Causing a stink: Low oxygen puts pond fish at risk

Saturday, August 13, 2005

Massive fish kills are common in summer months, conservationist says.

FRUITLAND, Mo. -- Mary Terry woke up Tuesday morning to a disgusting sight. Scores of fish, some of them about 3 feet long, were floating dead on the surface of the pond in her front yard.

Terry had a dirty job to do.

"I just couldn't take it," Terry said of the overpowering smell of fish carcasses rotting in the sun. "It smelled like somebody died. Actually, it was worse than that."

So Terry called the Missouri Department of Conservation, donned her rubber wading boots and some rubber gloves and started collecting dead fish with a leaf rake.

She filled five five-gallon buckets with the dead fish.

Terry has lived at her home just outside Fruitland for 25 years, but had never seen anything like this in her pond. Crawfish and muskrats damaging her levee was one thing, but such a massive fish kill was a surprise.

When Conservation Department fishery expert Brad Pobst saw the pond, he knew exactly what had happened. The pond's oxygen level had dropped too low, killing the fish.

"These kind of massive fish kills are almost always the result of low dissolved oxygen," said Pobst. "When the water gets warm in the summer, it just holds less oxygen."

Like land animals, fish too need oxygen to survive. Their gills filter dissolved oxygen out of the water. The fish in Terry's pond didn't have enough, so they suffocated.

Every year starting around July 1, the MDC's regional office in Cape Girardeau starts getting calls from its 16-county area about such fish kills. Overnight, a pond can be covered in dead fish. The larger fish go first, since they need more oxygen to stay alive.

The numerous huge grass carp floating on Terry's pond are an example.

Already this year the MDC has received about 30 calls of fish kills, said Pobst. There were even more last year, when a series of cloudy days prevented aquatic plants from producing as much oxygen as normal, said Pobst.

Several factors could contribute to depleted levels of oxygen, including large amounts of algae dying in warmer weather. But most of the time, these summer kills are the result of too many green plants in a pond -- the kind that rely on photosynthesis to survive.

Pobst said a danger level for plant coverage on a pond surface is about 30 percent. Terry's pond had 100 percent coverage.

Plant life is necessary for a healthy ecosystem, said Pobst, but only about a 15 to 20 percent coverage.

When plants cover a pond, two things occur to deplete oxygen. While they convert carbon dioxide to oxygen during photosynthesis, plants also use oxygen. When the sun goes down, the photosynthesis decreases and less oxygen is produced. The combination can deplete oxygen to dangerous levels for fish.

Just before dawn, pond owners can see the warning signs if their fish are in danger, said Pobst. If many fish are at the pond's surface seemingly gasping for air, that could mean the oxygen content is low.

In Terry's pond, her fish kill was caused by a particularly nasty plant called watermeal. Pobst said the plant can cover a one-acre pond in 30 days under the right conditions. Terry's pond was about a half-acre.

But pond owners do have proactive approaches that can be taken to prevent fish kills. Certain herbicides are safe for use in bodies of water, and aeration is a great help. But aeration can't be accomplished by simply pouring fresh water into the pond -- the water has to be a mist to gather more dissolved oxygen.

For Terry, it may be too late. The fish kill could be the last straw after having problems with muskrats and crawfish, too. She wants to break the levee.

"If I was rich, I'd get rid of that pond," said Terry.

The time may have come to do that. Pobst said the average productive life span of a pond is about 30 years; then they need extensive maintenance.

msanders@semissourian.com

335-6611, extension 182

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