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Illinois hunting and fishing

Chris Young/The State Journal-Register
Fish gasping for air may be a sign of oxygen depletion in ponds. Summer fish kills are common, especially in older, shallower ponds.

Fish kills haunt pond owners

July 28, 2010 at 10:01 PM


It’s a late-summer phenomenon that can make a pond owner’s heart sink.

Fish appear to be gulping for air at the surface of the pond, or they may show up dead around the shoreline for no apparent reason.

Alarm bells start to go off, and the pond owner frantically tries to determine what is going wrong.

Chances are it’s a common story.

In mid-to-late summer, high temperatures, a shallow body of water and dying algae combine to reduce oxygen levels in ponds to dangerously low levels. August is typically the busy month for fish kills, but this year they have started earlier than usual.

“With the unusually hot weather, there has been an increase in summer kills,” says Dan Stephenson, district fisheries biologist with the Illinois Department of Natural Resources.
Stephenson has been helping with administrative duties in the Springfield office lately, and he has fielded a number of calls.

“We received a number of reports statewide that I forwarded to the district biologists, and I’ve had quite a few in my district, too,” he says.

Stephenson says pond owners retire for the evening with no problems, only to wake up to dead or gasping fish.

“Ponds are at their lowest oxygen levels just before sunrise,” he says. “Pond owners are always astounded when I explain what’s happening.

“‘It’s never happened before,’” he says. “I hear it nearly every time.”

And if dead and dying fish aren’t enough of a shock, Stephenson says the big ones die first.

“Summer kills never get 100 percent of the total number of fish in the pond but generally do create a situation where the pond population is out of balance,” he says. “A summer kill gets the larger fish first, such as the grass carp (and) — if they are there — large channel catfish and bass

“The kill works its way to smaller and smaller fish, and the kill gets more severe.”

Why it happens
Just like people, ponds go through an aging process.

Fish kills often occur on older ponds that have become shallow as they fill with sediment over time.

“These kills occur on relatively small, shallow, old, fertile ponds or ponds that are older than 15 years in age and less than 12 feet deep,” Stephenson says.

One culprit is an overabundance of plankton algae, the microscopic plants that make pond water appear green — not the pond scum that is visible on the surface.

“These microscopic plants build up their numbers over the course of the summer, making the pond a pea-soup green,” he says. “With a few cloudy days or even overnight, these tiny plants die without sunlight and go to the bottom of the pond, where they begin to decompose.

“The bacteria that perform decomposition use up the oxygen, leaving little remaining for the fish,” Stephenson says.

The good news is summer kills only last a few days. But Stephenson says the factors that caused the kill don’t just go away.

Aeration can extend a pond’s life, but installing a pump and fountain to aerate a shallow pond may not happen quickly enough to stop a kill in progress.

“It makes a pond owner sick to see his fish population belly up,” Stephenson says.

But at this point, it may be best to break the dam, let the bottom dry out and then have the sediment removed and the pond deepened. This is equivalent to starting over, and it can be expensive.

Sometimes, though, there is little choice.

“If it’s happened once, it will likely happen again,” he says. “Ponds and lakes go through an aging process, and this is part of it that happens towards the end of the pond’s life.”

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