Time to Monitor Summer Fish Kills in Farm Ponds
A K-State wildlife specialist discusses reasons for summer fish kills in farm ponds and what can be done to prevent this problem.
August 6, 2015
MANHATTAN, Kan. – Many who swim in ponds during the summer notice how the water toward the surface tends to be warm, with cooler layers below. Water in ponds and lakes is stratified in the summer, and those layers usually don’t mix until the fall in a process called fall turnover.
A premature fall turnover, however, is one of the main reasons for summer fish kills, said Charlie Lee, wildlife specialist for K-State Research and Extension. Normal fall turnover happens typically by late September, so a turnover in July or August is considered premature.
“Oftentimes after a heavy, cold rain, the volume of water that washes into the pond causes a premature turnover,” Lee said. “What happens then is the oxygen levels are too low to support fish life because of the large amount of water that rushed into the pond.”
The oxygen deficient part of the pond is in that cooler layer, Lee said. This is because organic matter decomposes at the bottom of ponds and uses a lot of the available oxygen in the summer. Fish normally live in the top 4 to 5 feet of water, so if the cooler water prematurely rises to the top, the lack of oxygen near the surface can be lethal.
Sufficient oxygen is necessary for fish to survive, and Lee said the summer months often bring both increased oxygen production and increased oxygen use. Long days mean more sunlight and the opportunity for algae and aquatic plant production to create more oxygen. But, summertime also means increased animal and plant respiration and organic matter decomposition that require oxygen.
“In most cases, the oxygen produced in the daylight far exceeds the amount used in decomposition and respiration,” he said. “As long as the daytime oxygen production exceeds the nighttime oxygen use, summer fish kills due to insufficient oxygen are not going to occur.”
While vegetation in the pond helps supply oxygen, too much vegetation atop the pond can lead to summer fish kills. Like premature turnover, too much vegetation cover can cause lower levels of oxygen when the weather conditions are just right.
“Excessive vegetation itself doesn’t result in a fish kill,” Lee said. “Typically, problems arise in hot, cloudy and calm weather patterns. We get a reduction in sunlight, which reduces the amount of oxygen being produced while the oxygen consumption remains unchanged. Over time, that oxygen level will slowly decline.”
Ponds choked with vegetation can withstand a few cloudy days with no problems, but if the clouds persist for more than four to five days, lower oxygen levels could be harmful to fish, he said.
“Oxygen levels too low for largemouth bass and bluegill fall between 3 and 4 ppm (parts per million),” Lee said. “Channel catfish can survive on a bit lower, but typically I like to see oxygen levels above 4 ppm.”
Preventing summer fish kill
If you want to control the vegetation cover with herbicides, now is not the best time, Lee said. It is best to delay control until air temperatures are cooler, or better yet, control excessive vegetation in the spring.
“If you treat this time of year, you are going to get a massive die-off of that vegetation,” he said. “Once that die-off begins, we get a substantial amount of organic material on the bottom of the pond that begins to decay, so the oxygen consumption outpaces the oxygen production. The bacteria are using more oxygen. The plants are not producing any oxygen. You couple that situation with a calm, cloudy day or two, and you’re likely to get a fish kill.”
The best solution is prevention, Lee said, which starts with proper pond construction. As an example, small, deep ponds that experience a premature turnover would have a larger proportion of water with minimal oxygen compared to large, shallow ponds. Limit the amount of aquatic vegetation growing in the pond using timely management strategies. Lastly, consider installing an aeration system in your farm pond.
“Some use the windmill-type pond aerators,” Lee said. “Unfortunately, those are needed the most on those calm, hot days, and they cease to work. That’s not an effective solution unless you have some way to convert that wind energy to electricity or use a generator to make those windmills work on calm, hot days.”
“You can also try to use a 2-inch water transfer pump, spray that water up into the air with a nozzle and produce small droplets,” he added. “As those droplets fall back into the water, they pick up oxygen. But, usually once you see fish at the surface that are gulping and lacking oxygen, it’s probably too late.”
Lee pointed out that summer fish kill is often more noticed than winter fish kill—also the result of a significant decline in oxygen—due to periods of longtime ice cover.
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K-State Research and Extension is a short name for the Kansas State University Agricultural Experiment Station and Cooperative Extension Service, a program designed to generate and distribute useful knowledge for the well-being of Kansans. Supported by county, state, federal and private funds, the program has county Extension offices, experiment fields, area Extension offices and regional research centers statewide. Its headquarters is on the K-State campus, Manhattan.
Story by:
Katie Allen
katielynn@ksu.edu
For more information:
Charlie Lee – clee@ksu.edu or 785-532-5734
