Fish kill in Mason City pond

Dead fish line the banks of Black Pit in Lester Milligan Park in Mason City on Saturday. Officials say dying algae caused oxygen levels to drop drastically in the pond.

MASON CITY | Low oxygen levels were blamed for a second fish kill in North Iowa on Saturday.

Oxygen-deprived fish were spotted on Friday at Black Pit, a 16-acre pond in Lester Milligan Park in Mason City. Dead fish were found on Saturday.

More dead fish will likely be found Sunday after oxygen levels drop even lower overnight, said Iowa Department of Natural Resources Spokesman Kevin Baskins.

"When the sun is up (the lake) is still producing some oxygen," he said. "Once the sun goes down, that's when you'll really see the oxygen sag."

The low oxygen levels were caused by an algae bloom. Once the alage died, the decomposing plant material removed oxygen very quickly.

The same scenario was blamed for thousands of dead fish on Crystal Lake earlier this week.

Algae-filled water also can pose a danger to people. 

Dying algae produces a substance that can be toxic to humans and pets, so nasty-looking water should be avoided, Baskins said.

"When it really looks scummy (stay out), because a lot of times pets will go in there and ingest the water and they can get sick," he said. "In some cases we've even had pets die."

Officials don't know if Black Pit will recover on its own or need restocking. They'll continue to monitor the situation in the next few days.

State conservation officials posted a swimming advisory for West Twin Lake near Kanawha earlier this week due to an algae bloom.

It was one of 14 swimming advisories cautioning Iowans to stay out of the water at 11 state parks so far this summer.

Officials haven't said what impact the bloom had on West Twin Lake's fish.



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