The termfish kill, known also asfish die-off, refers to a localizedmass die-off of fish populations which may also be associated with more generalized mortality of aquatic life.[1][2] The most common cause is reduced oxygen in the water, which in turn may be due to factors such asdrought,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-madewater pollution.[3]
Fish kills are often the first visible signs ofenvironmental 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 fordrinking water production. Pollution events may affect fish species andfish 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. Iftoxicity is the cause, species are more generally affected and the event may include amphibians and shellfish as well. A reduction indissolved 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.
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 (oxygen depletion) is one of the most common natural causes of fish kills. The suffocating event may be brought on by factors such asalgae blooms, droughts, high temperatures[4] andthermal pollution. Fish kills may also occur due to the presence of disease,agricultural runoff,sewage discharges, oil orhazardous waste spills,hydraulic fracturing wastewater,sea-quakes, inappropriate re-stocking of fish,poaching with chemicals, underwaterexplosions, and other catastrophic events that upset a normally stable aquatic population.[2] Because of the difficulty and lack of standard protocol to investigate fish kills, many fish kill cases are designated as having an unknown cause.[5][6]
Oxygen enters the water throughdiffusion. The amount of oxygen that can be dissolved in water depends on theatmospheric pressure, the water temperature and whether the water is salty.[7] For example, at 20 °C (68 °F) and one atmosphere of pressure, a maximum of 8 mg/L of oxygen can dissolve insea water (35 mg/Lsalinity) 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 (5 mg/L) 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.[8] In temperate zones oxygen levels ineutrophic rivers in summertime can exhibit very largediurnal fluctuations with many hours of oxygensupersaturation during daylight followed byoxygen depletion at night.[9] Associated with thesephotosynthetic rhythms there is a matchingpH rhythm asbicarbonate 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/landfillleachate and many other sources.
Fish are subject to variousviruses,bacteria andfungi in addition to parasites such asprotozoans, 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 theShenandoah River basin in the spring, from the time water temperatures are in the 50s (°F) until they reach the mid-70s. So far, investigators suspect certain bacteria, along with environmental and contaminant factors that may causeimmune suppression.[10]
Infish farming, where populations are optimized for the available resources,parasites or disease can spread quickly. Inchannel catfishaquaculture ponds, for example, the "hamburger gill disease" is caused by a protozoan calledAurantiactinomyxon 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.[8]
Some early warning signs of fish suffering from disease or parasite infections include:[11]
Agricultural runoff,sewage,surface runoff,chemical spills andhazardous waste spills can all potentially lead to watertoxicity and fish kill. Some algae species also produce toxins. In Florida, these includeAphanizomenon,Anabaena andMicrocystis. Some notable fish kills in Louisiana in the 1950s were due to apesticide calledendrin.[12] Natural instances of toxic conditions can occur, especially in poorlybuffered water. Aluminium compounds can cause complete fish kills, sometimes associated with autumn turn-over of lakes leading to complex chemical interactions between pH,calcium ions and complexpolymeric salts of aluminium.[13]
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 aphosphate plant inMulberry, Florida, accidentally dumped 60 million US gal (0.23 million kl) of acidic process water into Skinned Sapling Creek, reducing the pH from about 8 to less than 4 along 36 miles (58 km) of creek, resulting in the death of about 1.3 million fish.[8]
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 ofbourbon whiskey into theKentucky River nearLawrenceburg. However, officials could not determine whether the fish kill was due to the bourbon directly or to oxygen depletion that resulted when aquaticmicrobes rapidly began to consume and digest the liquor.[8]
Cyanide is a particular toxic compound that has beenused to poach fish. In cyanide poisoning the gills turn a distinctive cherry red.Chlorine introduced as alkaline hypochlorite solution is also extremely toxic,[14] 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.
Analgae bloom is the appearance of a large amount ofalgae orscum floating on the surface of a body of water. Algae blooms are a natural occurrence in nutrient-rich lakes and rivers, though sometimesincreased nutrient levels leading to algae blooms are due to fertilizer or animal wasterunoff. A few species of algae producetoxins, 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.[15] When people manage algae blooms infish 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.[16] One of the more bizarre and recently discovered diseases produces huge fish kills in shallow marine waters. It is caused by theambush predatordinoflagellatePfiesteria piscicida. When large numbers of fish, likeshoalingforage 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-swimmingzoospores. If the fish remain in the area, continuing to provide nourishment, then the zoospores start secreting aneurotoxin. 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.[17] 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.[17] 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 pollutedland runoff increases.[18]
Red tide is the name commonly given to an algal bloom ofKarenia brevis, a microscopic marine dinoflagellate which is common inGulf 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.[19][20]The term "red tide" is also commonly used to describe harmful algal blooms on the northern east coast of the United States, particularly in theGulf of Maine. This type of bloom is caused by another species ofdinoflagellate known asAlexandrium fundyense.[21] These blooms are natural phenomena, but the exact combination of factors that result in red tide outbreaks is not fully understood.[22]
Just as an algae bloom can lead to oxygen depletion, introduction of a large amount ofdecaying biological material in general to a body of water leads to oxygen depletion asmicroorganisms use up available oxygen in the process of breaking downorganic matter. For example, a 10 miles (16 km) fish kill in September, 2010, in theSangamon 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 andmudpuppies.[23]
Excessiveanthropogenicnutrient enrichment of phosphorus and nitrogen allow for rapid growth and multiplication ofphytoplankton in theMississippi River. As phytoplankton continue to rapidly grow under optimal conditions, their biomass is almost doubled every 24 hours. In the water, higher concentrations of organic matter are present because of the high reproductive rate of the phytoplankton over a short period of time. The rapid growth of phytoplankton causesturbidity in the waters of the Mississippi and theGulf of Mexico. Turbidity is defined as the measure of water clarity by how much the suspended material, such as algae and phytoplankton, constrict the passage of sunlight through water. Hence, as phytoplankton begin to multiply more rapidly, turbidity in the river and gulf increases.[24] The increasing turbidity blocks plants from absorbing sunlight. The process of turbidity results in limited photosynthesis production, and sometimes even death from sunlight deprivation of the submerged aquatic vegetation that are affected by the opaque turbid water accumulating at the surface.
Furthermore, a significant detrimental outcome caused byeutrophication in the Mississippi River is the increased uptake of dissolved oxygen by bacteria, in response to higher concentrations of organic matter. After eutrophication starts and is in progress, the phytoplankton reach their maximumpopulation density and begin to die.[24] As the dead phytoplankton accumulate, detritus, or organic matter waste, forms at the surface along with other bacteria and algae. As more phytoplankton die, the higher the concentration of organic matter becomes; and with a higher concentration of organic matter, more bacteria will reproduce.
Consequently, as more bacteria, phytoplankton, and algae exponentially grow and multiply, the more submerged aquatic vegetation die, because they do not have access to sunlight due to eutrophication. Once this snowball-like course of action is in full motion, a dead zone has been created. As a result of the excess nutrient enrichment in the Mississippi River, dead zones appear in the Gulf of Mexico, created from the process of eutrophication. The dead zones in the gulf are mainly created by the nitrogen and phosphorus enrichment of the Lower Mississippi River.
Some species of fish exhibit mass simultaneous mortality as part of their natural life cycle.Fish kill due to spawning fatalities can occur when fish are exhausted fromspawning activities such as courtship, nest building, and the release of eggs ormilt (sperm). Fish are generally weaker after spawning and are less resilient than usual to smaller changes in the environment. Examples include theAtlantic salmon and theSockeye salmon where many of the females routinely die immediately after spawning.
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 inDelaware Bay was attributed to low oxygen as a result of high temperatures.[25]A massive (hundreds of thousands) fish kill at the mouth of the Mississippi River inLouisiana, 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.[26]
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), thelayers 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 thewater column and lead to fish kill.
Fish kills can also result from 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 thetilapia to bodies of water in Florida. Native to Africa'sNile River, the tilapia stop feeding when water temperatures drop below 60 °F (16 °C) and die when it reaches 45 °F (7 °C). Thus, tilapia that have survived and successfully reproduced in Florida are occasionally killed by a winter cold front.[8]
In January, 2011, a selective fish kill affecting an estimated 2 million juvenilespot fish was attributed to a combination of cold stress and overpopulation after a particularly large spawn.[27]
"Hundreds of thousands" of fish were found dead inTexas in June 2023 due to warming water.[28]
Underwater explosions can lead to fish kill, and fish withswim bladders are more susceptible. Sometimes underwater explosions are used on purpose to induce fish kills, a generally illegal practice known asblast 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.[29]
Droughts and overstocking can also result in inland fish kills.
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 oxygencarrying capacity of the water may also be reduced. Low river flows also reduce the available dilution for permitted discharges of treated sewage orindustrial waste. The reduced dilution increases the organic demand for oxygen further reducing the oxygen concentration available to fish
Overstocking of fish (or an unusually largespawn) can also result in inland fish kills. Fish kill due to insufficient oxygen is really a matter of too much demand and too little supply for whatever reason(s). Recommended stocking densities are available from many sources for bodies of water ranging from a homeaquarium or backyard pond to commercialaquaculture facilities.
Estimating the magnitude of a kill presents a number of problems.[30]
Some very large fish kills may never be estimated because of these factors. The discharge of red aluminiumsludge from a reservoir inHungary into the Marcai River is acknowledged as causing environmental devastation,[31] The loss of adult fish also can have long-term impacts on the success of the fishery as the following year'sspawning stock may have been lost and recovery of the pre-kill population may take years. The loss of food supplies or recreational income may be very significant to the local economy.[32]
Fish kills are difficult to predict. 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, mechanicalaeration and/or removal of decaying matter (such as fallen leaves or dead algae) may be reasonable and effective preventive measures.
Many countries in the developed world have specific provisions in place to encourage the public to report fish kills[33] so that a proper investigation can take place.[34] Investigation of the cause of a kill requires a multi-disciplinary approach including on-siteenvironmental measurements, investigation of inputs, review of meteorology and past history, toxicology, fish autopsy, invertebrate analysis and a robust knowledge of the area and its problems.[35]
The counts given below are all estimates. They tend to be underestimates, and may omit, for example, small fish, those removed by scavengers and those that settle to the bottom.[30]
| Event/Location | Date | Count | Species | Remarks |
|---|---|---|---|---|
| Gulf of Mexico (Corpus Christi) | 1935 | 22,000,000 | Caused byred tide. This event caused coughing, sneezing and watery red eyes in humans.[36] | |
| River Aeron | 1974 | 10,000 | salmon,trout | Discharge ofcreamery waste through poorly maintained sewer. Successful prosecution followed. |
| River Neath | 1976 | 50,000 | salmon,trout | Extreme drought left fish stranded in stagnant pools into which sewers drained. |
| River Ogmore | 1979 | 50,000 | salmon,trout | Spillage ofKymene from apaper mill on theRiver Llynfi a tributary of the Ogmore. Successful prosecution followed and substantial compensation. |
| Gulf of Mexico | 1986 | 22,000,000 | Gulf menhaden,striped mullet, various other species[37] | Caused byred tide.[36] |
| Rhine River | 1986 01 | 500,000 | Caused by spill from Swiss chemical warehouse[38] | |
| Texas coast | 1997–1998 | 21,000,000 | Caused by a bloom ofKarenia brevis[39][40] | |
| White River;West Fork, Indiana | 1999 | 4,800,000 | Caused by an automotive parts maker inAnderson, Indiana, which had discharged 10,000 gallons of the chemical HMP 2000 into the river. | |
| River Dee (United Kingdom) | 2000 07 | 100,000[41] | salmon,trout,perch | Unconfirmed link to release ofwhey into river |
| Klamath River | 2002 09 | 70,000[42] | salmon | Low flow of water due to drought and water diversions for agriculture led to heated and shallow water, increasing vulnerability to a gill disease. |
| Neuse River,North Carolina | 2004 09 | 1,900,000 | menhaden | "Natural upwelling" of an acknowledged polluted river.Hydrogen sulfide smell reported[43] |
| Taal Lake,Luzon, Philippines | 2008 01 05 | 50 metric tons | tilapia | May be linked tovolcanic activity and largefish farms |
| Liuxihe RiverGuangzhou People's Republic of China | 2008 09 09 | 10,000 | carp | Unknown[44][45] |
| Beaches atThanet, Kent, England | 2010 01 | 20,000 | velvet crab | 20000 + dead crabs – along with dead starfish, lobsters, sponges and anemones. Probably killed byhypothermia.[46][47] |
| Ting RiverFujian People's Republic of China | 2010 07 | >1,000,000 Enough to feed 70,000 people a year[48] | Part of theZijin mining disaster[49] | |
| Mississippi River;Plaquemines Parish, Louisiana | 2010 09 | 100,000[50] | redfish,trout,flounder | |
| Arkansas River;Ozark, Arkansas | 2010 12 | 100,000[51] | freshwater drum | Coincided with death of 5,000red-winged blackbirds that fell from the sky. |
| Chesapeake Bay | 2011 01 | 2,000,000 | spot croakers | Included some juvenilecroakers. Cold water stress was believed to be the cause.[52] |
| JiaxingXiuzhou District People's Republic of China | 2011 01 06 | 250,000 | bream,carp,murrel,silver carp,grass carp | Fish caught and transported to market held in large fish tanks fed with river water. Very rapid die-off and loss exceeded 100 tonnes. Only fish caught from a river underChina National Highway 320 east died.[53][54] |
| Redondo Beach, California | 2011 03 | millions | anchovies,mackerel,sardines and other small fish | Caused by oxygen deprivation[55] |
| Taal Lake, Batangas, Philippines | 2011 05 29 | 750 metric tons | Tilapia,milkfish | Caused by oxygen deprivation and largefish farms |
| Lingayen Gulf, Anda, Pangasinan, Philippines | 2011 05 30 | 500 metric tons | Milkfish | Oxygen depletion and change of water climate |
| Nordreisa,Troms, Norway | 2011 12 31 | several tons | herring | [56][57] |
| Guangxi, People's Republic of China | 2012 01 15 | 40 metric tons | Various | Caused by2012 Guangxi cadmium spill[58] |
| Menindee Lakes, NSW, Australia | 2018 12 | 1,000,000 | Murray cod, silver perch, golden perch, bony herring, carp | An independent assessment found that three significant fish death events were primarily caused by local hydrological and climatic conditions, shaped by a broader climatic, hydrologic and basin management context that placed the lower Darling River at risk of such fish deaths. The immediate cause was oxygen depletion when the weir pool destratified.[59] |
| Oder river,Poland | 2022 | 10+ metric tons | various:zander,catfish,gudgeon,loach,chub, etc. | 2022 Oder environmental disaster |
| Menindee, NSW, Australia | 2023 03 17 | Millions | Mostly bony herring | Oxygen depletion[60] |
| Brazoria County, Texas, USA | 2023 06 13 | Tens of thousands | Oxygen depletion[61] |
Hundreds of thousands of dead fish were floating west of the Mississippi River, in Bayou Chaland.
Dead drum fish floated in the water and lined the banks of a 20-mile stretch of the Arkansas River near Ozark, about 125 miles northwest of Little Rock, said Keith Stephens of the Arkansas Game and Fish Commission. A tugboat operator discovered the fish kill Thursday night, and fisheries officials collected some of the dying animals to conduct tests. ...
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