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Spawning
Steelhead spawn by constructing redds in gravel substrates typically found in pool riffle crests. The female scoops out a shallow depression with powerful movements of the tail and lays eggs within the redd. Accompanying males then fertilize the eggs, and the female quickly buries the redd with gravel. The egg development rate is highly temperature dependent and takes between one to two months. Eggs hatch in about 31 days at 50? F. Like other salmonids, steelhead hatch as alevins (yolk sac fry) and spend their first two to four weeks in the gravel before emerging into the stream.
Environmental conditions play a crucial role in successful salmonid spawning, egg incubation, and survival to emergence. The range of environmental tolerance of salmonids during this life stage is narrow, and many factors may limit survival. These factors include:
- Spawning gravel quantity and redd superimposition. Limited availability of spawning gravel is a problem faced by salmonids where access to spawning habitat has been blocked or suitable substrates have been dewatered. This problem can be further exacerbated in areas where limited habitat availability results in competition for space and leads to redd superimposition.
- Spawning gravel quality. Suboptimal spawning gravel quality can limit spawning and incubation success by rendering gravel unusable by spawners, creating unsuitable incubation conditions, and preventing fry from emerging after hatching.
- Water quality and temperature. During spawning, poor water quality or elevated water temperature may reduce the ability of adult salmonids to reach spawning grounds and successfully deposit eggs. Survival to emergence is dependent on successful incubation of eggs, which are especially vulnerable to low dissolved oxygen levels and high water temperature.
- Substrate mobility/scouring. Successful hatching and emergence require stable gravels in and around the egg pocket. Scouring of redd gravels can alter redd hydraulics and cause abrasion or displacement of eggs, resulting in reduced survival rates or direct egg mortality.
- Redd dewatering. Partial or complete dewatering of redds can result in low survival rates due to reduced delivery of water and oxygen and buildup of toxic metabolic byproducts, and may cause egg mortality due to desiccation.
Source: Napa County Resource Conservation District, Stillwater Sciences - Napa River Watershed Limiting Factors Analysis
