Smolts and Downstream Migration

Coho smolt prior to out migration. Photo courtesy of "Salmon Life Cycle" Web page, an educational site for elementary school students.

To enter saltwater, salmonids must undergo significant physiological changes through a process known as smoltification. Young salmonids (smolts) begin this process as they migrate downstream. They complete the transformation in the estuary, where they may remain for weeks feeding and growing while they prepare for saltwater entry. Different salmonid species undergo smoltification at different ages (coho salmon [Oncorhynchus kisutch] smolt as one year olds and steelhead trout [O. mykiss] are variable – smolting at one to four years) (view the Life History chart for more information). Unlike steelhead, coho must complete smoltification at a pre-determined age. If environmental conditions result in streams drying too soon, or a barrier prevents migration, coho will perish, while steelhead may wait another year for improved migration conditions. Since migration occurs primarily at night, artificial barriers, including flashboard dams, can prevent downstream migration.

Salmonids complete smoltification and enter saltwater in late spring or early summer. The estuary or lagoon habitat may be critical for completing final physiological changes necessary for going to sea. Healthy estuary environments are also highly productive nursery grounds and are important for providing prey resources to growing salmonids.

As salmonids enter saltwater the adult stage of their lifecycle begins. They will grow large in the ocean as they migrate in search of food resources. The ocean migration ends with salmonids returning to freshwater to spawn and begin the next generation.

General 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.

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.