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Fish Kills

Fishermen aren’t the only people concerned with keeping ponds healthy, and fish can serve purposes other than adorning the dinner table. Some are kept as pets, and others serve to keep ponds free of weeds that might take over. Regardless of their purpose, sometimes things go wrong with fish. Anyone who owns or uses a pond might be familiar with fish kills, a phenomenon that occurs when something goes awry in the water and an alarming number of fish go belly-up.

What can cause a fish kill? Most of the causes are natural (as opposed to something like a pesticide spill), and they can be difficult or impossible to predict. Even if you can see the conditions that cause a fish kill coming, there isn’t always much you can do to prevent it from happening

One of the most common reasons for a fish kill is low dissolved oxygen in the water. This can be an alarming scenario, because large fish are affected before smaller ones. Small fish may be seen gulping at the surface of the pond before a fish kill occurs, but the sudden appearance of many large, dead fish is certain to grab more attention!

Fish require a certain amount of oxygen in the water – around 5 parts per million, at least. Any lower than that and they begin to have trouble; when dissolved oxygen levels fall below 2 ppm, a fish kill may occur. Oxygen is added to the water by diffusion (wind and waves help oxygen to enter the water) and plants and algae (oxygen is a byproduct of photosynthesis). It is removed by organisms of all sorts, including fish, insects, bacteria, and even plants. During the night when plants are not photosynthesizing, they may remove oxygen instead of adding it to the water. Plants that die in a pond are food for bacteria, which use oxygen as they break down the dead plant matter. During the summer, long periods of cloudy weather may cause plants and algae to be unable to photosynthesize, causing them to take up oxygen and resulting in a fish kill. Thunderstorms may wash plant debris into a pond, resulting in an explosion of bacterial activity…resulting in a fish kill.

The temperature of the water also plays a role in determining how much oxygen is available. While warm water holds less than cool water, it rises to the top of a pond, where wind and waves help refresh it. During cold weather, that relatively oxygen-rich water can cool down. This causes it to sink and be replaced by low-oxygen water from the bottom of the pond. A pond turnover like this is more likely to occur during the fall.

To help prevent fish kills, avoid the introduction of large amounts of nutrients to the pond. Fertilizer or dead plant materials should be kept from entering the water, if possible. If you control weeds in your pond with herbicides, apply them to only one third to one half of the pond at a time. In small ponds, aerators may help to increase the amount of oxygen in the water.

These are not the only reasons for fish kills, but they are the most common. For more information, please see our publication at


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