Fish kills in Ohio ponds occur every year, and some years are worse than others. Fish kills can result from a variety of causes but the most common are weather, pond vegetation, and the interaction of the two. The critical factor is oxygen. Oxygen is essential to aquatic life and should be maintained at levels of 4 ppm (parts per million of oxygen per million parts of water) or higher.

Oxygen production resulting from photosynthesis is highest during summer. This is because warm water and long hours of daylight maximize the amount of algae and aquatic plants in ponds.
However, oxygen-using processes, including animal and plant respiration and organic matter decomposition are also at their highest levels in summer. In most ponds, the oxygen produced during daylight far exceeds the amount used by decomposition and respiration. However, there are summer situations that can occur that can lead to fish kills.

Many Ohio ponds become “choked” with vegetation during July and August. Problems can arise during a hot, calm, and cloudy weather pattern. A substantial reduction in sunlight causes the amount of oxygen produced to decline while oxygen consumption remains unchanged. Ponds can usually withstand several consecutive cloudy days but if clouds persist for longer periods, oxygen levels may decline to levels harmful to fish life.

A fish kill due to insufficient oxygen may begin to occur, particularly just before sunrise when daily oxygen levels are at their lowest. The first fish to die are the larger individuals as their oxygen needs are greater. A common result of this scenario is a partial fish kill, which leaves the pond’s fish community dominated by small fish.
Many owners wait too long before initiating any control measures.

This can lead to a serious oxygen problem if all the vegetation is treated simultaneously with an herbicide/algaecide. Soon after treatment, a massive die-off of the treated vegetation will occur. This causes a substantial build-up of organic material that begins to decay in the warm water. The breakdown requires large amounts of oxygen to complete. Oxygen consumption far outpaces oxygen production and levels begin to decline significantly. Couple this situation with a calm, cloudy day or two, and a fish kill can occur. Many ponds are stratified in summer, with a warm, upper layer of water on top of a colder, bottom layer.

These layers do not usually mix until fall. In Ohio’s productive ponds, a substantial amount of organic matter accumulates in the deeper areas. The decomposition of this matter results in the colder, bottom layer eventually losing most, if not all, of its oxygen by August. This is not a problem unless the pond turns over in August or September.

This premature mixing can be brought on by a heavy, cold rain. In many ponds, the volume of the colder, oxygen-deficient bottom layer of water exceeds the volume of the oxygenated, upper layer. In a heavy rain event, the cold rain water plunges through the warm upper layer towards the bottom.

This is because cold water is denser than warm water and therefore sinks. This rain event can cause the entire pond to mix. The mixing of oxygen-deficient water with the oxygenated water can occasionally cause oxygen to fall to levels lethal to fish.

Any pond that stratifies during summer will undergo a normal fall turnover as water temperatures decline. This process is described in Ohio State University Extension Fact Sheet A-7-01, Understanding Pond Stratification. You can go to: ohioline.osu.edu and search pond stratification for the fact sheet.

Readers with questions or comments for Steve Bartels may write to him in care of this publication.