Genetic Stock Composition of Subyearling Chinook Salmon in Seasonal Floodplain Wetlands of the Lower Willamette River, Oregon

We used genetic identification methods to examine the stock composition of subyearling Chinook salmon Oncorhynchus tshawytscha in floodplain wetland and main-stem habitats of the lower Willamette River, Oregon. Using it microsatellite DNA baseline of 13 standardized loci and 30 Columbia River basin populations, we analyzed 280 subyearlings collected in winter and sprint, 2005-2006 from wetland and main-stern river sites. Genetic stock identification analysis indicated that spring Chinook salmon originating from the Willamette River made up a substantial proportion of the samples and contributed 16-71% to sample mixtures representing the wetland habitat sites. Fall Chinook salmon from lower Columbia River sources were also present and contributed 58% of winter samples. Spring Chinook salmon from lower Columbia River populations were present in both wetland (17%) and river (16%) samples in spring 2005, and subyearlings from summer-fall-run populations in the middle and upper Columbia River contributed to spring wetland samples in 2006 (26%). The results suggest that floodplain restoration projects intended to improve fish habitats during winter and spring periods in the lower Willamette River may benefit Chinook salmon populations from the upper Willamette River, lower Columbia River, and tipper Columbia River summer-fall evolutionarily significant units.