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 Smith River Project > Habitat Needs > Spawning and Incubation

Chapter 5 -- Anadromous Salmonids and Their Habitats (continued)

Sequence of Habitat Needs for Anadromous Life Cycles (continued)

Spawning and Incubation

Successful spawning, incubation, and emergence of fry depends on quantity of suitable spawning habitat, number of spawning females, scouring of the stream channel during high flows, and other density-independent mortality during the egg and alevin stages. Spawning habitat is typically located in the swift shallow area between a pool and a riffle. Suitable substrate for spawning consists of large gravel and small cobbles which contain little sand. Anadromous salmonid species vary in their preferred gravel size for spawning. Chinook prefer to spawn in gravel that is up to six inches in diameter. This allows the chinook to spawn in gravel beds that other salmonids cannot use.

During spawning, a redd, or nest, is excavated in the gravel by the female. After the eggs and sperm are deposited, she covers the cavity with large gravel and cobbles. The incubation of eggs in the gravel is usually between 31 and 60 days depending on water temperature (Flosi 1994, Salo 1991, Sandercock 1991). As the eggs develop into embryos, water flowing through the gravel supplies oxygen and removes metabolic wastes. When spaces between the gravel become filled with sand, flow of water through the gravel is reduced. Reduction in water flowing to the incubating eggs can suffocation of some or all of the eggs or alevins. In some cases, even though fine sediments cover the spawning area, intergravel water flow may be sufficient, but there is a lack of openings for fry to emerge from the gravel and continue their life cycle.

Success or failure of incubation depends heavily on stream flow timing. Before incubation and emergence are completed, any given redd may be destroyed by scouring during high flows. A successful redd avoids scouring by high flows while also receiving sufficient stream flow to supply oxygen and remove metabolic wastes. Only a portion of the redds remain intact through the incubation period and successfully produce fry.

Although each year is unique, there are often two windows of opportunity when most successful spawning and incubation occurs. The first window begins with the onset of upstream migration by pre-spawners in the fall and lasts until large storms bring major scouring. A second window may begin in late winter, following the last major scouring event and extending until stream flows decrease and no longer cover the redds (Lisle 1996). In the Smith River, mainstem spawning by chinook occurs during low flows from October to January (Reedy 1995), that is, the "early window". Because mainstem redds are susceptible to scouring before fry emerge, the success of mainstem spawning by chinook is assumed to be variable. During years when heavy scouring does not occur until later in the winter or spring, many fry may emerge from early redds before scouring occurs and are likely to become the largest fish in their age class. Lack of spawning success in a particular area does not indicate anything about overall spawning success in the basin.

Typically, the most productive spawning gravels in a river system are found in the tributaries. In the Smith River system, important spawning areas are found in the North Fork subbasin, the upper part of the Middle Fork subbasin, the middle section of the South Fork subbasin, Mill Creek, and Rowdy Creek. Spawning also occurs in the lower mainstem (Reedy 1997) and is probably most successful in low flow years.

In steeper tributaries farther upstream, pockets of suitable spawning gravel are found in short sections of stream having gradients less than 3%. These isolated spawning areas in sections of relatively low gradient stream channel may produce enough fry to "seed" a very large area of adjacent rearing habitat. Therefore small low gradient areas in the upper watershed can be important in overall production of salmon and steelhead from the watershed. Especially in steeper channels, large woody debris plays a role in the formation of spawning habitat. Instream logs create structures that trap gravel and form spawning habitat. Thus the amount of spawning habitat varies as the amount of woody debris in the stream changes over decades.


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