Earlier this year there was a die-off of rainbow smelt in Lake Sakakawea.
Although the die-off appeared to be throughout the reservoir, it was not without precedence. Biologists report similar die-offs of smelt have occurred in Lake Sakakawea.
It is believed that this year's die-off was not persistent enough or lengthy enough to significantly affect the rainbow smelt population in the big reservoir.
Now, however, another anticipated problem has attracted the attention of fishermen and biologists alike. Numerous dead fish have been seen in the Missouri River below Garrison Dam, with many showing up as far south as Bismarck.
According to a release by the N.D. Game and Fish Department, the majority of these fish, including walleyes, likely died from passing through the dam. Record releases have been made in recent weeks both through the Garrison Dam intake structure and from the spillway gates.
"Entrainment, when fish pass through or over a dam or spillway, is often lethal to some fish species due to the extreme pressure change from quickly moving from deeper water to the surface, or simply the traumatic forces of expulsion," said Dave Fryda, Missouri River System supervisor for the department. "As a result, the highest densities of dead fish are right below the dam, with numbers decreasing downstream as fish decompose or are eaten by birds and other scavengers."
Dead or stunned fish have been common in the Tailrace area every year. However, a major change this year is that flows are much higher. Higher flows means more fish are passing through the dam more than terns, gulls and other birds can consume. That means more dead fish floating downstream.
"Entrainment has likely been occurring all summer, but we've seen a spike with the coldwater fish species in recent weeks," Fryda said. "As Lake Sakakawea has stratified, those fish are simply the most likely to encounter and move through the dam intakes. We can't explain why cisco far outnumber other fish being entrained, because smelt far outnumber cisco in the reservoir. The cisco are from the abundant 2007 year-class, and are now too big to provide much forage for predators in the lake."
Fryda said he is not too concerned about Sakakawea's loss of fish at this time, but will continue to monitor the situation. A big plus is the fact that smelt numbers have rebounded well recently and that they are spread throughout the entire reservoir.
"Hopefully, what smelt are lost in the next six weeks will not adversely affect the population," Fryda said.