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Migration
Steelhead spend part of their life in freshwater and part in saltwater, and therefore face a complex set of environmental and physiological challenges. Over time, steelhead have adapted to cope with changes in the natural processes which have shaped their evolution. In the Napa River basin, adult steelhead spawning runs typically begin in January and extend through April, depending on early and late season stream flow patterns. Fish that make the upstream migration early in the season (December - January) have the advantage that their young emerge sooner and grow larger in the first year of life. However, these small fish, also called fry, are more vulnerable to heavy winter storms, which can destroy redds (spawning nests) and wash away developing fry. Steelhead that migrate later in the season (February – April) run the risk of being stranded by low flows to carry them back to the ocean. Most populations contain a mixture of early and late spawning fish, which improves the overall odds of success from year to year.
Source: Napa County Resource Conservation District, Stillwater Sciences - Napa River Watershed Limiting Factors Analysis
