Why do fish die in recreational ponds? Thermal stratification, is the technical answer

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By JASON REYNOLDS

This summer, I have received many calls from landowners concerned with fish kills in their ponds.

While summer fish kills can be caused by a variety of reasons, here in Ohio many times it is caused by premature fall turnover. This process begins with thermal stratification of the pond.

Thermal stratification is a change in water temperature at different depths in a pond. Some of you may have noticed this when swimming in a pond or lake. If your pond is stratified, you may have felt that the water around your legs is noticeably colder than the water at the surface of the pond.

Thermal stratification of a pond usually begins in May or early June and ends in September or early October.

In early spring, the water temperature begins to warm to 32 degrees Fahrenheit. Winds blowing across the pond cause the water to circulate from top to bottom, maintaining a constant temperature and a good supply of oxygen throughout the pond.

As summer approaches, the water temperature will warm even more and the pond may begin to stratify if conditions are right.

If calm, hot summer days persist, stratification will strengthen even more and become harder to break up. The pond will have a warm upper layer and a colder bottom layer.

Little oxygen by August

If these calm, hot summer days continue, the colder bottom layer will lose most, if not all of its oxygen by August. This is the result of large amounts of organic matter decomposing and fish respiration.

The warmer upper layer of water will then contain most, if not all, of the oxygen in pond. This warmer upper layer will contain enough oxygen for the fish to survive. This is when a problem can arise.

In most ponds, the volume of colder, oxygen-deficient bottom layer of water exceeds the volume of warmer, oxygenated upper layer of water. So if we get a heavy rain, the cold rain water will plunge through the warm upper layer of water toward the bottom cold layer of water.

This is due to the fact that cold water is denser than warm water.

So when the cold water sinks to the bottom, it causes the whole pond to mix very quickly. The mixing of the oxygen deficient bottom layer with the oxygenated upper layer causes the oxygen to drop to levels that are lethal to fish.

Aerate your pond

One way to help prevent premature fall turnover is to install an aeration system. An aeration system will continuously add oxygen to the water, even on those calm, hot sunny days.

These systems help keep ponds from becoming stratified, so premature fall turnover will not occur.

You do not need to run an aeration system 24/7. Using the aeration system at night from May to September will prevent summer fish kills.

Installing an aeration system is one of the most effective ways to prevent summer fish kills.

Other options

There are other actions that pond owners can take to help prevent summer fish kills. To help limit aquatic plant growth, make sure your pond has shoreline slopes of 3:1. This will create a better balance of oxygen levels between day and night.

If you are looking to eliminate or reduce aquatic vegetation, make sure your application of herbicide and or algaecide is completed before July 1. If you are treating your pond after this date, treat a quarter of the pond every two to three weeks.

For information on pond stratification, contact our office at 330-332-8732.

Information can also be found in these fact sheets from Ohio State University Extension online at http://ohioline.osu.edu/a-fact/0007.html and http://ohioline.osu.edu/a-fact/0008.html.

(Jason Reynolds is a wildlife/forestry specialist with the Columbiana Soil and Water Conservation District. He is a Columbiana County native and a 2010 graduate of Kent State University with a bachelor’s degree in conservation.)

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