Upstream Migration: Return to Streams

After spending time in the estuary adjusting to fresh water, salmonids return to the stream where they were born to spawn and begin the next generation. This upstream migration is known as a run, and a run of salmon may take weeks to months before the entire population has entered freshwater and reached the spawning ground. The salmonids in San Mateo and Santa Cruz Counties return in early winter through spring, with coho salmon (Oncorhynchus kisutch) arriving first, followed by steelhead trout (O. mykiss) (view the Life History chart for more information).

Homing

The act of returning to the natal site is known as homing. Perhaps the best-known trait of salmonids is their ability to undergo homing, or instinct to return to the stream of their birth. Salmon return, generation after generation, to their birthplace to reproduce and die. Because each generation returns to the stream of its birth and spawning occurs among stream-mates, the genetic composition across regions may differentiate over time. These differences in genetic makeup result in unique stocks at different locations and each stock requires special management considerations. The National Oceanic and Atmospheric Administration National Marine Fisheries Service (NOAA Fisheries) recognized these genetic differences when it created the Evolutionary Significant Units (ESUs). Additionally, humans become guardians to individual groups of salmon and stewards of the land and water that provide a home to the returning fish.

A coho migrates upstream to the spawning grounds.

Salmonids are thought to rely on olfactory senses (sense of smell) to navigate successfully from the ocean, then upstream, to relocate the site of their birth (Allen and Hassler 1986). Chemical pollutants may interfere with normal olfaction and some chemicals, such as diazinon – a general use insecticide – are being studied to determine potential effects on homing (Scholz et al. 2000). Vision, response to celestial patterns, sensitivity to light, sensitivity of the acoustico-lateralis system (system of inner ear and cellular receptors that perceive movement and sound), and genetics may also be important for successful homing.

Some populations faithfully return to the same spawning bed each year, while others respond to changing conditions and may move up or downstream from their natal bed. Young salmonids learn to recognize their natal stream, a process known as imprinting, at a young age, perhaps as early as egg stage within the nest (Sandercock 1991). Moving and transplanting young fish may result in adults either returning to the original site, the new site, or "straying" to an entirely different stream. While straying is a natural occurrence for salmon (useful for colonizing new habitat, avoiding poor environmental conditions, and genetic exchange) it can be disruptive it can be disruptive if human-induced. Studies are being undertaken to determine the level of natural straying and the effects of hatchery management and juvenile outplacement on populations.

Upstream Migration

Upstream migration does not begin until the environmental conditions, including stream access, flow, and temperature, are appropriate for each salmon species. Migration is thought to be in response to increased or decreased water levels, with very high flows (greater than 8 f/s [2.44 m/s] for coho salmon) potentially preventing movement (Sandercock 1991). Water temperature is also important, both to trigger upstream movement and for survivorship during migration. Preferred temperature requirements depend on the species, but 60°F (15.6°C) or lower is required by all (Spence et al. 1996).

A series of pools provide resting habitat for salmonids during their upstream migration (Scott Creek Watershed).

In many coastal California stream systems, a sandbar may develop across the river mouth during the summer, which prevents salmonids from entering the stream until winter flow has breached the sandbar. Returning salmonids are known to gather near the mouth of streams to wait for access and ideal environmental conditions. Once migration begins, the salmonids move primarily during the day.

Salmonids are capable of overcoming great obstacles in their efforts to reach spawning grounds. Prodigious jumpers, coho have been observed jumping vertically more than two meters (6.5 feet). Steelhead are even stronger jumpers and are estimated to be capable of vertical jumps as high as 10 to 11 feet (Powers and Orsborn 1985; Spence et al. 1996). Although salmonids are powerful jumpers, overcoming a migration barrier is not just dependent on vertical gain, because horizontal movement is important as well. The "take-off" pool must be deep and long enough, with inflowing water velocity sufficiently low for a salmon successfully to clear a barrier. Even small elevation gains may prevent migration. Culverts often create migration barriers because their size and placement can result in elevation gain, small take off pools, and high velocity water flow. Many efforts are underway to identify and replace problem culverts throughout San Mateo and Santa Cruz Counties.

A salmonid's determination to migrate also forces it to cross shallow stream reaches, sometimes in as little as two inches of water, to continue on its upstream migration. A heterogeneous environment, with a series of pools and riffles, provides optimum habitat for migrating salmonids. The pools provide sheltering habitat for resting between migration efforts across shallower areas with rapid water movement.

References

Allen, M.A., and T.J. Hassler. 1986. "Species Profiles: Life History and Environmental Requirements of Coastal Fishes and Invertebrates (Pacific Southwest) -- Chinook Salmon." U.S. Army Corps of Engineers, TR EL-82-4. USFWS Biological Report 82(11.49), 26 pp.

Powers, P.D. , and J.F. Orsborn. 1985. "Analysis of Barriers to Upstream Fish Migration: An Investigation of the Physical and Biological Conditions Affecting Fish Passage Success at Culverts and Waterfalls." Albrook Hydraulics Laboratory, WSU. Final Project Report Part 4 of 4, Contract No. 198BP36523, Project No. 198201400 (BPA Report DOE/BP-36523-1), 134 electronic 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.

Scholz, N.L., N.K. Truelove, B.L. French, B.A. Berejikian, T.P. Quinn, E. Casillas, and T.K. Collier. 2000. Diazinon disrupts antipredator and homing behaviors in chinook salmon (Oncorhynchus tshawytscha). Canadian Journal of Fisheries and Aquatic Sciences 57:1911-1918.

Spence, B.C., G.A. Lomnicky, R.M. Hughes, and R.P. Novitski. 1996. "An Ecosystem Approach to Salmonid Conservation." ManTech Environmental Research Services Corp. TR-4501-96-6057.

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