The past several weeks I have had a number of calls about fish kills in local ponds. Every time, it has been large numbers of the biggest fish in the ponds that have died.
In the spring you will see a few large bream or bass dead in a pond. These occur in small numbers when the biggest, oldest fish in the pond are stressed when the water temperature warms, then cools.
However, from June to October, typically when you see large fish dead it is because of oxygen depletion in the pond. Oxygen depletion occurs when conditions create a greater demand for oxygen than the environment can produce.
There are a number of reasons why the oxygen demand in a pond can increase. The first reason is a large buildup of weeds in the pond. Algae and aquatic weeds produce oxygen during the day, but at night they consume the oxygen to stay alive. Therefore, the more plants in a pond, the more oxygen consumed at night.
The amount of oxygen produced by these plants can vary depending on environmental conditions. If we have several cloudy days, the amount of sunlight reaching the pond is reduced. If a pond has floating weeds such as duckweed and watermeal, they can block the sunlight.
Also, during a drought, more weeds grow as the water level is reduced in the pond. This weed growth further reduces the oxygen levels at night. The time of day when the oxygen levels are lowest is just before daybreak.
Another factor that contributes to oxygen depletion is having too many fish in a pond. Some ponds are overstocked from lack of fishing. Even ponds that are stocked properly can become overcrowded as water levels drop because of a drought. As the water level drops, the fish are crowded into a smaller area that has less oxygen-holding capacity.
One of the ways to help with this problem is to remove fish. If you have large catfish, they should be removed first. Larger fish require more oxygen than smaller fish.
During warm-weather months, pond water stratifies into three layers. The top layer is warmer and contains oxygen-producing algae. Wind action also contributes oxygen to the top layer. Most of the fish in a pond are in the top layer.
The middle layer is a relatively thin layer characterized by a rapid drop in temperature and oxygen levels. When you are swimming in the lake or a pond, this is the cold layer that you will feel sometimes.
The third layer, or bottom, is stagnant, cool water, with little or no oxygen. It doesn't mix with the upper level in warm weather. Oxygen depletion can occur when these layers mix during a "turnover." Fish kills will occur if oxygen demand in the stagnant bottom layer exceeds the oxygen available in the top layer. Turnover is triggered by heavy rains, strong winds blowing for long periods in one direction and rapid cooling in the fall.
There are warning signs that pond owners can watch for that will indicate possible oxygen depletion in a pond. The first is a change in pond color. A change from green to brown or grey may indicate that the algae have died. The second is cloudy weather for two to three days. Also, periods of high winds when dense blooms of algae are present can cause oxygen depletion. During the fall, you can have oxygen depletion when the air temperature cools rapidly.
The last indicator is fishfeeding behavior. If you feed them floating food and they stop feeding, it is an indicator of low oxygen levels.
If you have low oxygen levels in your pond you need to check your pond at daybreak, the time of day with the lowest oxygen levels. Fish suffering from low oxygen will come to the surface in the morning and gulp air. Then you need to act.
The best action is to aerate the water. The simple solution is to purchase an electric aerator that provides about three-quarters' horsepower per acre of pond area. Another option is a pump to shoot water into the air and let it fall on the pond surface. The water needs to fall more than two feet in order to pick up adequate oxygen.
The time of the year for fish kills is now, so keep an eye on your fish.
Columbia County Extension Agent Charles Phillips can be reached at (706) 868-3413 or by e-mail at firstname.lastname@example.org. The Extension Web address is www.ugaextension.com/columbia.
The Columbia County News-Times ©2013. All Rights Reserved.