Fish kill
The term fish kill, also known as fish die-off, refers to a localized mass die-off of fish populations in a body of water, which may also be associated with more generalized mortality of aquatic life. The most common cause is anoxia in the water, which in turn may be due to factors such as drought, harmful algal bloom, overpopulation, or a sustained increase in water temperature. Infectious diseases and parasites can also lead to fish kill. Toxicity is a real but far less common cause of fish kill, and is often associated with man-made water pollution.
Fish kills are often the first visible signs of environmental stress and are usually investigated as a matter of urgency by environmental agencies to determine the cause of the kill. Many fish species have a relatively low tolerance of variations in environmental conditions and their death is often a potent indicator of problems in their environment that may be affecting other animals and plants and may have a direct impact on other uses of the water such as for drinking water production. Pollution events may affect fish species and fish age classes in different ways. If it is a cold-related fish kill, juvenile fish 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. A reduction in dissolved oxygen may affect larger specimens more than smaller fish as these may be able to access oxygen richer water at the surface, at least for a short time.
Causes
Fish kills may result from a variety of causes. Of known causes, fish kills are most frequently caused by pollution from agricultural runoff or biotoxins. Ecological hypoxia is one of the most common natural causes of fish kills. The suffocating event may be brought on by factors such as algae blooms, droughts, high temperatures and thermal pollution. Fish kills may also occur due to the presence of disease, agricultural runoff, sewage discharges, oil or hazardous waste spills, hydraulic fracturing wastewater, sea-quakes, inappropriate re-stocking of fish, poaching with chemicals, underwater explosions, and other catastrophic events that upset a normally stable aquatic population. Because of the difficulty and lack of standard protocol to investigate fish kills, many fish kill cases are designated as having an unknown cause.Oxygen depletion
Oxygen enters the water through diffusion. 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 and one atmosphere of pressure, a maximum of 8 mg/L of oxygen can dissolve in sea water 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.Many cold water fish that live in clean cold waters become stressed when oxygen concentrations fall below 8 mg/L while warm water fish generally need at least 5 ppm of dissolved oxygen. Fish can endure short periods of reduced oxygen. 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. In temperate zones oxygen levels in eutrophic rivers in summertime can exhibit very large diurnal fluctuations with many hours of oxygen supersaturation during daylight followed by oxygen depletion at night. Associated with these photosynthetic rhythms there is a matching pH rhythm as bicarbonate ion is metabolised by plant cells. This can lead to pH stress even when oxygen levels are high.
Additional dissolved organic loads are the most common cause of oxygen depletion and such organic loads may come from sewage, farm waste, tip/landfill leachate and many other sources.
Diseases and parasites
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 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:
- Discolouration, open sores, reddening of the skin, bleeding, black or white spots on the skin
- Abnormal shape, swollen areas, abnormal lumps, or popeyes
- Abnormal distribution of the fish such as crowding at the surface, inlet, or pond edges
- Abnormal activity such as flashing, twisting, whirling, convulsions, loss of buoyancy
- Listlessness, weakness, sluggishness, lack of activity
- Loss of appetite or refusal to feed.
Toxins
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
of acidic process water into Skinned Sapling Creek, reducing the pH from about 8 to less than 4 along of creek, 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 whiskey 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.
Cyanide is a particular toxic compound that has been used to poach fish. In cyanide poisoning the gills turn a distinctive cherry red. Chlorine introduced as alkaline hypochlorite solution is also extremely toxic, leaving pale mucilaginous gills and an over-production of mucilage across the whole body. Lime produces similar symptoms but is also often associated with milk eyes.
Algae blooms and red tides
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 fish 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.Some diseases result in mass die-offs. One of the more bizarre and recently discovered diseases produces huge fish kills in shallow marine waters. It is caused by the ambush predator dinoflagellate Pfiesteria piscicida. When large numbers of fish, like shoaling forage fish, are in confined situations such as shallow bays, the excretions from the fish encourage this dinoflagellate, which is not normally toxic, to produce free-swimming zoospores. If the fish remain in the area, continuing to provide nourishment, then the zoospores start secreting a neurotoxin. This toxin results in the fish developing bleeding lesions, and their skin flakes off in the water. The dinoflagellates then eat the blood and flakes of tissue while the affected fish die. Fish kills by this dinoflagellate are common, and they may also have been responsible for kills in the past which were thought to have had other causes. Kills like these can be viewed as natural mechanisms for regulating the population of exceptionally abundant fish. The rate at which the kills occur increases as organically polluted land runoff increases.
Red tide is the name commonly given to an algal bloom of Karenia brevis, a microscopic marine dinoflagellate which is common in Gulf of Mexico waters. In high concentrations it discolors the water which often appears reddish-brown in color. It produces a toxin which paralyses the central nervous system of fish so they cannot breathe. Dead fish wash up on beaches around Texas and Florida. Humans can also become seriously ill from eating oysters and other shellfish contaminated with the red tide toxin.
The term "red tide" is also commonly used to describe harmful algal blooms on the northern east coast of the United States, particularly in the Gulf of Maine. This type of bloom is caused by another species of dinoflagellate known as Alexandrium fundyense. These blooms are natural phenomena, but the exact combination of factors that result in red tide outbreaks is not fully understood.