Heat Stressing Some North Dakota Fish Populations

Much higher-than-normal July temperatures are stressing fish populations across the state.

Scott Gangl, fisheries management section leader for the North Dakota Game and Fish Department, said this summer biologists have documented fish kills in approximately a dozen water bodies throughout the state.

“Warm days followed by very warm nights are not conducive to maintaining healthy fish populations,” Gangl said. “Most fish need cooler water temperatures to thrive in summer, and they are just not getting it even at night. And with continued hot, dry and calm weather conditions forecasted, we expect to experience more fish kills in the next few weeks.”

Fish kills have been reported statewide. Some of the more notable water bodies and species impacted this summer include Stump (northern pike), lower James River (pike), Skjermo (yellow perch), Brewer (many species), Hoskins (many species) and Northgate (rainbow trout).

Gangl said fish species accustomed to cooler water, such as rainbow trout and northern pike, have been particularly vulnerable to warm temperatures in some locations. However, other species have also been affected as local conditions degrade into what biologists consider traditional summerkill conditions.

Traditional summerkills are caused by a combination of high temperatures and low dissolved oxygen. Warm water holds less dissolved oxygen. In addition, algae and plants begin to decompose, which also consumes oxygen.

According to Gangl, it’s important to note that though a lake may have suffered a fish kill, it doesn’t mean all species have died. In fact, most fish kills are only partial, killing only some of a few species.

Any observed fish mortality on area lakes should be reported to a local Game and Fish Department district office.