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Steelhead
Steelhead is the term commonly used for the anadromous life history form of rainbow trout (Oncorhynchus mykiss). Steelhead exhibit highly variable life history patterns throughout their range, but are broadly categorized into winter and summer reproductive ecotypes. The relationship between anadromous and resident life history forms of O. mykiss is poorly understood, but evidence suggests that the two forms are capable of interbreeding and that, under some conditions, either life history form can produce offspring that exhibit the alternate form (i.e., resident rainbow trout can produce anadromous progeny and vice versa) (Shapovalov and Taft 1954, Burgner et al. 1992, Hallock 1989). The fact that little to no genetic differentiation has been found between resident and anadromous life history forms inhabiting the same basin supports this hypothesis (Busby et al. 1993, Nielsen 1994).
Steelhead found in the Napa River watershed belong to the Central California Coast evolutionarily significant unit (ESU) (NMFS 1997). This ESU extends from the Russian River to Aptos Creek, and includes tributaries to San Francisco and San Pablo bays eastward to the Napa River, excluding the Sacramento-San Joaquin River basin. Winter runs of steelhead occur in the Napa River mainstem and tributaries. Critical habitat is designated to include all river reaches and estuarine areas accessible to listed steelhead in coastal river basins from the Russian River to Aptos Creek, and the tributaries to San Francisco and San Pablo bays (NMFS 2000).
Source: Stillwater Sciences, Napa River Watershed Limiting Factors Analysis
