Juvenile Salmon Ecology in Tidal Freshwater Wetlands in the Lower Columbia River Estuary

The lower Columbia River estuary is an important migratory corridor for threatened and endangered out-migrating juvenile salmonids. The tidal freshwater portion of the estuary provides nurseries for juvenile salmonids transitioning from freshwater to saltwater environments. This study examined the spatial and temporal occurrence of juvenile salmon in the tidal freshwater reaches of the lower Columbia River and characterized their genetic stock, condition, contaminant exposure, and origin (hatchery produced [marked] or presumed wild [unmarked]). Fish were sampled at 19 sites from 2008 to 2016. Chinook salmon (Oncorhynchus tshawytscha) were the most abundant species in all reaches; coho (O. kisutch), chum (O. keta), and sockeye salmon (O. nerka), and steelhead (O. mykiss) and cutthroat trout (O. clarkii) were also observed. Unmarked Chinook salmon juveniles were present throughout the year, while marked juveniles were present primarily from May through July and in highest proportions in the middle and upper reaches. Chinook salmon stocks from both the lower Columbia River and the interior Columbia River basin were present, with interior Columbia River basin stocks most abundant in the upper reaches. Chemical contaminant concentrations were generally lowest in salmon from the upper reaches, upstream of urbanized areas of the estuary. Our results reveal seasonal and spatial patterns in salmon habitat occurrence, provide baseline data for habitat restoration, and comparisons against future changes in anthropogenic conditions and climate.