Lake Pahoja Experiencing Natural Fish Kill
Posted: 09/14/2012
Lake Pahoja in Lyon County is experiencing an ongoing fish kill that likely began Wednesday night.  

Fisheries biologists from the Iowa Department of Natural Resources investigated the kill on Thursday at the 70-acre lake and found low levels of oxygen and elevated levels of ammonia.

Mike Hawkins, fisheries biologist with the Iowa DNR, said all the evidence points to fall turnover happening at a bad time as the cause.

“When a lake experiences turnover in the fall, nutrients that have been trapped in deeper water are suddenly available causing an algae bloom.  In this case, the turnover followed a series of warm days and reports of a heavy blue-green algae bloom, something that is not typical,” said Hawkins.

The two events together may have created enough biological demand for oxygen that levels fell critically low, he said.

“There are other instances when a fall turnover causes problems in a lake, but this seems to be a rare set of circumstances,” said Hawkins.

Although the fish kill is substantial, many live fish were seen by investigators.  Oxygen levels are beginning to rise and many of these fish will survive.

“It is really too early to determine how this fish kill will affect the fishery and fishing on Lake Pahoja.  Fish kills rarely kill all fish and fisheries normally fill back in very quickly through natural reproduction,” said Hawkins.

Lake Pahoja typically forms a thermocline that separates a zone of warm surface water from a zone of cooler deep water at about 12 to 14 feet.  This thermocline prevents the two zones from mixing during the summer.

It is not unusual for biological processes in the deeper water to use up all available oxygen and then begin to build up large amounts of nutrients.

As the surface water cools in the fall, a windy day can break up the thermocline causing the lake to mix, releasing large amounts of nutrients causing an algae bloom.