Hatchlings: Alevin (yolk-sac fry)

Newly hatched salmon, known as alevin, remain hidden within the gravel redd until they have absorbed their yolk sac. These young fish are not yet capable of moving to new habitat, and are sensitive to their environment.

After incubating within the redd, eggs hatch into alevin (yolk-sac fry). Alevin remain in the gravel, often migrating deeper into the bedload, and continue to grow by absorbing their yolk sac, rather than feeding. Appropriate water temperature and oxygen levels are critical for this life stage. High concentrations of fine sediment within the gravels can be extremely detrimental to growth and survivorship. Sediment not only reduces water movement to developing alevin, thereby reducing oxygen availability, but may also prevent their emigration from the gravel. A study of coho salmon (Oncorhynchus kisutch) found only an 8 percent survival to emergence when the gravel was composed of 70 percent sand (Sandercock 1991). Development time for alevin may range from two to seven weeks after which they emigrate from the gravel and emerge as fry (juveniles). Emerging primarily at night, a single cohort of fry may take over a month to leave the gravel. Fry who have not completely absorbed their yolk sacs, but have emerged, are called button-up fry or swim-up fry.

Reference

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.