Spawning Gravel Characteristics

Salmon, trout and steelhead all require gravel for spawning. The quality and characteristics of the gravel have a direct correlation with the survival rate of the offspring. Fish look for specific qualities in the gravel in which they lay their eggs to increase the chances of successful reproduction. The gravel area is chosen by the female who builds a nest in the gravel before depositing the eggs.

Location

• Spawning gravel is generally categorized as small, pea-size rock. The small rock naturally deposits in shallow riffled areas in rivers. The larger rocks are heavier, more difficult to move and deposited in deeper water. Gravel shelves are areas of shallow gravel deposits followed by an immediate drop in depth. The shelf areas are common spawning grounds because the deep water provides a staging area with some protection from predators and the gravel provides a spawning area.

Size

• Numerous studies have examined the preferred spawning gravel size with inconsistent results. The only conclusive data available represents the negative presence of fine sediment particles. The typical spawning gravel for cutthroat trout is varied but most studies represent and average size of less than 100 mm per particle.

Sediment

• Natural spawning areas are located in freshwater with little sediment. Salmon and steelhead travel long distances to locate suitable spawning areas with low amounts of sediment. Ocean run steelhead and salmon travel as far inland as northeastern Idaho in the Clearwater Drainage to spawn. Traditional spawning grounds that are currently blocked by dams are also found in remote reaches of Nevada near Great Basin National Park and in the Jarbridge River system. Sediment particles are on the rise in many river systems because livestock grazing destabilizes banks and releases the sediment. Spawning in area with low oxygen levels and over 17.5 percent sediment shows a significant reduction in the success of incubation, according to a 1994 study of the Olympic Peninsula watersheds by Douglas Morrill of the University of Washington. Salmon, steelhead and trout seek gravel deposits with low amount of fine sediment, but are forced into sediment rich deposits if clean gravel is not accessible or available.

Availability

• Availability ultimately determines the spawning gravel chosen by the fish. Clean, well-oxygenated areas with low amounts of sediment are preferred but the fish will spawn in less than adequate gravel beds when better options are not available. The availability of preferred spawning gravel is impacted by livestock grazing and bank erosion, dams and competition with hatchery fish. Bank erosion increases fine particle sediment deposits, reduces stream velocity and decreases oxygen in the river. Dams block access to the clean gravel beds in the high reaches of many river systems and hatchery fish compete for spawning gravel and reduce the success of wild gene reproduction.