Numerous dead fish have been observed in recent weeks along the Missouri River channel from Garrison Dam to south of Bismarck. The majority of these fish likely died from passing through the dam, Game and Fish Department fisheries biologists report.
Although many species are being observed, including walleye, most of the dead fish are cisco. With record releases coming from both the intake structure and spillway, just about any species can fall victim to what biologists call entrainment. “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, Department Missouri River System supervisor. “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.”
Entrainment through Garrison Dam is not uncommon and often fish are cleaned up by birds before they drift out of the Tailrace. However, higher flows in the Missouri River this summer means more fish passing through the dam, outnumbering what the abundant fish-eating birds can consume.
“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 out number other fish being entrained, because smelt far outnumber cisco in the reservoir.”
Fryda said fisheries biologists aren’t too concerned with the losses through the dam at this time. “The cisco are from the abundant 2007 year-class, and are now too big to provide much forage for predators in the lake,” he said.
Biologists are hopeful that as releases from Garrison Dam are ramped down in the weeks to come, far fewer of all species will be lost. In particular, the potential loss of smelt is a larger concern. “The smelt population in Lake Sakakawea has increased substantially in recent years and is currently spread throughout much of the reservoir due to abundant coldwater habitat,” Fryda said. “While we may lose some smelt through the intake structure, the entire population in Lake Sakakawea is not confined to a small area near the dam. Hopefully, what smelt are lost in the next six weeks will not adversely affect the population.”
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