8/7/2007

Excessive Heat Taking its Toll on Some Fish Populations

High temperatures, including many days in triple digits, are starting to have an influence on fish populations in some North Dakota lakes.

Scott Gangl, fisheries management section leader for the state Game and Fish Department, said die-offs have been reported at Northgate Dam, Burke County; Kettle Lake, Williams County; Raleigh Reservoir, Grant County; Mooreton Pond, Richland County; and the Bois de Sioux River in southeastern North Dakota. Most of the dead fish have been trout.

“This isn’t surprising to us,” Gangl said. “The warm days, followed by warm nights over the last few weeks, have really warmed some of our lakes. Since trout are adapted to cold water, they can be very sensitive to warm summer water temperatures.”

It’s unknown at this point how many trout died, but preliminary investigations indicate the die-offs were substantial. “Our biologists will know more once they have a chance to net the lakes,” Gangl said. “However, despite the trout die-offs, we’ve heard of anglers catching bass, bluegill, and crappies at these lakes.”

Fish kills during summer are caused by a combination of high temperatures and low dissolved oxygen. Warm water holds less dissolved oxygen. Meanwhile, algae and plants that flourished early in the summer will die and begin to decompose, which consumes oxygen. “This combination creates a recipe for lethal conditions for fish,” Gangl said. “Trout are usually the first to die because of their sensitivity to warm water. But if conditions degrade far enough, more tolerant fish species will also die.”

Gangl asks if anyone sees fish mortality on area lakes to report it to a local Game and Fish Department district office.