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Spawning
Chinook salmon spawn primarily in riffles and pool tailouts. Substrate size and intragravel flow conditions are important factors affecting Chinook salmon spawning distribution and incubation success (Harrison 1923, Hobbs 1937, McNeil 1964, Cooper 1965, Platts et al. 1979). Median particle sizes of spawning substrates used by Chinook salmon have been found to range from ˝ inch (1.3 cm) to 3 inches (7.6 cm) (Kondolf and Wolman 1993). The presence of fine sediment and sand in the bed can reduce intragravel flow in the redd and is detrimental to egg survival and development (McNeil 1964, Cooper 1965).
During spawning, the female Chinook salmon excavates a nest, referred to as a “redd,” into the gravel and cobble substrate. As she excavates the nest, she deposits eggs, which the male fertilizes, into several pockets in the redd and covers the eggs with gravel. Chinook salmon redds are large, typically 110–190 ft2 (10.2–17.7 m2) in size (Healey 1991). The female remains at the redd to defend the site from excavation by later-arriving salmon until she dies, usually within a few days after spawning. The fertilized eggs incubate in the gravel for a period of 6–13 weeks, depending on water temperature (Vernier 1969, and Heming 1982, both as cited in Bjornn and Reiser 1991). The larvae that hatch from the eggs, called “alevins,” are equipped with yolk sacs that provide nourishment. These larvae remain in the substrate until the yolk sac is absorbed, approximately two to three weeks, then swim up through the gravel substrate and begin rearing in open water. After emerging, fry either disperse downstream or move to stream margins or backwater areas near their natal redd.
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
