Pond managers may see evidence of sick and dying fish from February through April. Usually, it is a few fish per day, often catfish but maybe a mixture of species. The die-off may continue for several weeks. Fish often show external signs of distress, including patches of fungus, red sores and sometimes large ulcers in the head or back. All sizes are affected, and upset pond owners want to know who or what to blame.
Don’t worry. It is highly unlikely that someone poisoned your fish. The problem is most likely the pond water itself. Many locations in Mississippi have underlying geologies that are low in dissolved ions. Alkalinity, which is a measure of the negatively charged ions dissolved in the water, acts as a buffer for pH changes in the water.
Alkalinity encourages primary productivity in ponds and helps prevent wide swings in pH.
Similar to alkalinity, water hardness is the overall concentration of positive ions. Hardness reduces fish stress in ponds and provides elements important for animal survival, such as calcium and magnesium.
To promote good fish growth and health, experts recommend alkalinity and hardness values of at least 20 milligrams per liter. When values are below this threshold, fish tend to grow slowly, the pond is not very productive, and fish may become stressed and die. External signs of disease are usually secondary, which means the fungus or bacterial ulcer you see on dead or dying fish is not the cause of the problem. The fungus or bacteria have opportunistically invaded an already sick or stressed fish.
The solution to poor water chemistry is relatively simple. To overcome low ion concentrations, pond managers must add ions. The best way to accomplish this goal is to apply agricultural limestone. Liming provides major benefits if you are growing sport fish in a pond with poor water chemistry. Limestone reduces the acidity of the bottom soils, which makes nutrients more available. That increases the alkalinity and hardness of the water, which reduces stress on the fish over the winter. Additional limestone typically eliminates cold water fish die-offs for several years. However, ions will be flushed gradually from the pond, so liming may be needed every three to five years to keep water chemistry at appropriate levels.
Landowners who are concerned their ponds may have low alkalinity and hardness should contact their county Extension office to have the total alkalinity tested. Be sure to ask for alkalinity, not pH, as pH can fluctuate hourly in ponds with poor alkalinity.
If alkalinity is below 20 milligrams per liter, liming would be beneficial to the fish population. For more information, call your local county agent and request Extension Publication 1428, “Managing Mississippi Farm Ponds and Small Lakes.”[Editor’s Note: Extension Outdoors is a column authored by several different experts in the Mississippi State University Extension Service.