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.

ish kills are an unfortunate reality for ponds and lakes. Most fish kills are prompted by a change in the seasons, but some can be attributed to human related factors such as the improper use of chemical herbicides/pesticides or polluted runoff. No matter the cause, experiencing a fish kill can be very traumatic.

Source: Brad Vollmar of Vollmar Pond and Lake Management

Winter Fish Kills

Different factors play a role in winter fish kills, but is ultimately the result of low dissolved oxygen levels causing the fish to suffocate. Ice covering the water contributes to this problem:

Richard Kremer from Wisconsin Public Radio wrote an article about this years fish kill in the Wisconsin area, “Heavy ice and snow cover on lakes across Wisconsin has the Department of Natural Resources watching for higher than normal fish die-offs. During winter, oxygen levels decrease in smaller lakes that aren’t fed by moving water. That’s because ice keeps air out and heavy snow keeps underwater plants from making oxygen. Fisheries ecologist Paul Cunningham says it’s normal for some fish to die each winter, but with this year he expects a higher than normal fish kill. Read the Full Article via the Sawyer County Record.


Fish Kill Aeration Solutions:

Luckily, steps can be taken to help prevent fish kills from happening. We will introduce you to some long term solutions as well as some emergency solutions that can help save some of your fish if you are experiencing a fish kill.

Keeton Industries has lines of both electric and solar aerators that use subsurface aeration diffusers that can move 1,000-2,500 gallons of water per minute, depending on the depth of the lake. The diffuser creates a laminar that draws oxygen depleted water from the bottom, oxygenates it and transports it to the surface. This action mixes stratified water while increasing dissolved oxygen levels and protecting fish, aquatic organisms and beneficial microbes from suffocation.

Aeration, in combination with our innovative line of beneficial microbes and aquatic dye, work to create an excellent environment for fish to survive and thrive.

Contact Keeton Industries today for aeration solutions to fish kills specific to your pond.