Fish Communities and Juvenile Salmon in Tidal Freshwater -- Implications for Restoration

We conducted research to characterize landscape-scale differences among juvenile salmon and resident fish communities in shallow tidal freshwater habitats (<5 m) of the lower Columbia River.  Approximately 25% of the overall catch was comprised of non-native taxa.  Species richness was similar across large spatial scales; however, there were periodic differences in fish community composition among sampling areas. Juvenile salmon were present in all habitats sampled throughout the year. Juvenile Chinook salmon (Oncorhynchus tshawytscha) were the most abundant salmon encountered in our study and varied in stock composition, marked (known hatchery) and unmarked status, size, and timing of migration. Seasonally, the highest Chinook salmon densities and the smallest average lengths were observed in spring.  The second highest densities for Chinook salmon were noted in winter, when there was a bimodal size distribution of Chinook salmon indicating temporal overlap of salmon life history strategies in tidal freshwater. The genetic stock composition of unmarked Chinook salmon varied temporally with spatially explicit patterns linked to location of capture within the river. We found very different stock compositions of unmarked and marked juvenile Chinook salmon.  It is clear that juvenile salmon are associated with shallow tidal freshwater habitats, although exploitation varies seasonally by factors such as stock and life-history stage.  The spatial and temporal patterns exhibited by fish encountered in our study suggest the need for year-round sampling to obtain a holistic understanding of life-history strategies and better evaluate the effectiveness of restoration in shallow water habitats.  Our work will continue to focus on elucidating differences in juvenile salmon use among habitat types to inform ecosystem restoration and planning in the Lower Columbia River.