Metal picket fences are built across streams by Idaho Fish and Game staffers several times a year to assist with fish sampling, Fish and Game Fisheries Biologist Kat Gillies-Rector said.
These picket weirs capture and sample migrating salmon and trout, she said. Picket weirs were used for centuries to capture fish for subsistence, commercial and scientific purposes. Native Americans used picket weirs in streams and small rivers to capture migrating salmon for food. Similar structures have also been used to capture returning salmon for commercial harvest and brood stock at hatcheries, Gillies-Rector said.
A picket weir is made of two panels of pickets that fence off a stream and funnel fish to a trap box. Fish swim to the panels, then swim along them until they find the opening to the trap box. Fish that swim into the trap are unable to swim back out because of “fingers” placed on the opening to the trap. When biologists arrive, they close off the opening with more pickets, then net fish out of the box to take data, she said.
Four weirs are placed in the Upper Salmon River each fall to capture and tag bull trout. The captured fish are weighed, measured and tagged with a passive integrated transponder. The tags each can identify the fish when they are captured in the future and when they swim over permanent antennas located on the river.
Fish biologists started using PIT tags in the 1980s to track salmon and steelhead in the Columbia River. Now PIT tags are used widely to track animals on land and in water, Gillies-Rector said.
Bull trout can make large spawning migrations, sometimes more than 150 miles. In the Upper Salmon River, many fish spend most of the year living in the river or a large nearby lake. During high water they move into a small streams where they wait to spawn in the fall. After spawning, trout swim downstream and into the weirs.
Operating weirs on spawning streams helps Fish and Game monitor bull trout spawning populations. Biologists find out when fish spawn, how many are spawning and whether some spawn many years in a row, she said. They also allow biologists to track whether habitat restoration and stream reconnection projects improve resident trout populations.