Spawning

Creation of the Redd

The female salmon selects the spawning site, known as a redd. Riffles located directly downstream from pools provide good circulation of well-oxygenated water through the gravel and are preferred spawning locations (Sandercock 1991). Coho salmon (Oncorhynchus kisutch) are known to seek out areas of ground water upwelling as preferred habitat as well (Sandercock 1991; Spence et al. 1996). A female may choose to excavate and spawn in multiple nests within a site. The entire area used for spawning is considered the redd (individual nests are sometimes called redds as well). Once a site is selected, a female salmon will defend it against other females, as well as males until she is ready to spawn. One or more males may spawn at the redd, but one dominant male will guard the female, with the female's acceptance, while she is preparing the nest. The dominant male guards against other males, including jacks (two-year old males), in an attempt to be the only male to spawn. He occupies the closest position to the female while spawning.

Females may excavate nests for up to five days, digging in multiple sites. With her head pointed upstream, but at an approximately 45 degree angle to the current, a female salmon turns on her side and begins flexing her body. The alternate lifting of body and tail pulls sediment (gravel, silt, sand) away from the streambed and allows the current to carry it downstream so that a depression is formed in the gravel.

Habitat Conditions

Habitat requirements for successful spawning include suitable stream bottom substrate (gravel), water quality (temperature, dissolved oxygen, and sediment), water depth, and water velocity. Coho salmon and steelhead trout (O. mykiss) require similar conditions of cool, flowing, well-oxygenated water, but differ in their specific needs (see table below). Spawning gravel size may range from fine gravel to coarse rubble, but the amount of fine sediment is important. A high percentage of fine sediment (silt and sand) is undesirable spawning habitat and may result in lethal conditions. Fine sediment reduces inter-gravel water flow, and may prevent adequate oxygen from reaching the embryos and removal of metabolic wastes. Female coho salmon will not spawn in gravels with excessive quantities of fine sediment (Sandercock 1991).

Water quality is important for both spawning and egg survival. Salmonids require cool stream temperatures (less than 57°F), high oxygen levels (greater than 8.0 mg/L) and clean water (Spencer et al. 1996). Cool, moving water is often oxygen-rich water and provides the type of water quality for successful spawning and juvenile survival.

The amount of water, which dictates water depth and water velocity, is also important to spawning success and egg survival. Females choose sites with appropriate water depth and velocity, but once spawning is complete, the developing embryos remain within the nest for a month or more. Appropriate water flow must continue after spawning; too much water may excavate the nest, while too little can reduce survivorship by raising water temperature and reducing oxygen flow.

Spawning Habitat Requirements

Information for this table was taken from Sandercock 1991 (coho requirements) and Barnhart 1986 (steelhead requirements).

Spawning

When ready to spawn, the female salmon swims over the depression she has created for the nest and arches her body downward to gain close contact with the remaining gravel within the depression. The dominant male swims along-side the female and spawning commences with eggs and sperm released nearly simultaneously. Non-dominant males, including jacks, may also swim along-side the female or dominant male and release sperm. Because the specific gravity of the eggs is higher than water and the hydro-dynamics of the excavated depression create a back eddy, the eggs are not swept out of the nest, but instead sink into the depression. After spawning, the female uses tail slap motions to excavate an area approximately six inches upstream from the depression and thereby covers the nest with gravel.

The female may continue to build nests and spawn within a redd site after initial spawning. Successive nests are built upstream, in a row, from the initial site. Quantity of spawning habitat required by one female may be quite large and is dependent on body size and the number of nests built. Field measurements indicate an average coho redd size of nine square feet (Sandercock 1991). However, this figure does not account for the entire area required for spawning. Coho females are territorial and will chase off females attempting to spawn in adjacent habitat.

In streams with limited habitat, females spawning late in the season may excavate the redds of earlier spawners. For successful spawning, a stream must have enough spawning habitat to support the entire population. Therefore, the spawning area required depends not only on environmental conditions (stream bottom composition, stream gradient, and water velocity) but also on the number of spawners present.

Death after Spawning

Spawning coho salmon.

Coho salmon are semelparous, which means that they reproduce one time and then die. Steelhead, on the other hand, can prolong their life cycle. They are iteroparous and may return to their spawning grounds over many years.

For coho, death does not occur immediately after spawning. Both females and males may spawn in multiple sites with multiple partners before they are "spawned out." After the final spawning, females and males can live for over a week before they die. Females often remain near the redd, guarding it from other females who are searching for the appropriate conditions for spawning. By dying at or near the spawning site, salmonids fertilize the stream system by providing nutrients from their decomposing bodies.

References

Barnhart, R.A. 1986. "Species Profiles: Life Histories and Environmental Requirements of Coastal Fishes and Invertebrates (Pacific Southwest) -- Steelhead." U.S. Army Corps of Engineers, TR EL-B2-4. USFWS Biological Report 82(11.60), 21 pp. View on-line document.

Busby, P.J., T.C. Wainwright, G.J. Bryant, L.J. Lierheimer, R.S. Waples, F.W. Waknitz, and I.V. Lagomarsino. 1996. "Status Review of West Coast Steelhead from Washington, Idaho, Oregon, and California." U.S. Department of Commerce, National Oceanic and Atmospheric Administration. NOAA Technical Memorandum NMFS-NWFSC-27, 275 pp. View on-line source.

Sandercock, F.K. 1991. The History of Coho Salmon (Oncorhynchus kisutch). In Pacific Salmon Life History, edited by C. Groot and L. Margolis. Vancouver: University of British Columbia Press.

Shapovalov, L., and A. C. Taft. 1954. The life histories of the steelhead rainbow trout (Salmo gairdneri gairdneri) and silver salmon (Oncorhynchus kisutch) with special reference to Waddell Creek, California and recommendations regarding their management. California Department of Fish and Game Fish Bulletin 98:1-375.

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