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Hot weather means more fish kills

September 03, 2011 at 03:27 PM

The State Journal-Register

This summer’s hot weather has contributed to a higher number of fish kills.

Dan Stephenson, a fisheries biologist with the Illinois Department of Natural Resources, said there are no official stats on fish kills, but his calls are running at least 25 percent higher than normal.

“I had two or three today,” he said Friday. “It’s been a bad year.”

Summer kills almost always are caused by a lack of oxygen in the pond.

Stephenson said callers frequently think a chemical spill was responsible.

“They always assume the worst,” he said. “They assume there is a chemical spill that has come over from the neighbor’s property, or an aerial sprayer has been spraying the neighbor’s field and there’s been drift.

“They’re mad, and they want to sue their neighbor,” Stephenson said. “So I walk them through the steps.”

What type of pond is affected?

“Typically, summer kills occur on relatively small ponds of less than two acres, that are less than 12 feet deep, more than 15 years old and highly fertile,” he said.

What causes a fish kill?

“Low oxygen levels are the reason,” Stephenson said. “Water holds less oxygen as the temperature rises.”

Add to that heat stress on fish, and the bacteria that are part of natural decomposition processes consume oxygen at a time when levels are at their lowest.

Are there any warning signs before the fish start to die?

“The pond owner has little or no warning that there is a problem,” Stephenson said. “He comes out in the morning to find dead fish or fish on the surface gasping for air — a behavior called piping.

Are the dead fish large or small?

“One of the first things I ask is, ‘Are the dead fish large or small?’ Low oxygen tends to affect the larger fish first, and as the situation gets worse, it works it way to the smaller and smaller fish.

“Whereas a chemical kill gets the smaller fish first and works its way to the larger fish as the chemical concentration increases.”

Are all the fish killed?

“A summer kill never gets 100 percent of the fish, but usually leaves the predator and prey population out of balance,” he said.

What’s next?

“Two things need to happen then,” Stephenson said. “If the kill is severe enough, I recommend the pond owner finishes the kill with a fish toxicant called rotenone.

“I have to issue a permit so the pond owner can purchase it, and then I come out to apply it,” he said. “In a couple of weeks, the pond is ready to restock with the correct species in the right balance.”

How can another kill be prevented?

“The pond owner will need to install an aeration system because once a low-oxygen summer kill occurs, it is likely to occur again,” Stephenson said. “It is all part of the aging process.

“An aeration system slows the aging process, but of course doesn’t reverse it. A low-oxygen event is a symptom of a deeper problem that will need to be addressed if the pond is to continue to function.”

Chris Young can be reached at 788-1528.

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