Supplementary file 1_Intensive monitoring of an Interior Snake River steelhead population reveals complex life history diversity and highlights the challenges in assessing freshwater productivity.zip

Estimating viable salmonid population parameters (VSP, abundance, productivity, spatial structure, and diversity) of Endangered Species Act listed salmon and steelhead (Oncorhynchus spp.) in the Columbia River Basin is critical for assessing recovery efforts and the effectiveness of habitat restoration. We used two decades of data from a status and trends project (watershed-scale) and an experiment testing the effectiveness of stream restoration (headwater tributary-scale) to assess population parameters of summer run steelhead (O. mykiss) in Asotin Creek, WA. We assessed these parameters at the watershed-scale using screw trap and weir captures, and at the tributary-scale using passive integrated transponder tag interrogation systems and juvenile tagging. Asotin Creek adult spawner abundance varied from 2005-2023 (Average = 480, CV = 74.2, range = 83-1,260), and it consisted of > 97% wild steelhead over the last 10 years. We identified 24 life history strategies, which were dominated by fish spending 2 years in freshwater and 1–2 years in the ocean. Juvenile emigrants exhibited three movement pathways as they transitioned from headwaters to the hydrosystem: direct migration (no overwintering) and overwintering 1–2 years in the mainstem Asotin, or the Snake River, before entering the hydrosystem at Lower Granite Dam. Surprisingly, emigrant production from Asotin Creek persisted, even in years with low adult abundance and no adults entering the tributaries (average 44,550; CV = 31.1; range 22,587–73,749). There was evidence of density dependance as the emigrants/female spawning cohort decreased with an increasing abundance of females; however, there was high variability in the estimates. The diversity of life history traits (especially movement timing and age structure), high variance in population metrics, and equipment failure resulted in uncertainties in abundance estimation methods by influencing captures (weirs and screw trap) and remote detections (PIT tag arrays), which influenced population productivity estimates. However, the combination of capture and remote tag detection methods helped to identify these issues, better understand the Asotin steelhead population, improve our ability to assess the effectiveness of ongoing tributary restoration, highlight the difficulty of monitoring population productivity, and demonstrate the importance of decadal time series to understand the population status of steelhead.