2021-2024 Juvenile Chinook Salmon Rearing and Growth on the Lower American River, California

The primary goal of the Central Valley Project Improvement Act is to at least double natural production of Chinook Salmon ( Oncorhynchus tshawytscha ) in California Central Valley (CCV) streams on a sustainable basis. Beginning in 2008, semi-annual gravel augmentation and off channel habitat enhancement projects were implemented on the Lower American River (LAR) by Reclamation and USFWS, in partnership with the Sacramento Water Forum, as part of the LAR habitat enhancement program. The overall vision for the habitat enhancement program is to restore ecosystem processes by rehabilitating and enhancing critical channel, floodplain, and riparian habitats for juvenile and adult anadromous salmonids, thus promoting the recovery of healthy and diverse Chinook Salmon and steelhead ( O. mykiss ) populations. Off channel rearing habitat has been identified as a limiting factor in meeting CCV salmon population goals. While numerous studies have documented positive relationships between off channel habitat and juvenile rearing on the LAR, it is not known how long juveniles remain in off channel habitat compared with the main channel, whether rearing in off channel habitat enhances growth compared with the main channel, whether the relative value of off channel habitat for juvenile production depends on how long it has been inundated, or whether juvenile diet in off channel habitat differs from that in the main channel. To better understand these relationships, we conducted a mark-recapture study under a range of hydrological conditions between 2021-2024 using passive integrated transponder (PIT) tagged wild juvenile Chinook Salmon coupled with seining surveys at five off channel restoration sites on the LAR designed for rearing, as well as nearby main channel control sites that were also suitable rearing habitat. First, we investigated whether side channel habitats in restored reaches support higher Chinook Salmon densities than main channel control habitats. Density was estimated as the number of fish captured per unit water volume during seine hauls. Our dataset of 4,886 captures from 2021 to 2024 revealed notable year-to-year variability. In 2021 and 2024, main channel habitats had slightly higher juvenile Chinook Salmon densities, while no significant differences were detected between restored and main channel control sites in 2022 and 2023. We also evaluated community composition by comparing the proportions of native and nonnative fish between habitats using overall community data from 2021 to 2024. Native species comprised 87% of the fish assemblage, with juvenile Chinook Salmon and representing 60% and 15% respectively. Non-native fish were rare, with a total of 207 individuals across all years of study. The majority of these were Wakasagi ( Hypomesus nipponensis ) that were present during the 2023 high flow year only. Although the Upstream Control site consistently exhibited higher O. mykiss densities compared to other sites; the restored sites maintained a consistently higher native to non-native ratio. Finally, we assessed juvenile Chinook Salmon growth and minimum residence time using a mark-recapture study of PIT tagged fish in 2024. Because only one tagged fish was recaptured from restored sites, we were unable to compare growth and residence between restored and unrestored habitats. Instead, we compared spatial differences between upstream and downstream main channel sites. Growth rates ranged from 0.47 to 0.51 mm per day, with no significant differences detected between upstream and downstream locations. Average minimum residence time for recaptured fish was 12.5 days at main channel control sites. In summary, our study documented juvenile salmonid utilization of restored habitats and provides growth rates and residence times for wild Chinook Salmon in the LAR, which will help resource managers understand in-river rearing benefits for this species and inform decision support tools and life cycle modeling efforts. Fish community data from restored habitats indicated improvements in native species composition and prey availability. Overall, these findings offer important insights to guide habitat modeling and management practices for in-river Chinook Salmon populations in the CCV.