Juvenile salmon downstream migrations tend to occur in groups and pulses, which may
correspond to increased flow events. The many variables and consequent interactions associated with the migratory behavior of young salmon are complex and not well understood. Factors which may have influence on young salmon migration include water temperature, flow, barometric pressure, turbidity, flooding, rainfall, wind, species, stock (e.g., fall-run or spring- run), life history state, degree of smoltificaiton, parental origin (e.g., hatchery or wild) size of juveniles, distance from the ocean, and food availability.
- Entrainment – Loss of young anadromous fish in unscreened diversions is almost
universally cited as a significant contributing cause for declines in fish populations. The passage of the federal Central Valley Project Improvement Act in 1992 authorized a
program to prioritize and screen the diversions in the Sacramento Valley thought to be
having the greatest impact on anadromous fish. Since then, almost all of the priority
diversions have been screened to prevent entrainment.
- Predation – Predation can be a serious problem contributing to losses of young
anadromous salmonids. Common juvenile salmon predators in the Sacramento Valley
include the native Sacramento pikeminnow and rainbow trout and the non-native striped
bass and largemouth bass.
- Food – Recently, more attention is being given to the lack of food present in the
mainstem Sacramento River for outmigrating juvenile salmon. The disconnection of the
river from the historic floodplain has severely limited the production of appropriate food for juvenile salmon. Recent projects, such as the Nigiri Project and Fish Food on Flood Fields Pilot Program are looking to mimic the historic role the floodplains played in producing food for juvenile fish.