The bulky contraptions float listlessly downstream of three dams in Oregon’s Willamette Valley. The buoyant, metal devices hold large screws that the water flow turns. This twist of the screw – creating a creaking, rasping, scraping sound – generates enough hydraulics to keep small fish from escaping the slowly spinning, cone-shaped collectors – or, screw traps.
“They funnel fish from the upstream end of the cone where they come in at, back into a live box at the rear end and that allows you to collect the fish for sampling,” explained David Trachtenbarg, Portland District, U.S. Army Corps of Engineers fish biologist.
The Corps placed these traps below Cougar, Lookout Point and Big Cliff dams this year to help fish biologists collect data about fish passage at these tall dams. Downstream fish passage is difficult because dam height, combined with reservoir-level fluctuation, makes downstream fish passage difficult – an issue the Corps is working to fix.
“Operating the screw traps will allow us to get an indication of whether the operation that we are doing at a given project is providing that benefit,” said Brad Eppard, Fish Passage Section chief. “Is it getting fish past the dam.”
Screw traps are rudimentary tools that give Corps biologists basic data about the fish and the Corps will use them for the next couple years. The data is partially influenced by other factors, including traps only spans a small portion of any water body and they are designed to primarily capture smaller fish that cannot swim out of the device.
“It’s a way to look at and to evaluate migration timing,” said Eppard. “From a big picture perspective, it’s to look at things like run-timing, to examine fish condition and to collect what species of fish are out there.”
Fish biologists check the screw traps daily to examine, categorize, measure and release any fish caught in the traps and they are especially keen on Chinook and steelhead salmonoids. Even though the screw traps are basic tools, they do provide useful insight. They allow for Corps staff to estimate how operations at these dams are impacting fish passage and what changes the Corps can make with the goal of increasing passage success.
Corps biologists plan on using screw traps through at least 2022 to evaluate fish migration and may also use more refined methods, such as active tag studies to help give more precise data on fish passage, depending on funding.
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