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Juvenile Rearing
The period of fry emergence varies depending upon the timing of adult arrival and incubation temperature, but typically occurs from January through May. Chinook may disperse downstream as fry soon after emergence, early in their first summer as fingerlings, in the fall as flows increase, or after overwintering in freshwater as yearlings (Healey 1991). 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).
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Water temperature is an important factor affecting incubation and juvenile rearing success. Temperature directly affects survival, growth rates, and smoltification. Temperature also indirectly affects vulnerability to disease and predation and further influences juvenile growth indirectly, through its impacts on food availability.
Following emergence from the gravel, juvenile salmonids must begin feeding and competing for resources under varying environmental conditions. Factors that may limit survival of rearing juvenile salmonids include:
- Availability of summer rearing habitat. During summer, when flows are typically lowest and water temperatures highest, pools, substrate interstices, and other complex habitats provide rearing salmonids with important refugia from high temperatures and predation. A lack of summer rearing habitat can reduce the success of juvenile salmonids already faced with reduced food availability, increased competition for food and space, and increased predation.
- Availability of overwintering habitat. Displacement or mortality caused by high winter flows frequently limits production of juvenile salmonids that do not have access to protected microsites associated with LWD, large substrates such as boulders, interstitial spaces, offchannel habitat, or other features that provide velocity refuges. Certain habitat elements, such as substrate interstices, may also increase winter survival by providing resting or hiding sites for fish when water temperatures are coldest.
- Stranding by low flows. Stranding can cause direct mortality of juvenile salmonids when low flows or rapidly receding water levels isolate fish in disconnected or dewatered habitats, subjecting them to predation, desiccation, or other hazards.
- Displacement by high flows. Extremely high flows, especially in areas devoid of bed or bank roughness elements, can displace rearing salmonids and lead to reduced rearing success or mortality.
- Predation. Predation limits population success through direct mortality. Predation pressure on rearing salmonids may be increased by removal of instream and overhead cover, low flows, migration barriers, and changes in channel geometry.
- Food availability. An inadequate food supply can cause increased interspecific and intraspecific competition, and may lead to reduced fitness and, in some cases, mortality.
- Interspecific interactions between native species. Interspecific interactions between native species, which include competition for food and space, are usually related to reduced availability of food and suitable habitat. Juvenile salmonids may suffer reduced fitness and population success may be limited by these interactions.
- Competition with introduced species. Introduced species can compete for food and space with native salmonids, reducing access to these important resources and potentially limiting fitness and survival.
- Water quality/ temperature. The quality and temperature of stream water has a direct impact on the success of rearing juvenile salmonids. Prolonged periods of elevated water temperature, as well as acute or chronic water pollution, can lead to direct and indirect mortality of juvenile salmonids.
Photo: Jonathan Koehler
