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State warns about potential for fish kills from heat

A receding shoreline Fox River Kane County displays signs drought this week. | Michael Smart~Sun-Times Media

A receding shoreline on the Fox River in Kane County displays signs of drought this week. | Michael Smart~Sun-Times Media

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Updated: August 11, 2012 6:15AM

The Illinois Department of Natural Resources says the combination of low water levels and high temperatures puts fish at risk, especially in ponds and in river backwaters.

Those conditions have set the stage for fish kills this summer in water bodies from small ponds to large backwater lakes along large rivers in the state, according to a press release issued by the department.

The IDNR said it already has been responding to reports of summer fish kills from private pond owners and is anticipating more calls in the coming weeks.

Summer fish kills are reported almost every year, and most are caused by low oxygen levels in the water, the release said. 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. Algae blooms are also typical and further deplete oxygen levels in bodies of water, according to the department.

The IDNR Division of Fisheries receives thousands of 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, the release said, noting that 99 percent of 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 IDNR 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, so management may be needed to restore that balance, Stephenson added. Local IDNR fisheries biologists can be contacted by pond owners for recommendations if they experience a summer fish kill.

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, the release said. But 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 when it rains, according to the IDNR.

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