DNR: Fish kill at Swan Lake due to extreme heat

Authorities with the Iowa Department of Natural Resources say a recent fish kill at Swan Lake near North Liberty is likely the result of extreme heat.

At least a dozen large fish appeared to be floating at the surface of the marsh on Swan Lake Rd. on Tuesday morning.

Paul Sleeper, the DNR's fishery biologist, said the fish likely died as a result of low oxygen levels in the water because of extreme temperatures and low water levels in the marsh. Because this particular marsh tops out at about five feet, he said the body of water would be more prone to such issues.

"It has just been so warm for so long," Sleeper said. "Usually we will have some cool nights that cool (the water) off a bit, but with nights in the 70's it doesn't have a chance to cool off at night and it just keeps getting warmer and warmer."

He added, however, that Swan Lake does not naturally inhabit fish, and the large fish floating at the surface -- which included Grass Carp, Bullheads and Buffalo fish -- likely entered the marsh during the 2008 floods or from people dumping fish into the marsh.

Tim Thompson, wildlife biologist with the DNR, said the kill in this particular body of water could actually be good for the area as Grass Carp -- one of the fish discovered on the surface -- eat away at aquatic vegetation that produces seeds for other animals in the ecosystem.

He added that hot temperatures can also cause plants to die off and decompose, removing even more oxygen from the water as a result of the decomposition process.

Though Sleeper said larger bodies of water in the area have normal water levels at this time, he added the DNR is concerned with naturally shallow areas, adding river levels in the area are already low.

"We're not worried  about some of our bigger lakes right now running out of water or anything," Sleeper said. "Certain species are cooler water species and more susceptible to the heat. We're just kind of keeping our fingers crossed we don't have any major issues in some of our bigger lakes."

He said the DNR dealt with a fish kill in a shallow area of the Coralville Lake, near the I380 and Highway 965 bridge, where water temperatures reached 90 degrees. Roughly 95 fish were lost in the kill.

Regardless, Sleeper said fish kills are not necessarily a common occurrence.

"We don't see this every year," Sleeper said. "We've seen it occasionally in the past but this is definitely an exceptional year with the temperatures."

Sleeper said the kill likely happened Monday evening, as the fish don't appear to have begun decomposing, and many of them had not yet been eaten by other animals.