Embryos in the Redd: A New Generation is Born

Salmonid eggs developing in a redd. Photo courtesy of "Salmon Life Cycle" web page, an educational site for elementary school students.

Nests of salmonid eggs, or redds, are not cared for by adult salmon, which die shortly after spawning. Because of this, the location and structure of redds are extremely important. The placement and design of the redd ensures that the embryos (fertilized eggs) are exposed to ideal temperature and oxygen conditions from the flowing stream, unless conditions change. Incubation time for embryos to develop into the next life stage (alevins) depends on water temperature, but studies have shown that coho salmon (Oncorhynchus kisutch) embryos can develop into alevins in as few as 38 days at 50ºF (10ºC) (Shapovalov and Taft 1954). Therefore, maintenance of optimal stream conditions plays an important role in embryo survival. Winter flooding can scour gravel beds and excavate redds, or deposit fine sediments that reduce between-gravel water movement and suffocate eggs. Without adequate water movement, oxygen levels become critical or lethal for the developing embryos. Low flows are equally detrimental, with increased temperature and inadequate oxygen resulting in high mortality. Eggs and alevins survive well with dissolved oxygen concentrations of 8.0 mg/L or greater (Spence et al. 1996). Biological interactions reduce survivorship as well, including bird and insect predators, and fungal infection. Although the developing embryos do not require parental care, they do rely on the environment to provide the necessary incubation conditions for survival.

References

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.