Hot, cloudy weather can spell trouble for pond fish

The Kansas City Star
Associated Press

When Mary Terry checked in on her half-acre pond earlier this month in southeast Missouri, she was confronted by hundreds of floating, dead fish.

“It was devastating for me. I’d never seen anything like that in my life, or smelled anything like it,” she said.

What happened in Terry’s waters happens about 200 to 400 times a year in privately owned Missouri ponds, according to the Department of Conservation. A number of fish die at once, often because they’re not receiving enough oxygen through the water.

There’s no indication that the fish kills are on the rise this summer, officials said. Statistics won’t be compiled until the end of the year. But Missouri’s drought conditions certainly aren’t helping matters. Hot days combined with cloudy ones can mean trouble.

Usually, fish in a pond take in dissolved oxygen through their gills, but that can be more difficult for fish in a shallow pond in hot weather. That’s because it’s harder for a gas to stay dissolved in a liquid when it’s warm, explained Danny Brown, a fisheries management biologist.

Aquatic plants, which usually produce more oxygen than they need, produce less on cloudy days, and they use up oxygen at night. Several days of overcast weather also can cause pond algae to die. Algae also use up oxygen as they decay.
All that can mean less oxygen for fish in a pond, and if they don’t get enough, they’ll die.

Terry used a leaf rake to pull the dead fish out of her pond, located on her property east of Fruitland. She filled five, five-gallon buckets with their remains. Her pond was covered in watermeal, a free-floating plant that can block sunlight, reduce oxygen and upset a pond’s natural balance if it grows too densely.

“I can’t hardly even look at it,” she said of her pond. She’s considering filling in the pond, if she can afford it. Making it safe for fish again would require too much work, she said.

Not all ponds are equally at risk.

Deep ones usually provide fish with cold layers of water holding more oxygen. Standing vegetation – which can filter runoff, nutrients and silt – is usually preferable to a mowed, fertilized lawn at a pond’s edge and algae growth should be monitored to make sure it’s not excessive, explained Leanna Zweig, the resource scientist who investigates fish kill and pollution for conservation. The right sizes and numbers of fish are also important.

The conservation department has resources to help people with questions about their ponds.

A store and online business based in St. Louis County, called PondMarket, fielded more questions about fish ponds following some recent storms than due to this year’s drought. That’s because many pond owners in the region have electric devices, pumps or products that circulate water, helping add more oxygen into pond water. When storms led to temporarily lost electricity, some pond owners were anxious that their fish weren’t getting enough oxygen.

One warning sign: “The fish seem to be at the surface and seem to be gasping for air, which they are,” explained Brigitte Burchett, PondMarket’s president.

Some products can add more oxygen temporarily, she said, but overall education to maintain a proper pond balance is key. She has dealt with customers who come in after the fish have died.

“It’s really heartbreaking,” she said.