What Causes Fish Kills in Fish Ponds

Understanding Fish Kill in Your Pond

In this article, we offer helpful information relating to fish kills. This article will come in handy for pond owners, those of you that wish to own a pond, or for anyone interested in fish! Though the particular study we reference in this article is based in Alabama, people from other areas will also benefit a great deal. We discuss several fish farming principles, including the possible reasons for their uncommon deaths.

Decreasing Fish Numbers

Most fish farmers face the serious issue of decreasing fish numbers. Fish kills will result in a steady decrease in the quantity of fish in your pond, ultimately leading to a shortage of fish. Understanding the cause of fish kills and how to prepare for it is very important for pond owners.

Dissolved oxygen shortages are among the leading causes of fish loss. In order for your fish will thrive, your oxygen levels must be at or above the required amount. 

There are several ways to determine if an oxygen depletion type of fish kill is occurring. Many of your fish will be swimming near the surface of the water in an attempt to get oxygen. The larger fish will start dying before the smaller ones. Often, the fish kill will begin to make the morning hours or night hours. The fish will start dying quickly, generally within a few hours. The water will start to turn brown or develop a foul odor.

Oxygen depletion fish kills normally occur because the phytoplankton or tiny green plants start dying. A second possibility could be the water from the deepest part of the pond merges with the surface water. A balanced pond, due to the phytoplankton, will have a green color. These plants oxygenate the pond. If there are long periods of time where the pond goes without the sun, the phytoplankton can die suddenly since they use more oxygen than they produce. When the phytoplankton dies, the fish consume what’s left of the oxygen and begin to die at a fast rate.

Phytoplankton and Fish Kills

To prevent oxygen level shortages caused by the dying phytoplankton, suitable fertilization should be used with your pond maintenance. On the other hand, over-fertilizing may result in water that looks like green soup, with visibility only up to 2 to 3 inches as opposed to the desirable 12 to 18 inches of visibility. Excessive fertilization will also cause dense growth of phytoplankton. Whenever there is an overgrowth of plants in the pond, there are many herbicides available that can be used to balance the population. Ensure you follow the instructions when using such pesticides as they can be harmful to your fish. An ideal combination for pond maintenance is fertilization with the use of an aeration system.

Another cause of fish kills is the mixing of the deeper water level with the surface water level. If you receive a lot of rain, it’ll mix with the pond water while it sinks to the bottom of the pond. It will then displace the oxygen-deficient water, mixing it with the surface parts of the pond. This causes a decrease in oxygen levels, resulting in fish kills.