Juvenile Salmon (Fry and Fingerlings)

Growth

A juvenile coho salmon. .Image is a still from the "Fresh Waters Flowing" videotape, courtesy of Cedar Films (Producer) and SalmonWeb (Distributor).

Young juvenile salmon, known as fry, are approximately one inch (30mm) in fork length upon emergence from the gravel. These young fish grow rapidly and can reach 2.5 inches (64mm) within six months after emergence. Coho salmon (Oncorhynchus kisutch) spawn before steelhead trout (O. mykiss), so they tend to be larger than steelhead juveniles. However, steelhead grow rapidly and there is little difference in size by the end of the first year growing season (Barnhart 1986).

Juvenile growth takes place in freshwater environments. In less than a year, some steelhead go through the process of "smolting" (see the Smolts section) and enter the ocean in the late spring, while coho and most steelhead remain in-stream for the summer to spend an additional year or longer growing in freshwater. During the winter, coho and steelhead growth slows or stops as water temperature decreases and food becomes scarce. Salmonids again grow rapidly during the spring of their second year. These larger, second year juveniles are known as fingerlings and are approximately three inches or greater in length. During their second spring, coho begin outward migration from the streams and undergo the physiological changes necessary to enter salt water (see the Smolts section). The remaining steelhead may out-migrate at this time, but commonly spend an additional year in freshwater before smolting and going to sea (see the Life History table for more information).

Predation Pressures

Fry and fingerlings are susceptible to numerous predators, such as birds, mammals, large invertebrates and fish (including larger juvenile salmonids and non-native species). Immediately after emergence from the redd, fry continue to rely on bottom gravel for shelter, and hide between and under the stones. After a few days, fry begin sheltering near stream banks, within undercut banks, root wads, overhanging vegetation and downed trees and limbs (often referred to as woody debris). These young fish tend to congregate in quiet backwaters, side channels and small creeks, sometimes migrating upstream to reach rearing areas. Rearing habitat must have sufficient water inflow, especially during the summer to early fall dry season, to prevent habitat loss. Coho and steelhead experience greater predation pressure as pool water level decreases and crowding increases, shelter decreases, and access by predators increases.

Salmonid Predators

Source: ¹Spence et al. 1996; ²Tabor et al. 1993; ³Sandercock 1991.

Territory

Undercut banks, logs, stones, and overhanging vegetation, as shown here in Soquel Creek, increase in-stream structure and create diverse microhabitats for territorial development by young salmonids.

As growth continues, juvenile salmonids distribute themselves throughout the stream and establish territories. Large individuals win and defend the preferred habitat, which provides better access to food and superior protection, and thus enables them to grow faster than smaller individuals. Coho and steelhead co-exist within stream reaches, but segregation occurs when coho defend territories in pools and near riffle areas, while steelhead predominately hold territory within the riffles. The number of suitable territories increases with increased structural complexity within the stream system. Riffle-pool habitat type coupled with large stones, logs, undercut banks and overhanging vegetation provide diverse microhabitats. This heterogeneous environment is required to sustain a large salmonid population.

Feeding

Once a territory is established, a juvenile remains relatively stationary within the water column, moving only to defend territorial boundaries and to feed. Salmonids are visual feeders, and dart quickly to capture moving or flying prey before returning to the resting area within each territory. Juveniles do not feed after dark and spend nights resting on the bottom. Rarely feeding off the bottom or on non-moving food, salmonids depend on a steady supply of adult and juvenile insect prey drifting downstream, flying over or falling in from terrestrial sources. A diversity of habitat types is required to create highly productive streams, with riffles creating productive aquatic invertebrate prey habitat and water movement transporting prey downstream to juveniles in pools.

Willows and alder overhang Pescadero Creek. Inputs from leaves and branches provide nutrients required to support a diverse in-stream insect population.

Terrestrial insects, both larval and adult stages, are a large dietary component. Salmonids are opportunistic feeders and will change prey preference in response to seasonal peaks in insect populations. Studies show that dipterans (flies), especially chironomids (midges or blood worms [larval stage]) are often principal prey items. Terrestrial insects require overhanging vegetation, so streams with good riparian coverage have high inputs of terrestrial insect prey (Cloe and Garman 1996).

Riparian vegetation not only provides habitat for terrestrial insects, but it is also critical to aquatic invertebrates. Vegetation, primarily as leaf fall, contributes nutrients, especially nitrogen, needed for a properly functioning aquatic food web. The type and amount of vegetative input influences the aquatic macroinvertebrate assemblage. For example, broad leaf species, such as alders, supply the preferred food of "detritivores" (such as stoneflies), while high inputs of leaf litter create an important food resource for "shredder" insects (such as craneflies) (Wipfli 1997). Although not all aquatic insects directly provide food for salmonids, a diverse, healthy in-stream food web contributes to high quality salmonid habitat and increases aquatic prey. Thus the availability and composition of salmonid prey depends upon a healthy riparian zone. Management of riparian vegetation for the appropriate quantity, structure, and composition of vegetation can have important consequences on terrestrial and stream productivity.

Summer Survival and Water Temperature

Deep pools with overhanging vegetation, undercut banks, and structure complexity created by large wood or boulders, as shown here in Gazos Creek, provide good summer habitat.

California is characterized by its Mediterranean climate, with cool, wet winters and warm, dry summers. San Mateo and Santa Cruz Counties do not receive runoff from winter snow pack and depend entirely on annual rainfall. Typically, storms produce rain only from November through early May, so that by late summer and early fall many months have passed with no rain input into the stream systems. This results in summer conditions that may not be viable for the rearing of salmonids. Water temperatures increase as flow decreases and salmonids can quickly reach their temperature and oxygen limitations. Coho prefer stream temperatures between 54–57°F (12–14°C) (Sandercock 1991), and steelhead prefer 57–61°F (14–16°C) (Spence et al. 1996). While both species can tolerate slightly higher temperatures, growth stops at 68°F (20°C) and temperatures in excess of 77°F (25°C) are lethal (Spence et al. 1996). As water temperatures increase, the capacity to hold oxygen decreases, so warmer water is oxygen-poor water. Two habitat requirements are needed to keep streams cool:

1. Riparian vegetation supplies overhanging cover that shades the water and helps prevent increased temperature on sunny days.
2. Water input from headwaters, springs, and ground water provide fresh, cool water inputs into streams throughout summer. This water has been stored underground and protected from solar heating. These cold-water inputs help to keep water temperatures low.

Not only do juveniles have to survive increases in water temperature, but the lack of summer-time rain also results in low summer river flow. Pools become isolated from one another and tributaries no longer flow above ground to the river's mainstem. These pools may be adequate for successful production of salmonids if the water temperature remains low and there is sufficient cover from predators. Riparian vegetation cover, depth of pool, pool habitat complexity (including presence of large woody debris), and underground water seepage all contribute to survival.

Winter Survival

Stream productivity decreases during the cold, short daylight period of winter. Without adequate food, salmonid foraging activity decreases and growth slows. Water turbulence also increases in the winter from active rainfall. With the increase in water flow comes an increased danger of downstream displacement and death. As a result, salmonids rearing in riffles may move into pools during the winter. Territorial behavior breaks down and salmonids live together in the safer pool environment where cover is provided by large wood, undercut banks and exposed roots. However, winter storms also increase movement of suspended sediments downstream and may fill in deep pools, resulting in degraded over-wintering habitat for young salmonids. In addition, salmon are visual hunters and cannot feed when winter storms increase turbidity.

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.

Cloe, W.W., and G.C. Garman. 1996. The energetic importance of terrestrial arthropod input to three warm-water streams. Freshwater Biology 36:105-114.

Sandercock, F.K. 1991. The history of coho salmon (Oncorhynchus kisutch). In Pacific Salmon Life History, edited by C. Groot and L. Margolis. Vancouver: UBC Press.

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.

Tabor, R.A., R.S. Shively, and T.P. Poe. 1993. Predation on juvenile salmonids by smallmouth bass and northern squawfish in the Columbia River near Richland, Washington. North American Journal of Fisheries Management 13:831-838.

Wipfli, M.S. 1997. Terrestrial invertebrates as salmonid prey and nitrogen sources in streams: Contrasting old-growth and young-growth riparian forests in southeastern Alaska, USA . Canadian Journal of Fisheries and Aquatic Sciences 54:1259-1269.

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