Outmigration behavior and survival of juvenile Chinook salmon (Oncorhynchus tshawytscha) in response to deep drawdown of the Lookout Point Project, Middle Fork Willamette River, Oregon

Abstract

An acoustic telemetry study was conducted during August 2023–February 2024 to evaluate outmigration behavior and survival of juvenile Chinook salmon (Oncorhynchus tshawytscha) in the Middle Fork Willamette River, Oregon, during an experimental operation that was designed to facilitate downstream passage through two reservoirs and two dams. The experimental operation consisted of lowering the water surface elevation of Lookout Point Reservoir by nearly 100 feet between August and December 2023, and passing water through regulating outlets at Lookout Point Dam. This operation was intended to reduce residence time for juvenile Chinook salmon in Lookout Point Reservoir so that these fish would enter the free-flowing Willamette River as quickly as possible. During our study, acoustic-tagged juvenile Chinook salmon were released weekly during late August to late October to determine how fish responded to the drawdown. Data collected during the study were analyzed using a temporally stratified multistate mark-recapture model. We found that Lookout Point Reservoir became isothermic during the drawdown and water temperature exceeded 18 degrees Celsius during most of September 2023. This appeared to adversely affect juvenile Chinook salmon because the proportion of tagged fish that were subsequently detected in the forebay of Lookout Point Dam following release at the head of Lookout Point Reservoir during August 30–September 29 ranged from 0.01 to 0.05 for weekly release groups. Detections increased to 0.44–0.52 for fish released later in the year when water temperatures decreased. We found that fish size was a significant predictor of survival as fork length was positively related to survival probability in reservoir and free-flowing river reaches of our study area, but negatively related to survival probability for fish passing Lookout Point Dam. We also found that increased regulating outlet flow at Lookout Point Dam resulted in increased survival probability for juvenile Chinook salmon and water temperature was inversely related to survival. Results from this study suggest that the drawdown failed to create conditions that facilitated downstream passage and survival of juvenile Chinook salmon through the Lookout Point Project. Our analysis provides insights into several key factors that influence survival. This information can be used by resource managers when considering revised operations that may lead to improved outmigration survival in the future.