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Juvenile Rearing
Juvenile steelhead spend between one to four years in freshwater with two years being most common in central California streams. Juvenile steelhead feed primarily on aquatic insects and other invertebrates in fast water feeding lanes (riffles) and grow rapidly if food is abundant. When sufficient stream flows are sustained to support large aquatic insect populations throughout the year, juvenile steelhead can reach lengths adequate to out-migrate (smolt) in one year. However, in streams with very low summer flows, steelhead grow very little during mid to late summer, and usually require two years or more to grow large enough to migrate to the ocean. Juvenile steelhead survival is positively correlated with smolt size, so larger steelhead smolts have higher chances of surviving than smaller smolts. In streams that lack perennial surface flow, reduced smolt size would be expected, and hence a reduction in smolt survival in the estuary and ultimately the ocean.
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.
Source: Napa County Resource Conservation District, Stillwater Sciences - Napa River Watershed Limiting Factors Analysis
