Species-Specific Spatial and Temporal Distribution Patterns of Emigrating Juvenile Salmonids in the Pacific Northwest

The vertical and horizontal distribution of juvenile salmonid migrants on approach to the dams influences bypass success in rivers. Accordingly, fish distributions have been studied for nearly three decades. These studies, however, have not been integrated and summarized in a single body of work to determine overall patterns in the spatial distribution of emigrants. We reviewed peer-reviewed and gray literature to summarize species-specific trends in the horizontal and vertical distributions of emigrating salmonids as measured by several different methods. We found that there were no species-specific differences in horizontal distributions and that fish were often oriented with the river thalweg. There were weak differences between species in vertical distributions, e.g., juvenile yearling steelhead were shallower during the day than yearling Chinook salmon. For sockeye, coho, and subyearling Chinook salmon, the data were limited or conflicting. Studies were purposefully designed to measure distributions at certain dams under particular environmental conditions for specific, local purposes. The non-standard sampling design has hampered the development of testable hypothesis on fish distributions in the Snake and Columbia rivers. Recent advances in individual-based models are offering the potential to forecast fish distributions near dams and facilitate improved bypass system design.