As juvenile salmon Oncorhynchus spp. and steelhead O. mykiss migrate downriver to the ocean, they are subject to both natural- and human-induced mortality. Avian and piscivorous predators contribute to total natural mortality along with other factors (Vigg and others, 1991; Collis and others, 2001). Hydroelectric projects on the Snake and Columbia Rivers also can be major sources of mortality for migrating juvenile fish. Impoundments caused by dams may indirectly contribute to salmonid mortality by slowing the migration of juvenile salmonids (Raymond 1968, 1979; Plumb and others, 2006), thereby increasing energy expenditure during migration and allowing greater opportunity for predation. Furthermore, passage through dams can be a major source of direct mortality (Mesa, 1994; Whitney and others, 1997) that is cumulative for salmonid populations that negotiate multiple dams.
