Our office gets many questions from people about fish kill in their ponds and chances are that you or someone you know has experienced such a kill. Of the causes for this, fish suffocation due to lack of oxygen is the most prevalent.
Oxygen is incorporated into pond water by two processes. Photosynthesis, which is the process in which plants produce oxygen, contributes about 80% of oxygen production in ponds. The remaining amount of oxygen is incorporated by wind and wave action, which is known as absorption.
Fish kills due to low oxygen levels usually occur during the summer and winter. These two phases are known as summerkill and winterkill.
Summerkill and winterkill are processes that are different from each other but have the same result of critically low oxygen levels. Summerkill is most likely to happen in shallow ponds that are heavily vegetated and have high accumulations of decomposing organic matter. It can be triggered by the following situations.
The summerkill process is explained in figure 1. As displayed in figure 1, the bottom layer has no oxygen. When aquatic vegetation such as algae (also known as moss), pondweeds and other vegetation die, they decompose. Decomposition uses oxygen which in-turn increases the size of the bottom layer without oxygen, encouraging a fish kill.
Unfortunately, you cannot control all factors that contribute to fish kills. However you can control some of these factors.