Fall is homecoming season for a variety of migrating fish species, including fall-run Chinook salmon on the Stanislaus River. After two or three years of living in the ocean (or even longer), the fish are now making a long journey back to the streams where they were born. In our version of rolling out the red carpet, we prepare for the salmon to arrive by putting up a fence: not to keep the fish out, but to make sure we can count every single one that returns to the river. The fence-like structure is a called a resistance board weir, and our crews have installed one on the Stanislaus River every fall for 12 years, thanks to support from the Oakdale Irrigation District, South San Joaquin Irrigation District, and Tri-Dam. The weir guides the fish swimming upstream through a single opening, where they are counted with the help of our Riverwatcher automatic fish counter (housed in the rectangular “cage” being constructed in the photo above). This is how we get the weekly numbers that we share with classrooms so they can graph the salmon migration in progress.
These tools lets us know exactly how many salmon have returned to the river, and allow us to track other species, such as rainbow trout/steelhead, striped bass, and even beavers. The Riverwatcher records photos and videos of every fish, which help determine fish sex, size, general health, and whether the fish came from a hatchery. We also use the information to help study how environmental factors like river flow and water temperature affect the migration of these fish. Once a salmon swims under the Golden Gate Bridge into San Francisco Bay, it still has to swim more than 150 miles before it reaches our weir. These fish are truly marathon swimmers!