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ID: 2571

Pinnix, W. D., S. P. Boyle, T. Wallin, T. Daley, and N. A. Som. 2022. Long-Term Analyses of Estimates of Abundance of Juvenile Chinook Salmon on the Trinity River, 1989-2018. Arcata Fisheries Technical Series Report TS 2022-40, report for the Trinity River Restoration Program (TRRP). U.S. Fish and Wildlife Service, Arcata, California. Available:

This report presents long-term analyses of smolt abundance data from two monitoring sites on the mainstem Trinity River, California near Pear Tree Gulch (2003-2018), and near Willow Creek (1989-2018), that were established by the Trinity River Restoration Program. We present trend analyses and production estimates (juveniles-per-spawner [JPS] and biomass-per-spawner [BPS]) for naturally produced juvenile Chinook Salmon (spring and fall-run combined), and trend analyses of hatchery produced juvenile Chinook Salmon. The relationship of these population parameters following implementation of spring hydrograph, or Record of Decision (ROD), discharges from Lewiston Dam is explored for pre-ROD (1989-2003) and post-ROD (2004-2018) time periods. Trend analyses from the Willow Creek site indicate a long-term positive trend in juvenile Chinook Salmon abundance. A model incorporating a change-point at 2004 fit the data better than a simple linear trend, which resulted in no evidence of trend (positive or negative) on either side of the change point, and similarly, no evidence of trend in abundance at the Pear Tree site was detected. Analyses of JPS at the Willow Creek and Pear Tree trap sites resulted in evidence for density dependent response at both sites and an increase in JPS following implementation of spring ROD discharge from Lewiston Dam. Although no evidence for trend in JPS was found at Willow Creek following 2004, evidence for positive trend in JPS was found at Pear Tree. We suggest that this increase in productivity is due to an increase in carrying capacity of the system due to increases in habitat available to juvenile Chinook Salmon during the spring emigration period. Analysis of biomass estimates of juvenile Chinook Salmon showed a weaker response post-ROD BPS, suggesting that although number of juveniles has increased, growth of juvenile Chinook Salmon may be muted after implementation of ROD flows, perhaps due to low water temperatures during critical growth periods (i.e. May-June). Weekly mean fork length of juvenile Chinook Salmon, both natural and hatchery origin, was smaller post-ROD. Apparent survival from Trinity River Hatchery to Willow Creek increased following ROD implementation, but no evidence of trend was apparent post-ROD at Pear Tree or Willow Creek.

First Posted: 2022-09-14 23:31:56

Post Updated: 2022-09-14 23:31:46