There are a few ways that pond owners could experience a major fish die-off during these hot, dry summer months. Warm water already holds less dissolved oxygen than cold water and, when coupled with too large a fish population and the following conditions, we can expect to have fish die-offs.
First is the overuse (or misuse) of aquatic herbicides. Make no mistake, these products are very safe to fish and the environment when used according to their instructions. However, if there is a large amount of vegetation in the pond that is treated all at once, the resulting death and decomposition of vegetation can rob the pond of oxygen.
All decomposing things consume oxygen, be that a dead squirrel on the side of the road or all the vegetation recently treated in your pond. The rule of thumb is to treat no more than a quarter, perhaps up to a third, of the target vegetation and then wait a couple of weeks before treating another portion.
Even without any induced weed control measures, ponds can have a natural phytoplankton die-offs. Nutrient-enriched ponds often produce dense blooms of microscopic algae (called phytoplankton) that give them a deep green colored appearance. Sudden phytoplankton die-offs can occur following consecutive days of cloudy, hot, windless conditions. When an algal bloom “collapse” occurs, the water can change colors quickly. The sudden die-off of algae will, as mentioned above, lead to a rapid decline in dissolved oxygen as bacteria decompose the dead algae.
Following suite with too much phytoplankton is excessive vegetation. Everyone knows that plants take in CO2 and give off oxygen, but few remember that plants also need and use oxygen. Consider a pond where 60 to 80% of the surface is covered with plants. These plants are both producers and consumers of oxygen.
And add to the excessive vegetation a long period of hot, cloudy, still (windless) weather conditions where water temperatures rise above 85°F. The cloudy days give less sunlight for plants to make oxygen. The still days where the wind doesn’t seem to move at all provides no wave action to aerate the pond. And the hot temperatures reduce the water’s ability to hold oxygen.
Ponds can usually withstand several consecutive hot days and nights, but if these conditions persist, oxygen levels may decline to levels harmful to fish. Experience has taught us that this type of summer fish kill is more common in shallow ponds that are heavily vegetated.
Lastly, turnovers can follow the blessing of a heavy rain. Most ponds deeper than 8 to 10 feet typically stratify in the summer. This results in a warmer, lighter, more oxygenated upper layer on top of a colder, denser bottom layer.
After a long, dry, hot spell, most folks are overjoyed when a cool front and a resulting storm brings some much-needed rain. Yet the cooler blowing wind coupled with the cooler rain may reduce the temperature of the surface such that the pond “turns over”. The oxygen-poor bottom layers mix with upper layers, resulting in low oxygen levels throughout the water column and possible fish kills.
When we analyze our ponds’ ecosystems, we learn that large amounts of organic matter may accumulate during the summer in the deeper areas of stratified ponds. By the latter part of the summer, oxygen in this bottom layer will be used up as this organic material decomposes. This mixing of these stratified layers can occur during the summer after a heavy cold rain.
In all these circumstances, low oxygen levels may be identified by close observation during the early morning hours. At sunrise during extremely hot weather, check your pond regularly for fish “gulping” at the surface. This gulping is the sign of oxygen stressed fish. Larger fish usually die first because their oxygen requirements are greater than those of smaller fish.
If you find evidence of fish deaths later in the day, take some time to carefully look for smaller fish alive and well near the edges. If you find them, then we can confirm that the cause was low oxygen levels and not some other toxin-induced die-off.
Solutions to low oxygen in the pond revolve around two simple options. First you could add more oxygen be purchasing an aerator. From fountains, to paddlewheels, to bubbling devices located on the bottom of the pond, these are an immediate fix for the oxygen and are often employed by commercial fish producers.
But guessing that most every pond owner is not in the commercial fish production business, the other option is to reduce the number of fish in the pond. Annually at the very least, invite friends or family over to catch fish and then hold a fish fry! This physical reduction of the pounds of fish will reduce the need for oxygen and will provide a responsible means of enjoying the fish that have been produced in your pond.
Cary Sims is the County Extension Agent for agriculture and natural resources for Angelina County. His email address is cw-sims@tamu.edu.
Educational programs of the Texas A&M AgriLife Extension Service are open to all people without regard to race, color, sex, disability, religion, age, national origin, genetic information or veteran status. The Texas A&M University System, U.S. Department of Agriculture, and the County Commissioners Courts of Texas Cooperating.
