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Idaho Fish and Game

A day at a rotary screw trap


Winter Blog 2023

Idaho Fish and Game biologists spend a lot of time collecting information about Idaho’s salmon and steelhead populations. These fish travel huge distances over their lifespan, so understanding those migrations are key to managing healthy populations. One of the best ways to do that is by collecting young fish as they leave their home streams on their journey to the ocean.  

Young salmon and steelhead heading to the ocean are called smolts, others are called parr. In many places, Fish and Game uses a rotary screw trap, which is basically a big cone that sits on pontoons floating on the river. When the cone is lowered into the water, the trap can collect some of the smolts as they swim downstream. The young salmon and steelhead scooped up in the cone are passed into a tank at the back of the trap.

Biologists then remove the captured fish to study them. Fish caught are counted, measured and tagged with a PIT tag before being released back to the river to continue their journey. Screw traps are an important tool to monitor the juvenile migration and estimate how many fish are passing the trap, and help understand if populations are healthy. 

The PIT tags allow biologists to follow young salmon and steelhead on their journey to the ocean and back again years later. The trap must be checked every day!

Want to learn more about Idaho’s wild salmon and steelhead? 

Here’s another story about how screw traps work. Find more articles on Fish and Game's Wild Salmon and Steelhead page.