Hot North Texas summers can be hard on fish in ponds. Oxygen depletion in ponds is one of the most common problems in hot weather. Warm water holds less oxygen than cool water.

Eddie Baggs

As water temperatures rise, so do the metabolic rates of fish, increasing their need for oxygen.

A combination of heat and cloud cover can create deadly conditions for fish, especially those in small ponds. The cloud cover will deprive the phytoplankton in the pond of the sunlight necessary to conduct photosynthesis, further reducing oxygen levels.

When the increased oxygen demand by the fish exceeds the rate in which oxygen is being replaced, a deadly depletion can occur.

Rapid temperature fluctuations can also cause oxygen depletion. Ponds are normally stratified during the summer months with oxygen-rich water on the top and cooler oxygen-deficient water on the bottom. Cool winds and rain can cause the two layers to mix rapidly, which can result in the rapid breakdown of organic material on the bottom of the pond. This can remove a large amount of oxygen from the pond and can result in “fish kill.”

A similar problem can occur if herbicides are used to control aquatic weeds over large areas of the pond during the hot summer months. The vegetation that is killed will decompose, consuming oxygen during the process. Aquatic herbicides are usually most efficient if used in the spring, when aquatic weeds are young. But if used in the summer, herbicides should be metered across segments of the body of water.

Two signs serve as warnings of impending oxygen depletion — fish swimming sluggishly near the surface and striking at the water surface, gulping for air. This behavior will be most evident in the early morning.

The color of a pond can also serve as a good indicator of problems — a change in color from green to brown may indicate a phytoplankton die off.

Oxygen depletion is often discovered too late. However, if detected early enough, a pond owner has the opportunity to act quickly and save the fish.

Aeration of the pond can help relieve oxygen depletion. In emergency situations, an outboard motor can be used to add oxygen to water. Run the motor in a fixed position just below the surface.

The addition of aerated water from the surface of another pond or a well can also stave off disaster. Pumps that pull the pond’s water from the surface and spray it back over the pond also work well. Pumping water from the bottom to the surface only compounds the problem.

EDDIE BAGGS, county extension agent with the Texas AgriLife Extension Service in Denton County, can be reached at 940-349-2880.