Get News Feed
Adult Upstream
Steelhead grow rapidly in the ocean and reach sizes much larger than resident rainbow trout. After spending one to three years in the ocean adult fish, typically between 15 to 30 inches in length, return to their natal streams to spawn. Unlike pacific salmon, steelhead do not always die following spawning, and are capable of spawning multiple times over its lifetime. The percentage of return spawning steelhead is very low, representing five percent or less of the total population. In intermittent streams, low flows during the peak spawning months (January through April) may prevent anadromous fish from reproducing during a given season.
As adult salmonids migrate upstream to spawn, they frequently must overcome a variety of natural and anthropogenic obstacles before reaching suitable spawning areas. These include:
- Attraction flows. The initiation of upstream migration by adult salmonids generally requires an environmental cue in the form of an “attraction flow,” which provides a chemical or other type of signal to the fish that upstream conditions are suitable for migration and spawning. Alterations in the timing, duration, or magnitude of attraction flows may disrupt successful spawning migration by anadromous salmonids.
- Physical migration barriers. Natural or man-made features such as dams, dewatered reaches, inadequate flows, “hanging” tributaries, natural falls, or culverts may compromise the success of spawning salmonids by preventing access to spawning habitat, or, in the case of partial barriers, by critically depleting the fish’s energy reserves as it attempts to get past the obstacle.
- Environmental migration barriers. Upstream migration by adult salmonids may also be blocked or curtailed by environmental conditions, such as elevated water temperatures, that prevent fish from reaching spawning grounds. If water temperatures remain prohibitively high, spawning may not occur or may take place in suboptimal habitats.
- Migration corridor hazards. Other hazards that may be encountered by adult salmonids as they migrate upstream include poaching and false migration pathways presented by bypasses and diversions. These hazards can interfere with spawning migrations and limit the success of salmonid populations.
Source: Napa County Resource Conservation District, Stillwater Sciences - Napa River Watershed Limiting Factors Analysis
