San Francisco Bay serves as a migratory pathway for two anadromous salmonid species: chinook salmon (Oncorhynchus tshawytscha) and steelhead (O. mykiss). Williams (2006) stated, “Chinook salmon and steelhead have highly variable life-history patterns, with age at spawning in Chinook varying from one year to seven years, and age at emigration to estuaries or the ocean ranging from a few days to two years. Steelhead have even more variable life histories and may omit ocean rearing altogether…” Both species spawn in gravel-bed, freshwater streams. Juveniles return (as smolts) to the ocean. A biologically profound difference between the two species is that chinook die after spawning once (semelparous), whereas steelhead have the capacity to survive the spawning run, return to sea, and spawn again in future years (iteroparous) (for recent reviews of biological literature see Williams 2006, 2012; for recent data and discussions of out-migration of juveniles through San Francisco Bay see Hearn et al. 2010, Jahn 2011a, Hearn et al. 2013). The following salmonid evolutionary significant units5 (ESUs) (NOAA Fisheries 2005a) have the potential to be near the project site.
Central California coast steelhead trout (Oncorhynchus mykiss) Federally Threatened,
California Species of Special Concern.Steelhead populations in what is known as the Central California Coast “evolutionarily significant unit” are listed as threatened under the Federal Endangered Species Act (FESA). Anadromous rainbow trout, or steelhead, occur in California from the Smith River in Del Norte County south along the coast to San Mateo Creek, San Diego County, and in streams of the San Francisco Estuary and Central Valley (Moyle, 2002).All Central Valley steelhead are considered winter steelhead. Busby et al. (1996) wrote, “Steelhead within this ESU have the longest freshwater migration of any population of winter steelhead. There is essentially a single continuous run of steelhead in the upper Sacramento River. River entry ranges from July through May, with peaks in September and February; spawning begins in late December and can extend into April.”
The timing of steelhead outmigration is less well known, but trawl capture data are consistent with a late-winter and spring migration (Jahn 2011a). Steelhead smolts, like chinook smolts, migrate mainly in deep water, but steelhead tend to wander into shallow water more than chinook (Jahn 2011a). Thus, there is some chance that steelhead, especially of the coastal ESU that spawn in some south Bay tributaries, may enter the Brisbane Lagoon in small numbers. Because numbers are low, and most smolts are