Army Corps continues to investigate 30 rainbow trout found dead near Idaho dam

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The US Army Corps of Engineers is continuing to investigate the death of approximately 30 rainbow trout near Idaho’s Dworshak Dam on the North Fork of the Clearwater River.

Fishermen first noticed dead and injured fish near the dam and alerted Idaho Fish and Game biologists, who went to assess the situation on December 23. Idaho Fish and Game then alerted the corps.

Dead and injured rainbow trout were found in the dam tailrace, the water channel moving downstream of a dam.

The US Army Corps of Engineers is investigating numerous rainbow trout fatalities on the North Fork of the Clearwater River in Idaho.

Laura Mahoney/USFWS – Pacific Region, Flickr

“We, as fish managers and our anglers, don’t like to see dams kill fish,” said Jonathan Ebel, fisheries biologist with Idaho Fish and Game. “We don’t see fish kills in the tailrace of the dam unless they are caused by the dam.”

However, the deaths and injuries to the steelhead could have other causes, according to an email from Dylan Peters, public affairs specialist for the Army Corps of Engineers’ Walla Walla District.

For example, fish could be dead or injured in the tailrace or in parts of the dam that were not in use earlier in the year. Then, Peters said, the fish could have been pushed to the surface when the dam’s turbine started.

Similarly, coho salmon that swam from the ocean in 2021 could have died in the area of ​​the non-functioning dam. Those fish carcasses could have been pushed to the surface when the turbine started up and misidentified, Peters said.

Ebel said fish kills and injuries at Dworshak Dam are quite rare but not unheard of, occurring every two to three years at the dam.

A major kill occurred in 2016, when the Army Corps discovered that non-routine turbine maintenance had killed 200 rainbow trout in the dam’s tailrace.

The 2016 deaths prompted the Corps to update its turbine testing procedures at Dworshak Dam, Peters said.

Earlier fish deaths appeared to occur when rainbow trout swam through tubes leading to the turbine when water flows were low, Peters said. The fish would then run into the turbine on start-up.

To help reduce mortalities after 2016, the Army Corps installed compressed air to lower the water level below the turbines, which creates a vacuum to prevent fish from reaching the turbine, Peters said. . As the turbine ramps up and reaches full speed, the water eventually becomes too high for rainbow trout to reach the turbine.

“No fish kills of this magnitude have been observed since the startup procedure was put in place in response to the 2016 incident,” Peters said.

A few dead fish were found in 2017 and 2018, Peters said.

After a preliminary investigation into the 30 recent rainbow trout fatalities, the Army Corps found that operators had correctly followed all turbine start-up procedures, which aim to reduce, but not eliminate, mortality fishes.

Adult rainbow trout migrated last summer from the ocean to the river system to spawn. By December, rainbow trout were wintering and would have started migrating upstream to spawn in the spring, Peters said.

To help reduce fish kills at the dam, Ebel said Idaho Fish and Game would like the Army Corps to limit unnecessary turbine switches until it knows exactly why fish are being injured and killed at the dam.

“Nobody likes to see that happen,” Ebel said.

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