Fish Kill: Causes and Prevention

The term “kill fish”, also known as “fish death”, refers to the local death of a fish population, which may also be associated with a more widespread death of aquatic life.The most common reason is the reduction of oxygen in water, which, in turn, can be due to factors such as drought, algal blooms, overpopulation or constant increase in water temperature. Infectious diseases and parasites can also lead to fish killing.

Toxicity is a real, but much more prevalent, cause of the killing of fish. The risk of the killing of a fish is often the first obvious symptom of environmental stress and is usually addressed by the environmental agency to determine the cause of death. Many species of fish have relatively low tolerance for environmental fluctuations, and their mortality is often a sign of environmental problems that may affect other animals and plants and may have a direct impact on other water uses such as drinking water. Pollution events can affect different species of fish and age groups. When it comes to kill cold fish, young individuals or species that are not resistant to cold can be selectively affected. If the cause is toxicity, commonly affected species, and the event may be amphibians and crustaceans. Melting down the oxygen may affect samples that are larger than smaller ones, as they can reach at least oxygen-rich water for at least a short period of time.

Kill fish can be due to a variety of reasons. The known causes of fish death are most often caused by pollution caused by the outflow of agriculture or biological toxins. Eco-hypoxia (oxygen depletion) is one of the most common natural causes of fish kills. Hypoxia events can be caused by factors such as algal blooms, droughts, high temperatures and thermal contamination. The risk of death can also occur due to illness, agricultural outflow, discharges, oil spills or hazardous waste, hydraulic water shocks, marine impacts, water stocks. Due to the difficulty and lack of a standard protocol for investigating fish kills, many cases of fish kills have been labelled as “unknown” causes.

Oxygen Depletion

Oxygen enters the water through diffusion. The amount of oxygen that can be dissolved in water depends on atmospheric pressure, water temperature and whether it is salty. For example, at a temperature of 20 ° C (68 ° F) and one atmospheric pressure, a maximum of 8 mg / l of oxygen in seawater can be dissolved (35 mg / l salinity) and a maximum of 9 mg / l can dissolve in fresh water. The amount of oxygen that can be dissolved in water is reduced by about 1 mg / l at each temperature increase of 10 ° C by more than 20 ° C. Many cold-water fish that live in clean cold waters are stressed when oxygen levels fall below 8 mg / l, while warm water fish need at least 5 ppm (5 mg / l) of dissolved oxygen. Fish can withstand short periods of reduced oxygen. The worst cause of fish kill is the smallest concentration of oxygen. Oxygen levels typically fluctuate even during the day and are exposed to weather, temperature, sunlight, live and dead plant and animal substances in water. In temperate zones, oxygen levels in eutrophic rivers during the summer can show very high daily fluctuations, with many hours of oxygen saturation during the day and then oxygen consumption at night. The rhythm associated with these photosynthetic rhythms is adjusted because the bicarbonate ion is metabolized by plant cells. This can lead to pH stress even when the oxygen level is high. Additional dissolved organic loads are the most common cause of oxygen depletion, and such organic waste may come from sewage, agricultural waste, tailings and many other sources.

Diseases and Parasites

In addition to parasites such as protozoa, flucas and worms, shellfish, fish are subject to various viruses, bacteria and fungi. They are naturally occurring in many parts of the water, and fish that are underlined for other reasons, such as spawning or suboptimal water quality, are more susceptible. Symptoms of the disease are: ulcers, missing scales or lack of mucus, strange growth or visible parasites, and abnormal behavior – lazy, unfavorable, suffocating on the surface of the water or floating head, tail or stomach.

In fish farming, where populations are optimized for available resources, parasites or diseases can spread rapidly. For example, in aquaculture aquaculture on a canal, for example “burger poisoning” is caused by a protozoan called Aurantiactinomyxon and can kill all fish in the affected pond. In addition to altered behavior, affected fish have puffy gills that are mottled and have the appearance of minced hamburger meat.

Some symptoms of early warning of fish suffering from diseases or parasitic infections include:

• Discoloration, open wounds, skin redness, bleeding, black or white spots on the skin.

• Incorrect form, swollen areas, abnormal solids, or popeyes.

• Incorrect fish distribution, such as surface pressing, inlet or joint edges (although falling to the surface at certain times of the day, such as early morning, is probably more a sign of low oxygen levels).

• Incorrect activity such as flashing, twisting, spinning, convulsions, loss of buoyancy.

• Insomnia, weakness, weakness, inactivity.

• Loss of appetite or refusal to feed.

Agricultural effluents, sewage, surface runoff, chemical spills and spills from hazardous waste can lead to fish and fish poisoning. Some algae species also produce toxins. In Florida they include Aphanizomenon, Anabaena and Microcystis.

Some of the mighty fish that were slaughtered in Louisiana in the 1950s were caused by a specific pesticide called endrin. Natural toxicity may occur, especially in poorly buffered water. Alumina compounds can cause complete kill of fish, sometimes associated with autumn landslides, leading to complex chemical interactions between pH, calcium ions and complex polymeric aluminum salts. The risk of death caused by humans is unusual, but sporadically spilled material causes direct toxicity or a change in water temperature or pH that can lead to fish kill. For example, in 1997, the Florida plant in Mulberry accidentally dropped 60 million galaxies (0.23 million kl) of acidic water in the Skinned Sapling Creek, reducing the pH from about 8 to less than 4 along the 36 miles (58 km) of the gulf , resulting in the death of about 1.3 million fish.

It is often difficult or impossible to determine whether a potential toxin is a direct cause of fish kill. For example, hundreds of thousands of fish died after accidental leaking amber whiskey to the Kentucky River near Lawrenceburg. Officials could not ascertain whether the kill of fish was directly attributable to boron or oxygen depletion, which prompted waterborne microorganisms to quickly consume and digest alcohol. Cyanide is a particular toxic compound that was used to fish. Cyanide poisoning changes the characteristic red cherries. Chlorine introduced as an alkaline solution of hypochlorite is also very toxic, leaving pale slimy gills and excessive mucus production throughout the body. Lime causes similar symptoms, but is often associated with the milky eyes.

Prevention and Investigation

Fish disorders are difficult to predict. Even if there are circumstances that contribute to kill fish, prevention is difficult because it is often impossible to improve conditions and fish can not be safely removed in time. In small ponds, mechanical aeration and / or removal of decaying substances (such as fallen leaves or dead algae) may be a reasonable and effective preventive measure.

Many countries in developed countries have specific rules to encourage the public to report fish kills in order to carry out a proper investigation. Studying the cause of death requires a multidisciplinary approach, including on-site environmental measurements, expenditure surveys, meteorology and history reviews, toxicology, fish autopsy, invertebrate analysis, and solid knowledge of the area and its problems.

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