The Texas summer weather can be hard on pond fish.
Oxygen depletion in ponds is one of the most common problems in hot weather because warm water holds less oxygen than cool water. Fish also have higher metabolic rates in warm water, which can also increase their need for oxygen.
These conditions, combined with a cloud cover, can create conditions that can be potentially deadly to fish, especially in smaller ponds. Cloud cover deprives phytoplankton of sunlight, which is needed to conduct photosynthesis, which in turn provides oxygen for fish in the pond. When the fish’s oxygen demand exceeds the rate in which oxygen is being replaced, a deadly depletion can occur.
Another weather-related condition that can result in oxygen depletion is when a sudden change in temperature occurs. During the summer months, ponds are normally stratified with oxygen-rich water on 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 a “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 has been killed will decompose and, in the process, use oxygen that is already at low levels.
Aquatic herbicides are usually most effective if used in the spring, when aquatic weeds are young. If used in the summer, only a third of the pond should be treated at one time.
Two signs can warn pond owners of impending oxygen depletion problems: first, the presence of fish swimming sluggishly near the surface, and secondly, fish striking at the water surface, gulping for air. This will be most evident in the early morning hours. Also, a change in pond color from green to brown may indicate a phytoplankton die-off, which may also signal oxygen loss.
Many times, oxygen depletion is 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 replenish some of the oxygen. In emergency situations, an outboard motor can oxygenate the water by running in a fixed position just below the surface. Running back and forth across the pond with a boat and motor will not substantially increase oxygen.
The addition of aerated water from the surface of another pond or a well can also lessen the effects of oxygen depletion. Pumps that pull the pond’s water from the surface and spray it back over the top of the pond also work well; pumping water from the bottom to the surface only compounds the problem.
These corrective measures are usually only temporary but will help until environmental conditions improve.
More resources on these subjects and many others are available on two relatively new AgriLife Extension websites, http://aquaplant.tamu.edu and http://fisheries.tamu.edu.
BRANDON BOUGHEN is the agriculture and natural resources county extension agent with Texas AgriLife Extension. He can be reached at 940-349-2894 or firstname.lastname@example.org.