Delaware County is dotted with ponds and lakes, adding to the local scenery and providing many excellent opportunities for fishing, swimming, boating, wildlife watching and agricultural use. In the summer, we occasionally receive phone calls from pond owners about dead fish. Fish mortality is a natural process, and an occasional dead fish is not unusual, but large numbers are a reason for concern. It is important for pond owners to understand conditions that can lead to a summer fish kill and take preventative measures.

Fish need oxygen to survive, and they obtain it through oxygen gas dissolved in the water. Summer fish kills are often attributed to low concentrations of dissolved oxygen (DO) in the water causing fish to suffocate. Dissolved oxygen is produced by algae and aquatic plants during photosynthesis, and oxygen is dissolved into water directly from the atmosphere and through wind and wave action. Dissolved oxygen levels can vary greatly within a 24-hour period and throughout the year. Low levels of DO may occur in the summer due to one or more of the following events:

• Sunlight is needed for plant photosynthesis so a stretch of cloudy, hot, and still days in succession can lead to dangerously low DO levels.

• While a pond can withstand a few hot, overcast, windless days in a row, too many can result in a die off of microscopic algae. Bacteria in the pond will then use up the oxygen as they work to decompose the algae.

• Herbicide treatment of a weed choked pond can create a massive die off of plants and algae that once were responsible for oxygen production. Again, bacteria will use up much of the oxygen remaining to perform their decomposing functions.

• Dramatic weather changes can disrupt the typical temperature stratification in a pond. Heavy cold rains can cause oxygen poor layers at the bottom of the pond to mix with the upper layers, resulting in low DO levels throughout the pond.

Your pond is a living breathing organism. Natural causes such as predation, old age, minor disease outbreaks, handling, or spawning stress may lead to occasional fish mortality. When a large number of dying fish are observed, it is often too late to stop the fish kill; however, there are activities pond owners can undertake to avoid summer kill.

One of the most effective ways is to install an aeration system which continuously adds oxygen to the water. If chemical control of aquatic weeds is necessary, treat with granular formations of herbicides in specified areas, preferably about one quarter of the pond every two to three weeks. This spreads decomposition over a longer period of time. Creating and maintaining a buffer strip around the edge of the pond safeguards against nutrients entering the water and contributing to algal blooms and aquatic weed infestations.

A great resource is the Ohio State University Extension Program in pond, fisheries, and aquatic management at https://senr.osu.edu/extensionoutreach/ponds-fisheries-aquatics. In addition to archived newsletters, you will find a link for the free Open Pond Clinic hosted by Eugene Braig via Zoom, Tuesday evenings from 4 to 6 p.m.

A variety of pond management fact sheets produced by the Ohio State University Extension can be found at https://woodlandstewards.osu.edu/publications/wildlife, including the Division of Wildlife’s “Ohio Pond Management Handbook.” The handbook provides information on pond construction, stocking, fishing management, aquatic vegetation, fish health, along with common pond problems and solutions.