The Lorain Soil & Water Conservation District provides leadership in a partnership effort to help people conserve, maintain and improve the natural resources and environment in Lorain County.


Agricultural Center
42110 Russia Road
Elyria, OH 44035-6813
Phone: 440.326.5800
FAX: 440.326.5807

Fish Kills in Ponds

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.

  1. Cloudy, hot and still days in the heat of the summer.
  2. Large-scale die-offs of tiny microscopic plants (phytoplankton).
  3. Sudden thermal turnovers or inversions caused by dramatic weather changes.
  4. Chemical treatment of algae or aquatic weeds, which result in excessive decay.

[Photo: Figure 1 – Loss of Oxygen Process Resulting in a Fish Kill]

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.

How To Prevent Fish Kills
  • Use proper pond construction: 8-10 feet deep and 3:1 shoreline slope to prevent excessive aquatic growth.
  • Install aeration and circulation systems. Some systems have the capability to transfer a significant amount of oxygen to the pond. Fountains are usually less effective compared to diffuser-type aeration systems.
  • Reduce excess nutrients from running into your pond from the watershed. But if runoff from your yard comes into the pond, you can reduce the amounts of excess phosphorous and nitrogen by reducing fertilizer applications. If possible, a wetland may be constructed upstream of the pond to filter excess nutrients.
  • Use proper vegetation control treatments – apply aquatic herbicides early in the growing season to reduce large die-offs of aquatic vegetation.
Ohio Pond Management Handbook – Ohio Division of Wildlife
OSU Extension Bulletin 374-99


August 16, 2017