For the second time in as many years, the Montana
Department of Fish, Wildlife and Parks is operating a fish weir on the
Madison downstream of Yellowstone with cooperation from Gallatin
National Forest and funding from PPL Montana.

The primary objective of the weir is to learn
about the characteristics of the fall rainbow trout spawning run" says
Pat Clancey from the FWP field office in Ennis. Most rainbow trout tend
to spawn in the spring, but the Hebgen Lake drainage has many rainbows
that spawn in the fall.

Via: West Yellowstone News LINK -

Spawning Trout Art by Shari Erickson