Get News Feed
Outmigration
Juvenile Chinook feed and grow as they move downstream in spring and summer; larger individuals are more likely to move downstream earlier than smaller juveniles (Nicholas and Hankin 1989). In the Sacramento-San Joaquin system, fall Chinook smolt outmigration generally occurs from March to July (Maragni 2001).
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
