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Discover & Learn > About Our Watersheds > Our Native Fish > Steelhead & Salmon > Steelhead > Migration > Outmigration

Outmigration

After spawning in winter most adult steelhead make the return migration to the ocean quickly. However, juvenile steelhead begin the physiological changes for smolting (migrating to the ocean) in late March through May. This late migration allows them to feed longer during the most productive time of the year, growing to sizes which increase the chances for survival. This is also a period of rapidly declining streamflows in California, making the downstream journey over barriers, shallow riffles, and drying stream reaches very risky. For many ephemeral or urbanized central coast streams the outmigration period is a primary limiting factor for steelhead populations. Adult access to good spawning and rearing habitat is unable to compensate for low smolt success during most years. If a stream’s lower reaches are completely dry during the outmigration period, steelhead populations are typically limited.

A variety of environmental factors may serve as outmigration cues to juvenile salmonids in streams. Outmigrating fish are subject to a range of conditions that influence their ability to successfully reach the ocean. These include:

• Adequate flows for outmigration. Juvenile salmonids undergo physiological changes and initiate outmigration when adequate river flows occur, usually during spring. Reduced flow duration or magnitude during the outmigration period can render some portions of the river corridor impassible and may subject emigrating juveniles to increased predation, thereby reducing the chances of successful outmigration.
• Water quality and temperature. Water quality and temperature may be especially important to outmigrating salmonids during low-flow periods. Lethal or sublethal effects may result from pollutants or prolonged exposure to high water temperatures.
• Predation. Predation, especially by introduced warmwater, piscivorous fish, is believed to be a significant source of mortality of outmigrating salmonids in some rivers. Outmigrant juveniles may also be subject to predation by terrestrial or avian predators.
• Diversion hazards. Water diversions, such as canals, pumps, and bypasses, can act as “blind pathways,” preventing fish from reaching the ocean. They may also be directly lethal to fish or may expose them to high water temperatures, pollutants, predation, or desiccation.

Source: Napa County Resource Conservation District, Stillwater Sciences - Napa River Watershed Limiting Factors Analysis