Fish kills could be a continuing summer theme

Cape Girardeau, Mo. – The Missouri Department of Conservation (MDC) has received a high number of contacts this summer regarding fish kills across the southeast region of the state. Record-high temperatures and drought conditions are the most likely cause. According to MDC Fisheries Management Biologist Dave Knuth, the heat and drought cause oxygen levels to decrease in the affected bodies of water.

“Fish and other aquatic animals need oxygen to live, and each body of water will be different in the amount of oxygen that it can hold or produce,” Knuth said.

Water temperature, aquatic vegetation, fertility, number of fish and the amount of sunlight the water receives influences how much oxygen is in the water.

Knuth explained as the water temperature increases during summer, water loses some of its capacity to hold oxygen. Increasing water temperatures lead to increased fish activity and oxygen consumption.

“This is why a majority of fish kills occur during the months of July and August when water temperatures exceed 90°F,” Knuth said, “The extreme weather this summer has caused ponds, lakes and streams to reach higher water temperatures than normal this time of year.”

Little precipitation has further lowered oxygen levels in these bodies of water. Drought conditions since the spring have caused low water levels in lakes and ponds and reduced flows in streams. Lack of water depth and flow reduces oxygen levels available to fish. Shallow ponds or slow flowing streams have warmer water temperatures, limiting cool-water refuges for fish and reduced oxygen levels. Therefore, shallower ponds are more susceptible to fish kills than deeper ponds.

Aquatic vegetation and algae can also influence oxygen levels because plants produce oxygen only during the day and take-up oxygen both day and night.

“As the amount of sunlight decreases, plants produce less oxygen.” Knuth explained, “During extended periods of cloudy weather, plants can affect oxygen levels by consuming more oxygen than produced.”

Knuth also pointed out plants are good for fish and ponds, however too many plants can cause problems.

According to Knuth, “balance is the key.”

Vegetation that covers 15 to 25 percent of a pond can provide great enhancements to the pond and its fish populations. If vegetation covers 30 percent or more of the pond, it can become a problem. Now is not the time to address an excessive vegetation problem. When water temps cool, chemical control is a good option.

“Vegetative treatments done now, when water is warmer than 85°F, will result in decomposing vegetation, which can further deplete oxygen levels,” said Knuth.

Low oxygen levels can be detected when fish are seen on the surface of the pond gulping air. This is commonly seen in the morning, when oxygen levels are at their lowest. A quick response, if possible, is essential for saving the fish in the pond.

“The most feasible method to provide oxygen to fish in a pond is to aerate the water,” Knuth said. He said this can be done by using any kind of pump and nozzle to spray water from the pond into the air to allow it to pick up oxygen before reentering the pond. Knuth also recommends agitating the surface of the water by running a boat motor or commercial surface aerator. Pond owners can also consider a diffused aeration system.

Fish kills are a natural occurrence, however, there are design steps pond owners can make when managing private ponds to help avoid fish kills in the future. Keep the watershed of the pond well vegetated with trees and shrubs to prevent excessive nutrient inputs. Ponds should be constructed with at least 8 feet of water to ensure ample depth and less than 25 percent aquatic vegetation coverage should be maintained.

“No one wants to see a fish kill, and they can be inevitable. However, if we take preventative measures managing our water ways, we can limit the impacts to our fish populations,” Knuth said.

For more information about fish kills in the Southeast, contact the MDC Cape Girardeau office at (573) 290-5730.