Effects of River Flow on Abundance of Young Chinook Salmon, American Shad, Longfin Smelt, and Delta Smelt in the Sacramento-San Joaquin River System

Annual abundance indices for young fall-run chinook salmon (Oncorhynchus tshawytscha), American shad (Alosa sapidissima), and longfin smelt (Spirinchus thaleichthys) increased directly with river flow rates during the spawning and nursery periods. Annual abundance of young delta smelt (Hypomesus transpacificus) did not vary with river flow. Several factors associated with flow could explain the relationships described for chinook salmon, American shad, and longfin smelt. The one factor common to all affected species was that dispersal of young increases when flows increase, which probably results in decreased density-dependent mortality.