A dense green-colored pond with fish swimming near the surface is a tell-tale sign of oxygen deprivation.
Unfortunately, by the time most pond owners notice the problem it is often too late to do much, warns Ryan Sproul, Delaware County Extension, Ag Educator.
“Pond owners who regularly check their ponds early in the morning are more likely to catch the warning signs of a future fish kill and be able to avert it,” Sproul said.
The green color is caused by excess growth of microscopic algae known as phytoplankton. Having a moderate amount of the algae is a good thing, as they produce most of the dissolved oxygen in the pond. However, major problems can occur when a pond owner cannot see an object submerged 18 inches or less at noon on a sunny day.
“The more dense the phytoplankton blooms, the greater the danger of it dying, and when that happens the decay process uses up all of the oxygen in the water,” said Marley Beem, Oklahoma State University Cooperative Extension aquaculture specialist.
Additionally, floating plants — such as duckweed that totally cover the pond surface — will cause low oxygen.
“Don’t let this happen,” Beem said. “If sunlight cannot get into the water column, then there will be no oxygen produced and your fish will suffocate.”
In an attempt to avoid suffocation, the fish will swim near the surface, gulping as they try to get some oxygen from the surface layer. When these signs are identified, feeding of fish should be suspended and finding a way to get oxygen into the water is important.
“A pump set can be used to draw up surface water and let it splash over boards or fencing,” Beem said. “A boat motor can be run with the propeller at a shallow depth. People sometimes try running a brush hog to get oxygen into the water.”
Emergency aeration equipment should be set up and ready to go before dead fish appear. Permanent in-pond aerators are an increasing trend among pond owners, but they may not always be practical.
“It’s important to realize that they are not cheap to install or run and they can sometimes worsen a problem or even create new problems if incorrectly installed,” Sproul said.
Sproul and Beem suggest the best long-term management option that can be done to reduce the chance of a fish kill is to reduce fertilizer runoff into the pond from lawns, livestock corrals and other sources.