Author: Brad Vollmar of Vollmar Pond and Lake Management
What Killed My Fish?
The first question everyone has when they notice fish floating in open water or washed up along the bank is: what killed my fish?
The most common causes of fish kills are oxygen depletion, algal blooms (could deplete oxygen or be toxic), pesticide toxicity and disease. By far, the most common cause of fish kills is oxygen depletion in lakes or ponds.
There are two general types of fish kills due to lack of oxygen in your lake or pond: summer fish kills and winter fish kills. Sorry for the people up north, but we are not going to dig into winter fish kill. For our clients in Texas summer fish kill is worth understanding.
There are two types of summer fish kill related to lack of oxygen in your lake or pond. The first we will discuss is called pond or lake turnover. These pond or lake turnover summer kills are most common in early and late summer.
Summer Fish Kills
During the summer, your pond or lake tends to stratify by temperature driven density differences with warm water on top and cool water at the bottom of your lake. Oxygen stratification is also very evident in the summer months. High oxygen concentrated water occupies the upper parts of a lake due to greater wave action and biological activity in this area. At lower depths, wave action has little affect, and biological activity is greatly reduced since the sun does not reach these depths.
So we have warm oxygen rich water on top and cool anoxic water on the bottom of your lake. Then a large rain event occurs and dumps a lot of cool rain into your lake. This cool rainwater is denser than the surface water, so it sinks to the bottom of the lake (remember the bottom of the lake is nearly void of oxygen). The new, cool rainwater displaces the anoxic water at the bottom of your lake. The displacement of anoxic water from the bottom of the lake essentially mixes throughout the entire lake, which lowers the oxygen levels of the entire lake. The sudden mixing of anoxic water is what kills your fish in a summer lake turnover. This sudden mixing can also be caused by strong persistent winds, which essentially breaks the natural thermal stratification and allows the anoxic water to be mixed throughout.
Large Oxygen Swing Fish Kills
The second summer fish kill type is called the large oxygen swing fish kill. This type of summer fish kill has little to do with forced mixing of the oxygenated water with anoxic water.
The large oxygen swing fish kill always occurs in lakes with large amounts of submerged aquatic vegetation, floating algae or planktonic algae (pea soup water).
With large amounts of aquatic weeds and algae, a lot of CO2 is consumed during the day by photosynthesis. In turn, photosynthesis produces large amounts of oxygen, so much that the lake oxygen concentration becomes supersaturated with oxygen. Supersaturated oxygen concentrations means the water is holding more oxygen than it should at a given temperature. In general, the more supersaturated the oxygen concentration the more plant material present. Your fish don’t usually die from oxygen supersaturation, although they can. This condition is called gas bubble disease.
Where your fish begin to die in the large oxygen swing fish kill is when these aquatic weeds and algae switch from photosynthesis during the day to respiration at night. What happens is the plants that supersaturated the water with oxygen begin to consume oxygen and produce CO2, essentially supersaturating the water with CO2. Fish are not like plants; they cannot switch from breathing oxygen to breathing CO2. You usually find your dead fish in morning; with large oxygen swing fish kill. Both types of summer fish kills can largely be prevented by proper supplemental aeration.
Fish Kills Chart
The table below will help generally determine which type of fish kill you experienced.
| Benchmark | Oxygen Deficiency | Algae Bloom | Disease | Pesticide Toxicity |
|---|---|---|---|---|
| Fish Behavior | Swimming Near Surface gasping for air | Abnormal Swimming | Abnormal Swimming | Abnormal Swimming |
| Time of Fish Kill | Nighttime into morning | Brightest part of day | Anytime | During period of additional stress besides disease |
| Size of Dead Fish | Large fish die first | Small fish die first | Small fish die first | Any size |
| Microscopic Algae Abundance | Algae dying | One dominant algal species | No effect | Pesticides may kill algae which may make fish kill look like oxygen deficiency type |
| Dissolved Oxygen Concentration | Less than 3 ppm oxygen | Supersaturated oxygen concentrations | No effect | Little effect |
| Fish Species Selectivity | Desirable game fish first rough undesirable fish are usually more resilient to low oxygen levels | All fish species affected | Usually one fish species | One fish species dies before others |
| Water Color | Brown or gray or black | Dark green or brown or golden | No effect | normal |
A few scientific principles will allow us to better understand the functions behind lake turnover fish kills. 1) Cool water holds more oxygen than warm water, and 2) cool water is denser than warm water.
Fish Kill Aeration Solutions
Contact Keeton Industries for aeration solutions to fish kills specific to your pond by clicking here.
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Keeton Industries would like to thank Brad Vollmar for sharing his research on our website.
If you are in the Fredericksburg, Texas area, contact Brad to manage your pond or lake.
Phone: 830.992.0928
Fax: 888.232.3614
Email: brad@texaspondmanagement.com
Address: 5265 White Oak Rd. Fredericksburg, Texas 78624-7196
Website
We have had a smaller backyard pond in our family for about 50 years full of bluegill and bass and other pond life. This summer has been the first in all these years that we have what looks like the”pea soup” algae that has not dissipated. To my great sadness this morning I awoke to hundreds of dead fish in the pond. This is truly devastating and do not have any idea of a solution. Please any advice would be greatly appreciated.
HI Eryn,
So sorry to hear about your pond problems. What you need to do is install an aeration system. If you were to add dye to the pond, you would most likely make it worse at this late stage of the summer. This is because the algae biomass is already present in the pond, so adding dye will just reduce the insolation (sun exposure) to the algae. It will then die, and quit producing oxygen and also will start absorbing massive amounts of oxygen as it rapidly decomposes. You really need aeration to get out of this problem, or just wait and let nature take its course. As a pond matures (becomes more eutrophic and productive) this problem will continue and likely get worse from year to year. The highly productive ponds are the most prone to this problem, because they have so much biomass (plants and animals) in them. If you have more questions or need help selecting an aeration system, please contact Kurt at 970-449-4653.
Hope this helps, and best of luck.
I had a large fish kill that I found on Monday when l went out to my farm to feed the Chanel Cat fish. I lost about 100 Cats, 50 Bass,and about 100 Perch as well as 9 Grass Carp that were about 4 ft. Long. The water had turned about a month ago it was pea green. The pond was down about 2 ft. Over the weekend we had 4.3 in.rain when I went out on Sunday water was clear. My question is do I need an araitor. In years past when pond drops I run my wellin it ,a1in stream 24/7 for about a month doing that I never had a problem.
Robert,
Sorry to hear about your pond issues. Unfortunately this is a call or email we receive all too frequently in the heat of summer.
The short answer is, yes, you’ll need to aerate your pond to avoid this situation again. Now for the long answer.
Fish kills are typically related to a pollution event and/or low oxygen levels in the pond. Low oxygen levels are by far the most common cause of mass fish die-offs in ponds. What changed suddenly in the pond to cause a fish kill? Let’s start with a brief definition of the layers of water in the pond.
Epilimnion: Top water layer of the pond. Typically has adequate oxygen for fish under normal conditions; absorbs oxygen from the atmosphere at the pond surface-especially on windy days. Also receives oxygen from photosynthesis of the green material in the photic (sun receiving) layer of the pond.
Thermocline: middle layer of water. This is a transition zone from the epilimnion to the oxygen poor hypolimnion. The thermocline in most ponds is very stable, and is an invisible divider between the habitable and uninhabitable water for fish in most ponds over 6-8’ in depth.
Hypolimnion: lower reaches of the water column. In many ponds this bottom water layer is extremely low or almost devoid of oxygen. The thermocline is almost impervious to vertical mixing, so the oxygen stays in the epilimnion- and the hypolimnion remains at a low oxygen level. Without the ability to absorb oxygen from the atmosphere at the surface, the decaying organic matter absorbs most of the available oxygen, resulting in an uninhabitable layer for fish. Extensive studies have been conducted regarding the oxygen levels in ponds. It is not unusual to see the hypolimnion with an oxygen level of under 1 ppm; where 4ppm or more is required by most fish species. Low oxygen levels in the hypolimnion also encourage the rapid accumulation of organic muck, sludge, or “goo” at the bottom of the pond. These low oxygen levels result in partial or incomplete digestion of organic waste products by the natural bacteria in the pond.
Here is a breakdown of the likely scenario that led to your fish kill:
1) In your case, you had a lot of planktonic algae (pea soup appearance) growth going in the pond. Algae produces oxygen in the daytime, which often results in adequate oxygen in the upper water layer (the epilimnion). Although algae actually consumes oxygen at night, it produced enough oxygen through the day in your pond that it carried the fish though the night. When it rained over 4”, the surface temperature of the pond dropped rapidly, causing the pond water layers to mix suddenly (commonly referred to as a “turn” or “flip”). The now-cool surface water was denser than the slightly warmer water below; so the surface water dropped to the bottom and mixed the pond. There are two major problems with that situation, firstly, your pond was thermally stratified- so the water at the bottom of the pond was already lacking in oxygen. The water that was at the top of the pond mixed with the low oxygen water below- as a result none of the water in the pond had enough oxygen to sustain your fish. This was compounded by the algae drop-out; when the algae dropped out of the photosynthetic zone it no longer received adequate sunlight, so most of it died. The dead algae was no longer producing oxygen, it started consuming oxygen rapidly in the decomposition process. What is the long-term solution to prevent this from happening again? A correctly sized and installed pond aeration system. It will gradually and gently mix and aerate the pond every day, and prevent the re-development of a low- oxygen water layer in the lower reaches of the pond.
2) What can you do right now? Unfortunately, the majority of the damage is already done. You now have to start the slow process of rebuilding your fishery. We recommend that you consider installing an aeration system at this time, so that this exact same scenario will not occur again in the future.
In conclusion, there are two or three dangerous times for oxygen related fish kills, depending on the climate in your area.
a) The situation you encountered, a highly fertile pond in the heat of summer, torrential rain caused a pond turn or flip
b) Fall -when the first strong cold front moves in, again causing the top of the pond to cool rapidly and plunge to the bottom, turning the pond over.
c) Winter in cold weather areas. When ice forms on the pond, it eliminates the exchange of gases at the surface. This causes a buildup of carbon dioxide, methane, and nitrogen as the oxygen is used up under the ice. Without the wind and wave action at the surface, the oxygen levels can go down pretty quickly. Combine that with low water temperatures and less sunlight, which will lead to the cessation of photosynthesis in the pond. The plant material will not produce oxygen like it does in the summer, however the oxygen demand and consumption will continue all winter.
All three of these situations can be avoided with the installation of a correctly sized pond aeration system.
Thanks,
Kurt
KeetonAquatics.com
Kurt Hansen
Sales Executive, Pond and Lake Products
i have small pond full of fish. water level was very low until we received 5 + in of rain. today many of my big catfish are dead or dying. they are going to edge like they are trying to get air. im trying to decide if this was because of rain or, i had treated area by pond with herbicide for poision ivy approx 6wks ago. water got in the treated area from high water level. too late for fish, but are my animals safe to drink from pond ?? and swim in it? dogs ,ducks,guineas?
Hello Glenna,
If you received 5 inches of rain quickly, I would suspect a pond “turn” or “flip”. The rain is cold, it hits the top of the pond then wants to go to the bottom because the colder rainwater is heavier than the warm surface water. That then mixes the anoxic (oxygen free) water in the bottom of the pond with the oxygen rich surface water. Unfortunately, the surface water cannot accommodate the oxygen requirement from the lower reaches of the pond. The fish then die from a lack of oxygen. Cure? Put in pond aeration so it doesn’t happen again next year.
Kurt
Hi, We had currently stock Tilapia in the middle of our shrimps ponds confined in a 0.5cm mesh net at 400sq m area with an average population of 1500 pcs. The size of the Tilapia is now aroung 150g to 250g in which some of them already layed eggs and hatched outside the pen. What happened is that, 1 day after period of heavy rain we began to observed dead Tilapia (Big & Small) floating inside the net pen and surprisingly there were also dead tilapia oustide the pen. To start the pre-diagnosis of the mortality caused, we monitor the water if it is has high ammonia or nitrite level but test kits reveal it has zero ammonia and nitrite. What possibly caused the mortality of the Tilapia? is the bloom of the plankton? Baterial Infection? I hope you can help us figure out to solve this problem. By the way we are using SHRIMP SHIELD for our shrimp pond. Indeed it is very good product.
Thanks.
Hi Wilson,
The first thing I suspect when a sudden fish kill comes after a heavy rain, is low oxygen levels. Have oxygen levels been checked, and do you have aeration in the pond? When heavy rains come, it can cause the pond to mix because the surface water is now colder than the water at the bottom. If the water at the bottom has a low oxygen level, it can reduce the overall oxygen level so the fish can’t survive.
Thanks for the question, Wilson. Hope this helps. Please contact us if we can further assist you.
Kurt
I live in Michigan and the cold weather, well the really cold weather is gone. We have had over 50 small bluegill die in the last week or so. Our pond is a good size pond with two flowing wells. We are surrounded by farm fields and for quite a while the liquid manure they put on the fields was quite an amount and was the worse smell. We noticed this spring that the run off water was brown coming out of our pipe for drainage around our house. It has never been this color. We have never had a problem in the 15 years we have owned this property. The bluegill are small but when I feed the fish I see the white gunk on the perch and bigger blue gill. This gunk was on 98% of the dead fish. Never seen anything like this. We also have something that lays eggs in the pond. They look like how frogs lay eggs but they are not any black dots in it, they are white. But they have been laid every year. So we are horribly confused and upset that we don’t have a clue what to do to save these fish. Some of them are over 10 years old and come right up to us to get fed..like a pet. Sorry I am a weeper. Thank you ahead of time if you have any idea we would appreciate it.
Hi Anita,
The weather there has been very wet lately, so the manure they are putting on the fields is leaching or being washed into the pond. This is causing a quick rise in ammonia and nitrates in the pond, which is stressing the fish. This lowers their immune response, and allows opportunistic fungi to grow on the fish. Once that problem becomes severe, the fish can’t handle the stress and die. The white gunk is a symptom of the fungus on the fish. Another likely problem is, the oxygen level in the well water. Although that water may be of good quality, it is unlikely that the well water contains any oxygen. This is because the soil and rock strata underground absorbs the oxygen before it gets to the pond. Then, if the pond gets a dose of manure runoff; without adequate oxygen the pollution cannot be handled (digested) by the pond.
There are two basic solutions that will improve the fish health, reduce mortality and improve water quality in the pond.
1) Keeton’s PondToss probiotic for fish. This product will introduce billions of beneficial bacteria into the pond water. They will adhere to the fish, and be ingested by the fish. This gives the fish a protective barrier of good bacteria (gram positive), that will inhibit the attachment of bad bacteria (gram negative). Another really good product is our Waste & Sludge Reducer, which will quickly consume the waste products coming into the pond from the manure. This product is so strong that it is being used in sewage lagoons to speed waste digestion.
2) Add pond aeration to the pond. In the cold winter climate of MI, the most effective way to increase the oxygen in the pond is by micro-bubble sub surface aeration. This will keep the oxygen level much higher than it is naturally, especially if you are getting ag runoff (which can consume a lot of oxygen as it breaks down in your pond). This will help to keep the fish healthy, and reduce or eliminate winter fish kill if that has been a problem in the past.
Call Keeton Industries for more information, or a specific recommendation for your pond!
Kurt
Does my pond have to be a minimum depth for an aerator and what would that be? Right now my pond is very low due to lack of rain.
Hi Patty,
Our standard diffusers are most effective in water depths of 4’ or deeper, however we can substitute perforated aeration tubing, which is very effective in water depths from 1’-4’. Call or email for a free, no-obligation aeration design and quote.
Thanks for the inquiry.
Kurt
Hi! I have a pond near my house that I’ve fished at for years. This morning I went back for the first time this season and found hundreds of blue gill dead around the shore along with a few carp 4+ feet and some bass. Saw a fish that was going insane (Swimming in circles running into a rock and jumping out of the water) as he got closer to me I noticed he seemed to have some sort of ripped tissue on the top of his back. Realizing the fish was clearly in pain or insane I picked it out of the water and euthanized it much to my remorse. Any idea what’s going on with the pond?
Thanks,
Jack Webster
Hi Jack,
Unfortunately that is an all-too common problem in older ponds. The main reason for a fish kill is inadequate oxygen, caused by a turn or flip of the water in the pond. To explain that more, most ponds have 3 main layers of water in them, differentiated by temperature and oxygen content. The top layer is called the epilimnion, which is warmer and commonly contains enough oxygen to sustain fish life under normal conditions. Below that is the thermocline, a thin water layer that marks the transition from the warmer surface water and the cool water at the bottom of the pond. The cool bottom water layer in the pond is called the hypolimnion. In most all ponds of over 5-6’ in depth, these three layers are present. The hypolimnion is typically the layer that causes an oxygen related fish kill. This is caused when a sudden change in surface water temperature causes the 3 layers to mix. The now cooled surface water becomes more dense, and plunges to the bottom of the pond, mixing with the hypolimnion. The problem is, the hypolimnion contains very little oxygen. In fact, due to the presence of silt and decomposing organic sediments, it may have an oxygen demand to satisfy. That demand then “steals” the oxygen from the epilimnion that is mixing in. The available oxygen is consumed by the bad water, lowering the overall oxygen level in the pond to a level inadequate to sustain fish. Common causes of a flip or turnover in your pond would be: seasonal changes in water temperature associated with spring or fall air temperature changes, or a cool or cold rainstorm that suddenly cools the surface water layer. There is one other common cause of low oxygen related fish kills, and that is ice coverage of the pond in cold climates. The ice blocks the sunlight to the green plant material, shutting down photosynthesis. The ice also eliminates the wind action on the water’s surface that naturally infuses oxygen into the epilimnion, and allows the release of the bad gases (carbon dioxide, methane, and hydrogen sulfide).
The good news is, all of the natural causes of low oxygen fish kills can be remedied by the addition of a pond aeration system. Keeton Industries offers a full line of 120 volt, 230 volt and solar powered aerators that can address the mixing and oxygen needs of your pond. The aeration will mix and oxygenate your pond on a daily basis, greatly reducing or eliminating the stratification that allows the hypolimnion to become low in oxygen. Give us a call or email us and we can design an appropriate aeration system for your pond. There is no cost or obligation for this design service.
Thanks for the question.
Kurt
Keeton Sales Director of Pond & Lake
I transplanted some iris from my sisters pond. At about 2 weeks later all my fish perished. Would that have something to do with it?
Carol, typically that would not cause a fish kill. If it did, it would likely have happened within 1-2 days of the work being done in the pond. The primary cause of fish kills is inadequate oxygen in the pond. The easiest remedy for that is installation of a correctly sized aeration system into the pond. To backtrack, transplanting plants into a pond could possibly cause a fish kill, by stirring up a lot of sediment and turbidity into the water. The sediment can prevent the fish’ gills from being able to absorb enough oxygen. This situation is typical when a pond is dredged, not when a few plants are transplanted. More likely, the pond was thermally stratified, then a sudden temperature drop or heavy rainstorm caused the pond to “flip” or “turn over”. This occurs when the warm surface water is quickly cooled, causing it to become more dense and plunge to the bottom of the pond. This mixes the upper layer of the pond (which typically has adequate oxygen levels) with the bottom layer of the pond (which frequently has virtually no oxygen, or inadequate oxygen). The anoxic water mixes with the good water, causing an oxygen crash and fish death. I speculate that is what happened in your pond.
Thanks for the question.
Kurt