Winter and Summer Fish Kills in Ponds, A-8-01 Ohio State University Extension Fact SheetSchool of Natural Resources2021 Coffey Road, Columbus, OH 43210Winter and Summer Fish Kills in PondsA-8-01William E. Lynch Jr.Extension Associate, Aquatic Ecosystem Management Eric R. NorlandAssistant Professor, Natural ResourcesExtension Specialist, Natural ResourcesFish kills in Ohio ponds are rare events considering the largenumbers of ponds and small lakes located in the state. However, they dooccur every year, and some years are worse than others. Visiting yourpond and seeing many dead fish floating around would no doubt cause youconcern, particularly if the fish population is to your satisfaction.Fish kills can result from a variety of causes but the most common areweather, pond vegetation, and the interaction of the two. The criticalparameter of these scenarios is oxygen level. Oxygen is essential toaquatic life and should be maintained at levels of 4 ppm (parts ofoxygen per million parts of water = milligrams per liter of water) orhigher. This fact sheet will focus on these three common causes and whatyou can do to minimize the possibility of a winter or summer fish killin your pond. If you experience a fish kill and you suspect it is theresult of disease or chemical spill, contact your county Extension Agentfor assistance.WinterkillThe Winter Oxygen Cycle During winter, contribution of oxygen from photosynthesis by greenplants and algae is greatly reduced. This is caused by the normal falldie-off of plants and algae due to cold water. Additionally, those greenplants that remain produce less oxygen because their metabolism slows incold water. This does not mean that oxygen levels are much lower inwinter. In fact, the reverse is true. In unfrozen ponds, high oxygenlevels will occur during winter because the oxygen needs are less incold water. Aquatic animal (primarily fish) metabolism andoxygen-consuming decomposition processes are greatly reduced in coldwater. Also, cold water contains more oxygen than does the warm water ofsummer. The strong winter winds also keep the pond water circulating andcontinually add oxygen during winter. These factors combine to preventwinterkill in unfrozen ponds even though plants are contributing littleoxygen during winter. Problems, if they are to occur, will happen onceice forms on the pond.How Winterkill Occurs Figure 1. Oxygen depletion scenario during winter ice cover. Oxygen levels are expressed as parts per million (ppm) and show generalized levels and trends.    Winterkill is the result of a significant decline in oxygen during along period of ice cover. Figure 1 illustrates the sequence of eventsthat can lead to winterkill. As long as the pond is open or partiallyopen, oxygen levels remain high. This is because diffusion of oxygenfrom the air and wind agitation add more oxygen to the water than isused for animal metabolism and plant decomposition. This is even truefor those very sheltered ponds that receive very little exposure towind.When ice forms, neither wind nor diffusion from the air contributeoxygen to the water. The sole source of oxygen becomes the small amountproduced by the few remaining algae and plants. If the ice remains thinand clear, oxygen production by plants and algae can continue as enoughsunlight can filter through the ice to allow photosynthesis to occur.This production can nearly compensate for that which is used byrespiration and decomposition that is continually ongoing. If the iceremains into mid-winter or thickens slightly, oxygen levels will beginto decline by 1-2 parts per million. This is because each day thepond plants and animals use slightly more oxygen than is produced. Atthis point, however, the pond still has sufficient oxygen.A serious problem develops when ice persists into late winter andthickens considerably or worse yet, is covered by snow. This reduces theamount of sunlight reaching the plants and algae, thereby substantiallyreducing the amount of oxygen produced. Since metabolism anddecomposition continue unabated, oxygen levels begin to drop and canapproach levels that threaten fish survival. In cold water, oxygenlevels less than 2-3 ppm for an extended time will begin to killfish. If levels drop to 1-2 ppm or lower throughout the pond, acomplete fish kill will result.Factors Influencing the Likelihood of a Winterkill Winter Severity—This is by far the most importantfactor in determining the likelihood of a winterkill. A mild wintermeans little or no ice and thus, no winterkill due to low oxygen levels.A harsh winter greatly increases the chances of problems. Ice thicknessand clearness is important. As mentioned previously, thickening ice overtime results in less and less light being available for photosynthesis.Perhaps the worst thing that can occur is for snow to cover the ice.Often, winterkill is avoided as long as there is no snow cover (unlessthe ice is very thick which rarely occurs in Ohio). Four inches of wetsnow on top of the ice nearly eliminates sunlight penetration and oxygenlevels will decline quickly.Pond Volume—The more water volume (i.e., gallons)in a pond, the less likely the pond will experience winterkill. This iswhy fish over-winter better in larger and/or deeper ponds. A one-acrepond that averages 6 feet deep will have more winter oxygen availablethan a one-acre pond that averages only 3 feet in depth. Decomposition—An important factor is the amount ofdecaying organic matter that is present on the bottom. Dead aquaticvegetation and tree leaves account for most of the organic matterundergoing decay during winter. A pond bottom covered with thesematerials is more likely to experience winterkill than a pond lackingsuch materials. This is why ponds having a very dense aquatic plantcommunity in summer are the very ponds most susceptible to winterkillduring harsh winters.Fish Biomass—The amount of fish (numbers and pounds) inthe pond during winter also influences oxygen decline under the ice.Even though fish metabolism has slowed during winter, they still requireoxygen. A pond that contains many pounds of fish will experience afaster decline in oxygen than a pond with fewer pounds of fish. This iswhy fish farmers closely monitor ponds in winter as they are maintainingfish biomass at levels that greatly exceed a normal pond.Worst Case Scenario—Pond owners should be most concernedduring a harsh winter in which ice cover persists, there is considerablesnow cover on the ice, the pond is small and shallow, and the pondcontained a large amount of aquatic plants the previous summer. Less iceand snow, fewer aquatic plants the previous summer, and the deeper thepond, the less likely a winter fish kill will occur.Winterkill Prevention Fortunately, winter fish kills can be prevented in most cases, due inpart to weather conditions. Even in Ohio's harsh winters, a warm spellcan partially thaw a pond for a few days. Oxygen levels quickly reboundwhen a pond becomes ice-free. One timely warm period of 2-3 dayscan greatly reduce the possibility of a fish kill.There are activities the pond owner can do to prevent a winterkill.The most important should occur during pond construction. Ponds shouldhave shoreline slopes of 3:1 to limit the amount of shallow water wheresummer aquatic plant growth occurs. The reduced summer aquaticvegetation means less decomposition and results in higher oxygen levelsunder the ice in winter. Because deeper ponds experience winterkillsless often, it is important to have a maximum depth of 10-12 feetif possible.What can be done for the existing pond? Any strategy that limits theamount of plant decomposition that will occur during winter isimportant. There are a number of methods to eliminate or reduce summeraquatic vegetation that will decrease the amount of oxygen-consumingdecomposition that occurs the following winter. Ohio State UniversityExtension Fact Sheets A-3-98 Controlling Filamentous Algae inPonds and A-4-98 Chemical Control of Aquatic Weeds along withthe Ohio Pond Management bulletin describe a variety ofstrategies for controlling aquatic plants.Another activity to be considered is the installation of an aerationsystem that helps keep a small area of the pond ice-free. Aeration notonly adds oxygen to the water directly via the bubbles and agitation,but the open area allows for considerable diffusion of oxygen into thepond from the air. The aeration system does not need to be runcontinuously all winter. Rather, turn it on when ice is forming on thepond. Leave it off when the pond is ice-free. Aeration can be usedsparingly in winter to minimize ice cover. Safety note: Aeration during winter generally prevents safe ice from developing anywhere on the pond. Finally, if your pond is ice-covered and aeration is not an option, fish winterkills can often be prevented by simply removing some of the snow off the ice. About 25-50% of the pond surface needs to be kept free of snow to maintain enough light to allow sufficient photosynthesis to occur. This is particularly effective in small, shallow ponds where the volume of water may not be sufficient to allow oxygen to last the winter. Safety note: Only remove snow when the ice is safe. Four inches of ice is a good rule to follow before stepping onto the ice to remove snow. Summerkill The Summer Oxygen CycleOxygen production resulting from photosynthesis is highest duringsummer. This is because warm water and long hours of daylight maximizethe amount of algae and aquatic plants in ponds. However, oxygen-usingprocesses, including animal and plant respiration and organic matterdecomposition are also at their highest levels in summer. In most ponds,the oxygen produced during daylight far exceeds the amount used bydecomposition and respiration, especially at night. As long as daytimeoxygen production exceeds nighttime oxygen use, summer fish kills due toinsufficient oxygen will not occur. However, there are summer situationsthat can occur that can lead to fish kills.How Summerkill Occurs Excessive Vegetation Scenario—Many Ohio ponds become"choked" with vegetation during July and August. Fortunately,excessive vegetation by itself will not cause a fish kill as the highamount of oxygen produced during daylight easily compensates forrespiration and decomposition. Problems can arise during a hot, calm,and cloudy weather pattern. A substantial reduction in sunlight causesthe amount of oxygen produced to decline while oxygen consumptionremains unchanged. Thus, oxygen levels slowly decline. Ponds can usuallywithstand several consecutive cloudy days but if clouds persist forlonger periods, oxygen levels may decline to levels harmful to fishlife, about 3-4 ppm for largemouth bass and bluegills. A fish killdue to insufficient oxygen may begin to occur, particularly just beforesunrise when daily oxygen levels are at their lowest. The first fish todie are the larger individuals as their oxygen needs are greater.Fortunately, a sunny day will quickly raise oxygen levels and end thefish kill. A common result of this scenario is a partial fish kill whichleaves the pond's fish community dominated by small fish.Late-Summer Herbicide Treatment—Many owners prefer tohave minimal vegetation but wait until the pond has become choked withplants before initiating any control measures. This can lead to aserious oxygen problem if all the vegetation is treated simultaneouslywith an herbicide/algaecide. Soon after treatment, a massive die-off ofthe treated vegetation will occur. The actual timing of the die-offdepends on the herbicide used and other conditions. But once die-offbegins, it proceeds rapidly. This causes a substantial build-up oforganic material that begins to decay in the warm water, and whichrequires large amounts of oxygen to complete. Coinciding with thedie-off is a decline in oxygen production as the plants responsible forthat production are now lying dead on the pond bottom. Oxygenconsumption far outpaces oxygen production and levels begin to declinesignificantly. Couple this situation with a calm, cloudy day or two, anda fish kill can occur.Premature Fall Turnover—Any pond that stratifies duringsummer will undergo a normal fall turnover as water temperaturesdecline. This process is described in Ohio State University ExtensionFact Sheet A-7-01, Understanding Pond Stratification. A fish killcan result in some ponds that experience a premature turnover in latesummer.Many ponds are stratified in summer, with a warm, upper layer ofwater on top of a colder, bottom layer. These layers do not usually mixuntil fall. In Ohio's productive ponds, a substantial amount of organicmatter accumulates in the deeper areas. The decomposition of this matterresults in the colder, bottom layer eventually losing most, if not all,of its oxygen by August. This is not a problem unless the pond turnsover in August or September. Figure 2. Oxygen depletion scenario during premature turnover during summer. Oxygen levels expressed as parts per million (ppm) and show generalized levels and trends.   This premature mixing can be brought on by a heavy, cold rain. Figure2 illustrates how premature turnover in summer results in a fish kill.In many ponds, the volume of the colder, oxygen-deficient bottom layerof water exceeds the volume of the oxygenated, upper layer. In a heavyrain event (several inches in just a few hours), the cold rain waterplunges through the warm upper layer towards the bottom. This is becausecold water is denser than warm water and therefore sinks. This rainevent can cause the entire pond to mix. The mixing of oxygen-deficientwater with the oxygenated water can occasionally cause oxygen to fall tolevels lethal to fish.A heavy rain event can also cause a stratified pond to mix in anothermanner. A large inflow of cold rain water from watershed runoff can alsocause a sudden mixing of oxygen-deficient water with oxygenated water.Depending on the volume of the two layers, a fish kill may result. Pondswith a very large watershed are more prone to this scenario.Premature turnovers are not a problem in June or after September. InJune, stratification has just occurred and the colder, bottom layerstill contains oxygen. While that oxygen level may be slowly declining,a sudden mixing does not result in low oxygen levels throughout thepond. Premature turnovers generally do not cause a problem untillate July. After mid-September, pond temperatures begin cooling. Coldersurface water holds substantial oxygen which allows the pond to moreeasily withstand turnover effects. Additionally, as temperaturesdecrease, the very distinct stratification diminishes which slowlyallows oxygen to be re-introduced into the deeper waters.Worst Case Scenario—The likelihood and severity of asummer fish kill is increased if the scenarios described above occur inconjunction with each other. For example, if a pond treated with anaquatic herbicide in late-July or August receives a heavy inflow of coldrain water, a fish kill is likely. This is because a very large quantityof dead plant material is decaying. A sudden turnover results in theupper layer oxygen being mixed throughout the pond where it is quicklyconsumed by the decomposing plants. The situation is further worsenedbecause the pond has fewer oxygen-producing plants.Factors Influencing the Likelihood of a SummerkillVegetation Biomass—The quantity of vegetation in Augustand September plays an important role in the likelihood of a fish kill.Vegetation-choked ponds are much more likely to experience problems thana pond with little or no vegetation.Weather—As described previously, weather plays a criticalrole in whether a summer fish kill will occur. A cool summer marked bybreezy conditions generally reduces the chances of a summer fish kill.Conversely, a calm, hot summer or a cloudy summer increases thelikelihood.Pond Size and Depth—These factors influence the likelihood and severity of a premature turnover causing a fish kill. Small, very deep ponds (more than 12 feet) are more susceptible to the effects of a premature turnover than are large, shallow ponds. This is because a small, deep pond has a higher proportion of the water that lacks oxygen as compared to a larger, shallower pond. If the ponds turn over, oxygen may decline to near zero in the deep pond but often remain high enough in the larger, shallow pond to keep fish alive. A large, windswept pond in an open area rarely stratifies because of the continual mixing and usually does not experience problems associated with a premature turnover. Summerkill Prevention Just as for winter fish kills, there are activities the pond ownercan undertake to prevent a summer fish kill. As noted previously, pondsshould have shoreline slopes of 3:1 to limit summer aquatic plant growthto the areas near shore. The reduced amount of summer aquatic vegetationmeans more balanced levels of oxygen between day and night. While oxygenproduction during the day may not be as great, plant respiration (oxygenconsuming) at night will be greatly reduced. This prevents pre-dawnoxygen levels from plunging to lethal levels for fish.In existing ponds, any early summer strategy that reduces the amountof plant growth is beneficial. This is particularly true for those pondshaving a history of dense aquatic vegetation and/or may have had summerfish kills in previous years. If an herbicide/algaecide is used toeliminate or reduce summer aquatic vegetation, applications should becompleted before July 1. Later applications run the risk of indirectlycausing a fish kill as described earlier.In situations where the pond owner decides late-season herbicidecontrol is needed, the pond should be treated in sections. Granularformulations of herbicides allow for control in specified areas. Treatabout 1/4 of the pond every 2-3 weeks, starting with the area wherecontrol is most important (e.g., swimming area). This spreadsdecomposition over a longer period of time as compared to total pondvegetation control with a liquid herbicide.Perhaps the most effective way to prevent summer fish kills is toinstall an aeration system. Aeration serves to minimize the risk of afish kill in several ways. First, aeration continuously adds oxygen tothe water, which is important if oxygen begins to approach criticallylow levels. Fish will find the oxygenated water around the aerator.Secondly, aeration often prevents summer stratification from occurring.If the pond is not stratified, premature turnover will not occur.Finally, aeration keeps nutrients suspended which promotes single-cellalgae growth. This reduces aquatic vegetation density due to the shadingeffect of green pond water. Most Ohio ponds do not require aerationduring normal summers, but more pond owners are installing them as apreventative measure. Aeration does not need to be a 365 days, 24 hoursa day activity. Using aeration at night during the May to Septemberperiod will prevent summer fish kills.A common type of aeration that many pond owners are installing is a windmill system in which wind power generates the aeration in the pond. While these systems can reduce the chances of a fish kill as long as a breeze is blowing, they are useless in those calm, cloudy, late-summer weather patterns that can lead to a summerkill. If these systems are installed, a pond owner would be well-advised to consider having the system equipped with the capability of using electricity to generate aeration as a back-up. Summary Winter and summer fish kills are uncommon, but they do occur on anannual basis. Fortunately, they are preventable with wise management. Acommon factor in fish kills is the presence of very dense aquaticvegetation. Any strategy that limits late-summer vegetation growth toless than 15-20% of the pond will greatly reduce the chances of awinter or summer fish kill. Weather is important in causing fish killsin both seasons. Very cold, calm winters lead to heavy ice cover whichcan, depending on other factors, lead to a fish kill. Similarly, calmsummers can increase the chances of a summer fish kill. Perhaps the bestmethod to prevent fish kills is to install an aeration system. Inwinter, they can limit ice cover and add oxygen. In summer, they preventstratification and maintain, if not increase, oxygen levels duringweather conditions that can lower oxygen to dangerous levels.If an aeration system is not installed and lethargic fish near thesurface are observed gulping for air, a serious situation exists andrequires immediate attention. Any method that can significantly aeratethe water may alleviate the crisis and prevent a fish kill. It may be assimple as using a 2- or 3-inch pump and spraying water up in the airwhich incorporates oxygen when it falls back into the pond. Whatever isused, aeration should continue (particularly at night) until conditionsreturn to normal. Keep in mind that if a pond exhibits fish in distress,it is likely to occur again that summer if similar weather conditionsoccur again. Unfortunately, once a fish kill is actually underway, it isusually too late to stop it. The key is prevention.Additional Pond Management Information Placing Artificial Fish Attractors in Ponds and Reservoirs;Ohio State University Extension Fact Sheet A-1.Pond Measurements; Ohio State University Extension Fact SheetA-2.Controlling Filamentous Algae in Ponds; Ohio State UniversityExtension Fact Sheet A-3.Chemical Control of Aquatic Weeds; Ohio State UniversityExtension Fact Sheet A-4.Muddy Water in Ponds: Causes, Prevention, and Remedies; OhioState University Extension Fact Sheet A-6.Understanding Pond Stratification; Ohio State UniversityExtension Fact Sheet A-7.Ohio Pond Management; Ohio State University Extension Bulletin374Controlling Weeds in Ohio Ponds; 41 minute videotape. VT50.Visit your county office of Ohio State University Extension forcopies of these resources. Disclaimer This publication contains recommendations that are subject to change at any time. These recommendations are provided only as a guide. The authors and Ohio State University Extension assume no liability resulting from the use of these recommendations. Click here to view the PDF of this Fact Sheet. All educational programs conducted by Ohio State University Extensionare available to clientele on a nondiscriminatory basis without regardto race, color, creed, religion, sexual orientation, national origin,gender, age, disability or Vietnam-era veteran status.Keith L. Smith, Associate Vice President for Ag. Adm. andDirector, OSU Extension.TDD No. 800-589-8292 (Ohio only) or 614-292-1868| Ohioline| Search| Fact Sheets| Bulletins| |
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