Outmigration of Juvenile Chinook Salmon in the Lower Willamette River, Oregon

We used direct sampling and radio telemetry to describe the outmigration of juvenile Chinook salmon (Oncorhynchus tshawytscha) in the lower Willamette River downstream of Willamette Falls from 2000 to 2003. Juvenile Chinook salmon were present all year, with peak densities occurring in winter and spring. Small, naturally-produced (and therefore ESA-listed) fish were present in December and January, a period when in-water work (e.g. dredging) is authorized. Small fish were likely spring-run stocks that outmigrated as subyearlings. Juvenile Chinook salmon were significantly larger at downstream sampling sites, suggesting growth occurs, or larger fish entering from the Columbia River use this area as rearing habitat. Radio-tagged fish (>100 mm fork length) migrated at a median rate of 11.3 km/d, and hatchery fish migrated significantly faster than naturally-produced fish (12.4 vs. 8.4 km/d). Fork length and river flow were significant predictors of migration rate. Radio-tagged fish were distributed evenly across the river channel regardless of year, time of day, or origin (hatchery or naturally produced). Except for a possible affinity for pilings, the distribution of radio-tagged fish appeared to closely follow the proportional availability of nearshore habitat types, suggesting they do not select for specific habitats during their outmigration. We recommend that additional work focus on subyearling fish, which may have more specific habitat requirements and are more vulnerable to predation and other limiting factors. Considering the large number of subyearling juvenile Chinook salmon present during winter, restricting in-water work to July–October may help protect and recover these stocks.