One Fish, Two Fish: Counting Salmon & Steelhead at the Rotary Screw Trap

Local fish populations give us clues to how our watershed is functioning as a whole.

What is swimming in our streams? By tracking local fish migrations and populations, biologists can find clues about the health of the river and our watershed.

Over 30 species of native and non-native fish have been observed in the Napa River through the use of a rotary screw trap. It can be tricky to track fish in the river; some species, like Steelhead, swim up the river to spawn at night or when the river flows high during storms. This makes it nearly impossible for scientists to track their movements.

The Rotary Screw Trap is a tool that helps scientists monitor fish in the river without disrupting their life cycles. The Rotary Screw Trap is an 8 foot cone shaped device that floats on the river. When lowered into the water, the current rotates the screw trap like a fan, funneling the fish into a holding area at the smaller end of the cone. The biologists at Napa County Resource Conservation District check the holding area for fish each day. They gather data and record observations about the the fish they find in the trap, including species, length, age, sex, and genetic samples. Some fish are marked or tagged for long term monitoring. After the scientists visually inspect the fish to assess overall health, they release them back into the river. The trap is installed during the peak migration season each spring, from March to May.

Biologists use the data collected from the rotary screw trap to estimate the populations of two important species of fish in our river: Chinook Salmon and Steelhead Trout. Salmon and Steelhead are a good indicator of the health of the watershed because they use all portions of the river and require clean, cool water. From the ocean, adult salmon and steelhead migrate up the Napa River to deposit their eggs into the gravel of the riverbed. After hatching, young salmon and steelhead live in the river for up to 3 years. When conditions in the river are right, the fish will migrate to the ocean where they will live another 3 years until they are mature enough to return back to the streams they were born to lay their eggs, or spawn.

How do the fish tell us that our river is healthy? When fish populations struggle to thrive, it is a sign that something is out of balance in our waterways. In California, the steelhead population is less than half the size it was 30 years ago. According to the Napa County Resource Conservation District, the major factor causing fish population decline is freshwater habitat loss and degradation. This results from three main factors: inadequate stream flows, blocked access due to dams, and human activities that add sediment and debris to waterways.  Natural landscapes gradually filter rainwater as it makes its way through the watershed, helping to preserve balance in watershed ecosystems. Farms and developed areas tend to move water more quickly through pipes and ditches, adding fine sediment and contaminants into streams and rivers. Slower water flow helps keep streams and rivers flowing longer through the dry season, increasing river habitat for fish. It also prevents erosion that causes excess sediment to be washed downstream, covering the gravel fish need to lay their eggs in. Contaminants in runoff from neighborhoods and farms can decrease the amount of oxygen in the water, making it difficult for the fish to breathe and develop as they grow. Contaminants can also contribute to algae growth and blooms that make waterways poisonous to fish, as well as people and pets.

What is good for fish is also good for people. Restoration work on the Napa River and the streams feeding into it helps improve water quality for fish populations. Preventing erosion that adds sediment to waterways improves spawning for fish and helps preserve farmland and prevent vineyard loss. Preserving riparian (waterside) areas helps filter and slow water naturally to prevent flooding and reduce contaminates downstream. Preventing harmful contaminants from entering waterways helps keep our water supply safe and clean. Monitoring fish tells us more than just what is swimming in our river; data gathered through projects like the rotary screw trap helps scientists determine the health of our watershed so we can keep our valley a healthy place for everyone to live.

Check us out! Books and resources available from the Napa County Library:
1). Run Salmon Run by Bobs & Lolo
2). The Magic School School Bus Goes Upstream: A Book About Salmon Migration by Nancy E. Krulik
3). La Migración del Salmón by Grace Hansen
4). The Salmon's Journey by Jon M. Fishman
5). Trout by Leo Statts

Go deeper! Explore these great resources online:
Fish Monitoring Factsheet
Fishbio Video: How Rotary Screw Traps Work
Napa RCD Video: Fish Diversity in the Napa River
Napa RCD: Steelhead Salmon
US Fish & Wildlife Services: All About Chinook Salmon

Click on the images below to learn more:



The location of the screw trap on this map is approximate. Each season it is placed on the river in a location above the tidal influence and accessed with permission on private property. Although you can't visit it on your own, Napa County RCD hosts tours in the spring, which are posted on their website.