In 2021, the U.S. Geological Survey (USGS) focused adult salmon survey efforts in the Snake River on deepwater redd searches and fish collection for parentage-based tagging (PBT) analyses. We use used a boat-mounted underwater video camera to count 93 deepwater redds at 17 of the 28 sites surveyed. Redd depths averaged 3.9 m. In conjunction with the Idaho Power Company, we collected genetic samples from 346 live fall Chinook salmon (Oncorhynchus tshawytscha) and 15 carcasses at 48 unique geographic locations that spanned 90 river kilometers. Seventy-two fish were recovered at Eureka Bar (river kilometer [rkm] 307.1) and High Range (rkm 332.3), which accounted for 20% of all collected fish in 2021. Most (291 fish) post-spawned salmon were collected during the first two weeks of November around the peak of spawning. A summary of 2021 PBT results produced by the Idaho Power Company can be found in Appendix A.2.

Beach seining and PIT tagging of subyearling fall Chinook salmon was conducted in Snake and Salmon rivers to obtain information on population metrics and growth as well as to provide data for ongoing life-cycle modeling. In the Snake River, we collected 13,710 subyearlings, tagged 6,299, and recaptured 981 (15.6%). Using 8-mm tags in 45–49-mm fish allowed us to represent an additional 40% of the juvenile population through PIT tagging beyond just using standard 9- and 12-mm tags. In the Salmon River, we captured 103 natural subyearlings with the majority (60%) of fish being captured at two sites: rkm 11 and 20. We tagged 27 subyearlings, and no fish were recaptured. In Lower Granite Reservoir, we captured 4,887 subyearlings, PIT tagged 2,585, and recaptured 312.

Many of the subyearlings we tagged in the Snake River were detected passing Lower Granite Dam, but no fish tagged in the Salmon River were detected. In total we detected 673 (7.6%) tagged fish at Lower Granite Dam, and detection rates varied by tag size and passage route. More subyearlings were detected passing via the removable spill weir (RSW) earlier in the season while more fish were detected passing through the juvenile fish bypass system (JBS) later in the season. In general, fish tagged with 12-mm PIT tags had higher detection rates than fish tagged with smaller tags. Survival to Lower Granite Dam was low and ranged from 0.195 to 0.228. Growth of subyearlings was similar between riverine and reservoir reaches but was slightly lower in Lower Granite Reservoir. Only 19 subyearlings were recaptured at Lower Granite Dam in early autumn that predominantly originated from the Clearwater River (1 fish was from the Snake River). Mean standard deviation (SD) fork length and mass growth rates were 0.92±0.10 mm/d and 0.06±0.01 g/g/d, respectively.