Steelhead/rainbow trout have one of the most complex life histories of any salmonid species, exhibiting both anadromous and freshwater resident life histories. Freshwater residents typically are referred to as rainbow trout, and those exhibiting an anadromous life history are called steelhead (NMFS 1998). The steelhead trout is an anadromous strain of rainbow trout exhibiting a general life cycle similar to Chinook salmon except that not all adults die after spawning and juveniles rear for longer periods in freshwater before migrating to the ocean. Viable naturally- produced runs of steelhead are found in the Sacramento River and some of its tributaries. Steelhead exhibit highly variable life history patterns throughout their range, but are broadly categorized into winter and summer reproductive ecotypes. Winter steelhead, the most widespread reproductive ecotype and the only type currently present in Central Valley streams
(McEwan and Jackson 1996), become sexually mature in the ocean, enter spawning streams in summer, fall or winter, and spawn a few months later in winter or late spring (Meehan and Bjornn 1991, Behnke 1992).
In the Sacramento River, adult winter steelhead migrate upstream during most months of the year, beginning in July, peaking in September, and continuing through February or March (Hallock 1987). Spawning occurs primarily from January through March, but may begin as early as late December and may extend through April (Hallock 1987). Individual steelhead may spawn more than once, returning to the ocean between each spawning migration. Spawning, egg incubation, and fry emergence occurs in a manner similar to that previously described for Chinook salmon. Peak emergence of steelhead fry occurs in the late spring or early summer.
Juvenile steelhead rear a minimum of one and typically two or more years in fresh water before migrating to the ocean during smoltification. Juvenile migration to the ocean generally occurs from December through August. The peak months of juvenile migration are January to May (McEwan 2001), although downstream movements of young-of-the-year steelhead have been reported in the lower Yuba River from late-spring through summer (HDR/SWRI 2007). The importance of main channel and floodplain habitats to steelhead in the lower Sacramento River and upper Delta is not well understood. Steelhead smolts have been found in the Yolo Bypass during the period of winter and spring inundation, but the importance of this and other floodplain areas in the lower Sacramento River and upper Delta is not yet clear. The specific timing of steelhead migration through the Delta is not well defined. Most of the outmigration probably
occurs during the wettest months. However, peak numbers of juvenile steelhead at the south Delta water export facilities (perhaps an indication of peak outmigration timing) occur during March and April (USFWS 1995).