Restoring habitat is only half the equation; knowing whether it's actually working for fish is the other half. CalTrout's South Fork Eel River Science & Monitoring Program was built to close that gap. Rather than relying on older methods that track only adult salmon returns, the program shifts the focus to juvenile Coho Salmon, Chinook Salmon, and steelhead, measuring their growth, survival, and movement through the watershed to understand how geology, land use, drought, and restoration work together, or against each other, to shape recovery.

To implement the program, CalTrout partners with UC Berkeley (including a joint CalTrout/UC Berkeley scientist position held by Dr. Gabe Rossi), Cal Poly Humboldt, NOAA's Southwest Fisheries Science Center, the California Department of Fish and Wildlife, the National Marine Fisheries Service, the Wiyot Tribe, and environmental consulting firms Stillwater Sciences and McBain Associates. The toolkit blends old and new. At the center is a fish weir, a technology with ancient roots, engineered around pikeminnow behavior: downstream migrants, including out-migrating salmon and steelhead smolts and spawned-out steelhead kelts, pass through freely, while fish moving upstream are herded into a pen where they can be sorted, native species released and non-native pikeminnow removed. That design takes advantage of research showing pikeminnow migrate seasonally within the system, dropping into deeper mainstem pools each winter before returning upstream to forage each summer.

Around that weir, the program layers three additional technologies. ARIS sonar camera stations, now in their third year of operation on the South Fork with two more stations on the mainstem and Middle Fork, use sound to document migration timing and population size across the basin, day or night, in water too turbid for a camera to see through. Juvenile Salmon Acoustic Telemetry System (JSATS) tags, miniature transmitters implanted in young salmon, are picked up by a network of receivers placed throughout the watershed, letting researchers track how fast fish move, how many survive the journey downstream, and where along the way they run into trouble. And where technicians once spent hours manually reviewing weir camera footage frame by frame, AI-powered counting algorithms now identify and tally fish automatically, turning what used to be a bottleneck into a fast, consistent stream of data.

A key thread of the research targets the Sacramento Pikeminnow, a species native to California but invasive in North Coast rivers like the Eel, where it preys on juvenile salmonids, competes with them for food, and disrupts their behavior. Since Sacramento Pikeminnow can grow large enough to consume trout up to 20 inches long, understanding pikeminnow abundance, distribution, and movement is critical to figuring out whether, and how, suppression efforts could help native fish recover. With CDFW funding supporting the weir's construction in spring 2023 and continued operation since, this science program is quietly generating the evidence base that restoration projects across the North Coast, and the Eel River specifically, depend on to know what's actually working.