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Use of Seasonal Instream PIT Tag Detection Systems in Lower Clearwater and Salmon River Tributaries

Peter Cleary, Mike Ackerman

Nez Perce Tribe, 45559 Highway 12, Orofino, ID 83544

Instream PIT Tag Detection Systems (IPTDS) have been used during the past decade to estimate natural origin steelhead abundance in Snake River populations upstream of Lower Granite Dam. Annual IPTDS-based monitoring in the basin has typically targeted populations contained within a single tributary, meaning their abundance can be monitored using a single IPTDS (or weir) located in that tributary below major spawning areas. However, two particular steelhead populations, in the lower Clearwater and Salmon rivers, spawn throughout several smaller tributaries flowing separately into the mainstem rivers, making them difficult to monitor. To improve adult steelhead abundance monitoring, small-scale seasonal IPTDS have been deployed in six small tributaries throughout these populations from 2019 to 2023, each operating for 1-2 years. Weirs were operated in combination with the seasonal IPTDS in three of the six tributaries, allowing for comparison of the two methods; abundance estimates from IPTDS data analysis, and weir mark-recapture methods.  The results of the two methods were similar. Each method provides its advantages. Data from weirs provide some biological and life history data not available from IPTDS monitoring.  Vice-versa, IPTDS detected some individuals missed by weirs. The seasonal IPTDS additionally detected some juvenile fish utilizing the tributary while emigrating downstream that would otherwise have been unobserved without the operation of the seasonal IPTDS. Use of seasonal IPTDS throughout the lower Clearwater and Salmon rivers has proved useful to improve adult steelhead abundance and distribution monitoring of major and minor spawning aggregates, information necessary for ESA status and trends monitoring.