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Hot weather taking its toll on fish

  • 8/8/2017 1:43:00 PM
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The end of summer is the worst time for underwater dwellers. When temperatures are high and oxygen levels are low, fish and other water critters are particularly vulnerable.

“We have received several reports of small summer fish kills at many lakes, ponds and a few streams throughout Iowa,” said Chris Larson, fisheries supervisor for the DNR in southwest Iowa. “We have also had some fish kills caused by pollutants.”

Seeing fish swim erratically and aquatic plants or algae dying are signs that a natural summer fish kill could occur.  As aquatic plants die and decay, they remove dissolved oxygen from the water.

“These partial summer kills rarely kill all fish in the pond or lake and in a couple of years it will be back in balance,” said Larson. Usually large fish are more likely to die from low oxygen. However, some small fish can be affected.

In the hot days of summer, even small amounts of polluted runoff can cause problems for fish and other aquatic organisms.

“Historically we see more fish kills in August and September,” said Ken Hessenius, supervisor at DNR’s Spencer field office. “We’ve investigated four fish kills in the last two weeks. So we want to encourage farmers, pesticide and manure applicators, and homeowners to be extra careful when applying chemicals, fertilizers, and manure.”

Take these simple precautions to protect your waters:

  • Avoid applying chemicals and fertilizers before it rains.
  • In town, remember storm drains go right to a stream – past spills have been caused by draining home swimming pools and rinsing chemical containers.
  • Follow pesticide labels for application rates and disposal. Some pesticides are toxic to fish at concentrations of less than one part per billion.


Call the nearest DNR field office or the 24-hour spill line at 515-725-8694 as soon as possible to report a fish kill. Quick reporting can help DNR staff identify the cause of a fish kill and potentially stop a fish kill in progress.