River environment effects on adult migration phenology and rate of spring-run Chinook Salmon

ABSTRACT Objective Our overarching objective was to better understand how river environment affects the migration phenology and behavior of adult Chinook Salmon Oncorhynchus tshawytscha in a watershed (Oregon’s Willamette River basin) where climate warming and other habitat impacts threaten the spring-run population. Methods We analyzed migration phenology of annual spring runs using a 23-year time series of daily adult Chinook Salmon counts at Willamette Falls (river kilometer 42, measuring from the Willamette River–Columbia River confluence) in relation to river discharge and temperature data at a nearby gauge site. We also examined stock-specific phenology and upstream migration rates with general linear models using monitoring data from 909 radio-tagged Willamette River Chinook Salmon to explore the effects of river environment and fish traits on movement through 13 main-stem and tributary reaches. Results Willamette River Chinook Salmon runs migrated earlier in warm, low-flow years. Mean annual river conditions in May were the best predictors of median run timing dates, which ranged from early May to mid-June. Radio-tagged salmon moved upstream faster when river temperatures were higher and discharge was lower. Tagged salmon moved much faster (∼25–50 km/d) in low-gradient main-stem reaches than in the steeper tributary reaches (mostly <10 km/d). Individual fish traits, including stock of origin, were generally not statistically associated with migration rate after statistically accounting for water temperature and discharge. Phenology and migration rate results from the Yukon, Columbia, and Snake River basins broadly aligned with those from the Willamette River basin. Conclusions Our study results offer a mechanistic explanation for why adult salmon migrations occur earlier in warmer years across a broad geographic range. The results also suggest that some spring-migrating populations may continue to trend earlier, a behaviorally plastic response with uncertain implications. Of particular concern are the risks presented by increased adult freshwater residency for spring-migrating populations like upper Willamette River spring-run Chinook Salmon.