DNR warns hot weather, low water levels likely will result in fish kills

Hot, dry weather and low water levels in Illinois will set the stage for fish kills this summer in water bodies from small ponds to large backwater lakes along large rivers. 

The Illinois Department of Natural Resources said calls are coming in from private pond owners about summer fish kills and more are anticipated.

Summer fish kills are reported almost every year and most are caused by low oxygen levels in the water. 

As temperatures rise and less rain falls, the water levels and oxygen levels drop in ponds, lakes, rivers and streams resulting in increased stresses on fish. 

Algal blooms are also typical and further deplete oxygen levels in bodies of water.

The DNR Division of Fisheries receives calls each year from private pond owners who notice dead fish in their ponds and assume that chemicals may have somehow entered their pond. 

This is seldom the case. 

Nearly all summer fish kills are due to the natural conditions that have reduced the oxygen levels below what fish can tolerate.

“Typically, the pond owner doesn’t notice anything unusual until one July through September morning, and then fish are either belly up or are gasping for air,” said Fisheries Biologist Dan Stephenson. “The largest fish are affected first. Generally, pond owners will see the large channel catfish die first, followed by bass then bluegill, and working its way down to the smallest fish as the oxygen levels get lower and lower.”
 
A summer kill seldom results in 100 percent mortality of the fish in a pond, but may throw the predator-prey relationship out of balance.

Stephenson said additional management may be needed to bring ponds back into balance.

Just like fish kill events in private ponds, summer kills happen in backwater lakes, rivers and streams as fish get trapped in pools that grow smaller and smaller as intense heat evaporates the water. 

Not much can be done to prevent the fish kill during times of persistent hot weather and lack of rainfall. 

Farmers and others who handle chemicals or animal manure can take precautions to prevent further impacts to surface waters and fish. 

They should check for discharges from chemical mixing stations and areas of livestock concentration to make sure those discharges cannot reach ponds and streams, especially in rainfall events.