There are many different causes of fish kills in private ponds, but some are much more likely than others.

There are many different causes of fish kills in private ponds, but some are much more likely than others.

Low oxygen in the pond is the most common cause of sudden fish kills, but other water-quality problems also occur, according to Dr. Andy Goodwin, associate director of the Aquaculture/Fisheries Center at the University of Arkansas at Pine Bluff. Fish kills because of infectious diseases or accidental introductions of chemicals are very rare.

“Oxygen depletions can be easily recognized because they usually happen very suddenly,” Goodwin said. “One day the fish are fine, the next day many are dead. The only way to be sure that it is an oxygen problem is to use scientific instruments to measure oxygen levels, but there are more practical approaches to diagnosing an oxygen problem.”

When oxygen levels are low, big fish die before small fish. And if you visit the pond shortly after sunrise, you will see the fish gasping at the surface or congregating in the shallows around the edge of the pond, he said.

Oxygen depletions are far more likely to occur in hot weather and right after storm winds or rain that cause sudden pond mixing. They commonly occur in ponds with very green water, heavy growths of aquatic plants, or in ponds covered with plants such as duck weed or water meal.

The best prevention is to avoid the introduction of excess nutrients such as fertilizers, animal waste and decaying plants. These can lead to excess plant production. Pond owners can control aquatic weeds by stocking grass carp which are legal in Arkansas but not in all states, or by the careful use of herbicides. Before using an herbicide, Goodwin urges pond owners to work with their county Extension agent to identify the plant and get advice on safe treatments. Some herbicides may be very toxic to fish in some kinds of water.

If an herbicide is used to kill plants in the heat of the summer, the death and decomposition of those plants can cause the oxygen depletion that you are trying to prevent, he said. Pond owners with electricity pond-side and a healthy pond budget can install electric aerators and run them during summer nights to prevent oxygen problems.

For everybody else, aquatic plants should be controlled by restricting livestock access, limiting fertilizer runoff and only fertilizing the pond when it is really needed.

The second most common fish kill problem in Arkansas is related to ponds with watersheds made of acid soils. In these ponds, and especially in the late winter, pond owners may see several fish die each day over an extended period of time, often after heavy winter rains. The rainwater and acid soils combine to make the pond water into a weak acid. This damages the fish’s skin and gills and leads to infections by common pond bacteria and fungus.

“Your county agent can help run a water test (alkalinity) to diagnose this problem,” Goodwin said. “If acid water is the issue, it is easily treated by the addition of agricultural limestone — not hydrated or quick lime.”

Typically, the only infectious disease problems that occur in ponds are occasional infestations by fish parasites. The two most likely to be noticed are yellow grubs and leeches. Yellow grubs have snails and birds in their life cycles and form small yellow cysts under the skin of fish. In large numbers they harm fish health, and make the fish less attractive for the table. Yellow grubs can be controlled by stocking grass carp to eat the weeds that are the food and habitat for snails, and by stocking redear sunfish, known as shell crackers, to eat snails. Leeches are more difficult to control and the only option is to live with them and hope that the conditions will change and their numbers will drop.

In Arkansas, more than 90 percent of all fish kills are caused by oxygen. Most of the remaining cases are acid water. “Infestations by yellow grubs and leeches look ugly, but rarely kill fish. Kills from accidental poisoning by agricultural chemicals are extremely rare and almost never occur,” Goodwin said. “If you notice that your fish are dying, are behaving strangely, or have sores or parasite problems, contact your county Extension office for assistance in determining the cause of the problem, and for help in planning a safe, legal and effective strategy to deal with the problem.”