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FLORIDA LAKE MANAGEMENT SOCIETY
by Justin Strynchuk LOW DISSOLVED OXYGEN FISH KILLS What Causes a Fish Kill?
There are many factors that can account for a sudden fish die off in a water body. Temperature, salinity, pH, ammonia, nitrate, carbon dioxide, turbidity, dissolved oxygen, pesticides, toxins, and diseases all must be considered when determining the cause of fish mortalities. It is important to determine the causes of a fish kill to ascertain if future kills are preventable and to suggest the best protocol for prevention. Even a relatively minor change in one environmental variable can lead to increased stress on aquatic organisms. What is the Most Common Reason for a Mass Die-Off of Fish? Low dissolved oxygen (DO) levels are the most common cause of fish kills. There are many factors that reduce water's ability to hold oxygen in a dissolved state, and the ability of a fish to use it. Temperature is an important factor. The amount of oxygen dissolved in water is inversely proportional to water temperature. As water temperature rises, the amount of dissolved oxygen decreases. Also important is the amount of other gases, solids, and chemicals already dissolved in the water. The more standard water is with other substances, the less "room" there is to hold dissolved oxygen. The DO in most open water bodies ranges from 0 to 13 milligrams per liter (mg/l), and varies throughout the day. Most fish do well in water which contains at least 5 mg/l of oxygen. Fish have specific oxygen level requirements, below which they will not reproduce, feed or survive. At oxygen levels below 3 mg/l most fish will show signs of distress. Some fish have adapted to low oxygen conditions, but even they can not survive long when the DO level drops below 1 mg/l. How Does Oxygen Get Into The Water? Most oxygen gets into the water from either direct mixing at the surface, or as a by-product of photosynthesis from aquatic plants. Aquatic plants both produce and use oxygen. On cloudy days plants produce less oxygen than on sunny days, and they tend to use oxygen at night. On windy days, oxygen is forced into the water due to mixing occurring in surface waters. As the water is mixed and circulated not only is oxygen introduced in the system, but waste gasses, such as nitrogen, carbon dioxide, and excess oxygen are released into he atmosphere. Where Does the Oxygen Go? Oxygen is used by vegetation, fish and other organisms. Additionally, many chemical reactions use oxygen, including those involved in the process of decomposition. Because many chemical reactions take place in water, there can be a substantial chemical oxygen demand (COD). Rainwater run-off often carries with it large amounts of sediments, organic debris, nutrients, and other chemicals all of which can increase the COD of the system. As the COD of a water body increases, the potential for a fish kill also increases, because less oxygen may be available for the fish to use. When is a Fish Kill Most Like to Occur? The period just before dawn is most often when there is an oxygen demand greater than the available supply. If the water body has a high COD, and if the day (or consecutive days) is not sunny or windy enough, a low dissolved oxygen "crisis" may occur. Fish may then become stressed and die. Why Do So Many Fish Die? The Organisms that require the highest DO levels begin to die first. After death, these organisms begin to decompose increasing the COD. As DO levels drop further, more organisms are stressed and die, and the cycle spirals downward. The process can be reversed at any time. Many times only certain numbers, sizes, or types of fish may die. What Can Be Done to Prevent Low DO Fish Kills? There are several things that can be done to prevent or reduce the effects of a dissolved oxygen fish kill. If low DO levels occur in a small lake, pond, or cove area, immediate first aid is usually the addition of emergency oxygen. Methods used most often include air and water pumps, or mechanical water agitation. Air pumps inject pressurized air taken from the atmosphere through the pump, into a diffuser submerged in the water body. This type of aeration is the most inexpensive of the commonly used methods. The drawback with this method is the relatively limited area of influence and the slow introduction of oxygen. Quite often, this type of system is used in fish farms, stormwater retention ponds, and small lakes. Water pumps shoot water into the air, where it releases gasses and takes on new oxygen, mostly through diffusion. This method costs more than air pumps, and may alter the water temperature, but can exchange an enormous amount of oxygen and other gasses. Mechanical agitation of water is usually initiated by a motor moving some type of blade, which then moves the water. Paddlewheels and outboard motors are good examples of these systems. These mechanisms add air from the atmosphere at the point of mechanical contact, and push a layer of aerated water across the surface. This system can quickly add oxygen over a large surface area, but at high operating costs. Summary The cycling of oxygen levels in aquatic ecosystems is a natural process, and usually follows some seasonal pattern. Fish kills have been recorded in Florida since the first fishermen, hunters, and pioneers settled the state. Disturbingly, the influx of nutrient and sediment laden stormwater from watershed development has contributed to a greater potential for fish kills. The most effective means of reducing fish kills is to educate the public. This can go a long way to minimize the nutrients and other chemical constituents of stormwater that contribute to low dissolved oxygen. For more information about what can be done, contact your local county extension office, the State of Florida Department of Environmental Protection, or your Water Management District Office. |
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 Coastal Resources |
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Lakes,
Estuaries and Lagoons |
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Mosquito Control |
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Natural Areas |
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Permitting and Regulation |
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