RICEVILLE - Several hundred dead fish turned up Monday in a seven-acre private pond about two miles north of Riceville.

Specialists with the Iowa Department of Natural Resources investigated but found no immediate cause for the fish kill. They noted dissolved oxygen levels were very low, however.

Oxygen was low where a small stream entered and where water left the pond. The water flows about a quarter mile to the Wapsipinicon River.

Dissolved oxygen levels upstream in the creek that feeds the pond and downstream in the Wapsipinicon River were well above the level fish need to survive.

"This one has us puzzled," said Cindy Garza of the DNR.

"Ammonia levels were normal. There was no sign of a pollutant or manure near the pond, and the algae was not dying off, which might cause oxygen levels to drop as the algae decays," she added.

"We did see a slight sheen on the water, which might indicate fuel or a petroleum-based product such as some pesticides," Garza said. " … The area has had several inches of rainfall in the last week, which could have washed something into the pond."

The landowner reported the fish kill after noticing the situation over the weekend. The fish were from 2 to 18 inches long.

DNR officials are testing water samples for pesticides, biological oxygen demand and ammonia and expect results back within a week.