often conditions cannot be improved and fish cannot be safely removed in time. In small ponds, mechanical aeration and/or removal of decaying matter (such as fallen leaves or dead algae) may be reasonable and effective preventive measures.

Fish kills are often the first visible signs of ecological hypoxia and are usually investigated as a matter of urgency by environmental agencies to determine the cause of the hypoxic event and whether or not the cause was man made, due to pollution, or naturally occurring. If it is a dissolved oxygen-related fish kill, large fish seem to be affected first. If it is a cold-related fish kill, juvenile fish and/or species that are not cold-tolerant may be selectively affected. If toxicity is the cause, species are more generally affected and the event may include amphibians and shellfish as well.

Oxygen Depletion

Warm water fish generally need dissolved oxygen (DO) concentration of at least 5 ppm (5 mg/l). Fish can endure short periods of reduced oxygen. However, fish might not recover from DO levels below 2 mg/l and begin to die when they fall below 1 mg/l. Depleted oxygen levels are the most common cause of fish kills. Oxygen levels normally fluctuate even over the course of a day and are affected by weather, temperature, the amount of sunlight available, and the amount of living and dead plant and animal matter in the water.

Oxygen enters the water through diffusion. Algae and aquatic plants also produce oxygen through photosynthesis. The amount of oxygen that can be dissolved in water depends on the atmospheric pressure, the water temperature and whether the water is salty. For example, at 20°C (68°F) and one atmosphere of pressure, a maximum of 8 mg/l of oxygen can dissolve in sea water (35 mg/l salinity) while a maximum of 9 mg/l of oxygen can dissolve in fresh water. The amount of oxygen that can be dissolved in the water decreases by about 1 mg/l for each 10°C increase in water temperature above 20°C.

Oxygen depletion can result from a change in any of several factors or unfavorable combination of factors.

drought

A drought can lead to lower water volumes so that even if the water contains a high level of dissolved oxygen, the reduced volume may not be enough for the fish population. Droughts often occur in conjunction with high temperatures so that the oxygen carrying capacity of the water may also be reduced.

turnover

Another cause of fish kills is pond turnover, sometimes called inversion or pond rollover. Turnover results in low dissolved oxygen levels and can lead to fish kill. It is usually only a serious problem in ponds 4 feet deep or deeper. During the late spring and summer, the pond water develops stratified layers as colder water sinks to the bottom. (Even though cooler water has more oxygen carrying capacity, this water in the bottom layer of the pond is not exposed to the plant or atmospheric sources of oxygen and so may have as little as 1 mg/l of dissolved oxygen.) Turnover occurs when the top layer of warm, oxygenated water mixes with the lower layer. A sudden cooling of the top layer of water causes the mixing. This sudden cooling may be caused by heavy rains, which in areas such as the Southeastern United States may be an every afternoon occurrence, a rapid temperature drop, a heavy cloud cover which affects the amount of sunlight, or a strong, steady wind blowing in one direction for a long period of time.

In some coastal areas, another type of turnover occurs in ponds or lagoons that have a direct connection to saltwater. Some of these connections are at too high an elevation for there to be regular tidal influence. On extreme spring tides, the ponds are flooded with saltwater, which is denser and therefore heavier than the freshwater of the pond. When the saltwater enters, it sinks to the bottom, displacing the bottom water that has a low dissolved oxygen level.

algae bloom

An algae bloom is the appearance of a large amount of algae or scum floating on the surface of a body of water. Algae blooms are a natural occurrence in nutrient-rich lakes and rivers, though sometimes increased nutrient levels leading to algae blooms are due to fertilizer or animal waste runoff. A few species of algae produce toxins, but most fish kills due to algae bloom are a result of decreased oxygen levels. When the algae die, decomposition uses oxygen in the water that would be available to fish. A fish kill in a lake in Estonia in 2002 was attributed to a combination of algae bloom and high temperatures. When people manage algae blooms in ponds, it is recommended that treatments be staggered to avoid too much algae dying at once, which may result in a large drop in oxygen content.

biological decay

Just as an algae bloom can lead to oxygen depletion, introduction of a large amount of decaying biological material in general to a body of water leads to oxygen depletion as microorganisms use up available oxygen in the process of breaking down organic matter. For example, a 10-mile-long fish kill in September, 2010, in the Sangamon River in Illinois was traced to discharge of animal waste into the river from a large dairy operation. The illegal discharge resulted in a complete kill of fish, frogs, mussels and mudpuppies.

water temperature

A fish kill can occur with rapid fluctuations in temperature or sustained high temperatures. Generally, cooler water has the potential to hold more oxygen, so a period of sustained high temperatures can lead to decreased dissolved oxygen in a body of water. An August, 2010, fish kill in Delaware Bay was attributed to low oxygen as a result of high temperatures.

A massive (hundreds of thousands) fish kill at the mouth of the Mississippi River in Louisiana, September, 2010, was attributed to a combination of high temperatures and low tide. Such kills are known to happen in this region in late summer and early fall, but this one was unusually large.

A short period of hot weather can increase temperatures in the surface layer of water, as the warmer water tends to stay near the surface and be further heated by the air. In this case, the top warmer layer may have more oxygen than the lower, cooler layers because it has constant access to atmospheric oxygen. If a heavy wind or cold rain then occurs (usually during the autumn but sometimes in summer), the layers can mix. If the volume of low oxygen water is much greater than the volume in the warm surface layer, this mixing can reduce oxygen levels throughout the water column and lead to fish fill.

overstocking

Fish kill due to insufficient oxygen is really a matter of too much demand and too little supply for whatever reason(s). Overstocking of fish (or an unusually large spawn) can thus lead to fish kill by itself. Recommended stocking densities are available from many sources for bodies of water ranging from a home aquarium or backyard pond to commercial aquaculture facilities.

Cold temperatures

Fish kills can result form a dramatic or prolonged drop in air (and thus, water) temperature. This kind of fish kill is selective - usually the dead fish are species that cannot tolerate cold. This has been observed in cases where a fish native to a more tropical region has been introduced to cooler waters, such as the introduction of the Tilapia to bodies of water in Florida. Native to Africa’s Nile River, the tilapia stop feeding when water temperatures drop below 60 degrees F and die when it reaches 45 degrees F. Thus, tilapia that have survived and successfully reproduced in Florida are occasionally decimated by a winter cold front.

In January, 2011, a selective fish kill affecting an estimated 2 million juvenile spot fish was attributed to a combination of cold stress and overpopulation after a particularly large spawn.

Spawning fatalities

Fish kill due to spawning fatalities can occur when fish are exhausted from spawning activities such as courtship, nest building, and the release of eggs or milt (sperm). Fish are generally weaker after spawning and are less resilient than usual to smaller changes in the environment.

Disease

Fish are subject to various viruses, bacteria and fungi in addition to parasites such as protozoans, flukes and worms, or crustaceans. These are naturally occurring in many bodies of water, and fish that are stressed for other reasons, such as spawning or suboptimal water quality, are more susceptible. Signs of disease include sores, missing scales or lack of slime, strange growths or visible parasites, and abnormal behavior – lazy, erratic, gasping at the water surface or floating head, tail or belly up.

For example, since 2004 fish kills have been observed in the Shenandoah River basin in the spring, from the time water temperatures are in the 50s (oF) until they reach the mid-70s. So far, investigators suspect certain bacteria, along with environmental and contaminant factors that may cause immune suppression.

In fish farming, where populations are optimized for the available resources, parasites or disease can spread quickly. In channel catfish aquaculture ponds, for example, the “hamburger gill disease” is caused by a protozoan called Aurantiactinomyxon and can kill all the fish in an affected pond. In addition to altered behavior, affected fish have swollen gills that are mottled and have the appearance of ground hamburger meat.

Some early warning signs of fish suffering from disease or parasite infections include:

1. Discoloration, open sores, reddening of the skin, bleeding, black or white spots on the skin
2. Abnormal shape, swollen areas, abnormal lumps, or popeyes
3. Abnormal distribution of the fish such as crowding at the surface, inlet, or pond edges
4. Abnormal activity such as flashing, twisting, whirling, convulsions, loss of buoyancy
5. Listlessness, weakness, sluggishness, lack of activity
6. Loss of appetite or refusal to feed

Toxins

Agricultural runoff, sewage, surface runoff, chemical spills, hazardous waste spills can all potentially lead to water toxicity and fish kill. Some algae species also produce toxins. In Florida, these include Aphanizomenon, Anabaena and Microcystis. Some notable fish kills in Louisiana in the 1950s were due to a specific pesticide called endrin.

Human-induced fish kills are unusual, but occasionally a spilled substance causes direct toxicity or a shift in water temperature or pH that can lead to fish kill. For example, in 1997 a phosphate plant in Mulberry, Florida accidentally dumped 60 million gallons of acidic process water into Skinned Sapling Creek, reducing the pH from about 8 to less than 4 along for about 36 miles and resulting in the death of about 1.3 million fish.

It is often difficult or impossible to determine whether a potential toxin is the direct cause of a fish kill. For example, hundreds of thousands of fish died after an accidental spill of bourbon into the Kentucky River near Lawrenceburg. However, officials could not determine whether the fish kill was due to the bourbon directly or to oxygen depletion that resulted when aquatic microbes rapidly began to consume and digest the liquor.

Underwater explosions

Underwater explosions can lead to fish kill, and fish with swim bladders are more susceptible. Sometimes underwater explosions are used on purpose to induce fish kills, a generally illegal practice known as blast fishing. Underwater explosions may be accidental or planned, such as for construction, seismic testing, mining or blast testing of structures under water. In many places, an assessment of potential effects of underwater explosions on marine life must be completed and preventive measures taken before blasting.

Reporting fish kill

Some states have hotlines established for the reporting of observed fish kill. Information is passed along to the appropriate regional official(s) for investigation.

Fish kill hotline in Florida to report observed fish kills

See also

• Aquatic biomonitoring

References

• Herring, and Scott (2002) Fish Kill in the Gulf of Oman: A space-based diagnosis NASA: Earth Observatory, featured article.

External links

External image
Red tide outbreak and fish kill

• Oxygen Depletion in Ponds, University of Georgia Cooperative Extension Service, Publication L233, 1993
• Fish Kills - Their Causes and Prevention, Virginia Tech, Virginia Cooperative Extension Publication 420-252, 2009
• Fish Kills in Coastal Georgia Ponds and Lagoons, Georgia Department of Natural Resources
• "The Fish Kill Mystery" - Erica F. Kosal, North Carolina Wesleyan College (2003)
• Maryland DNR page on Pfiesteria (sometimes toxic dinoflagellate)