A fish kill affecting adult rainbow smelt is ongoing in portions of the upper half of Lake Sakakawea.
Dave Fryda, Missouri River System fisheries supervisor for the North Dakota Game and Fish Department, said the cause of the die-off has been documented in the state numerous times in the past.
“The vast majority of the dead and dying smelt have physical signs of Columnaris bacteria,” said Fryda, who mentioned dead and dying smelt have been observed from Four Bears Bridge to about the Little Missouri River Arm, including the Van Hook Arm of Lake Sakakawea. In addition, he said there are likely some in other areas.
Columnaris bacteria are present in all water bodies, and outbreaks typically occur when rapid water temperature changes occur at a time when the fish are stressed, such as after spawning.
“The smelt recently spawned in Lake Sakakawea, and were recovering from that stress when we experienced very high temperatures which boosted the water temperature in the shallow bays where the smelt spawned,” Fryda said.
Smelt affected by Columnaris often develop visible skin irritations that have the appearance of fuzz or mold. Although there is no known cause for concern when in physical contact with these fish, Game and Fish suggests people leave the fish alone.
Lake Sakakawea had a similar smelt die-off in 2016, Fryda said, and in other instances in the past when the overall smelt population was very high. He said the current smelt population is the highest it’s been for decades, so that is likely part of the reason the bacteria has spread over such a wide area.
The effects on the population won’t be known until later this summer when fisheries crews assess the adult smelt population.
“Although the visible impression left by the dead smelt has anglers concerned, the overall impact of this year’s die-off will likely be minimal,” Fryda said. “With a population estimate approaching 200 million smelt in 2017, a high abundance of healthy smelt remains in the lake.”