Snake River Fall Chinook Salmon Research

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Snake River fall Chinook salmon were listed as “threatened” under the Endangered Species Act in 1992. At that time, little was known about the spawning, rearing, migration, and life history of this species. This long-term research and monitoring project has produced much of the contemporary knowledge on fall Chinook salmon that has been used by fish managers to implement recovery measures. The population has responded positively to these measures but will likely remain a conservation-reliant species.

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Hells Canyon on the Snake River

Hells Canyon on the Snake River. Photo credit, Ken Tiffan, USGS - Western Fisheries Research Center. (Public domain)

Snake River fall Chinook salmon mainly spawn and rear in Hells Canyon on the Snake River and in the Clearwater River basin. They are unique in that they complete the freshwater portion of their life cycle in main-stem habitats unlike other salmonids that use smaller tributaries. Initial work on this project focused on understanding the spawning and rearing requirements of these fish in main-stem habitats as well as identifying important spawning and rearing sites. Because fish must pass eight dams en route to the ocean as juveniles and as returning adults, much research was later directed at determining appropriate spawning and migration flows. Research increased our understanding of the relationships between downstream movement behavior and water velocity, turbulence, and fish physiology that in turn helped explain life history diversity in this species. 

Other research explored the growth differences between fish rearing in riverine and reservoir habitats that shed light on the food web that supports juvenile salmon. Growth is higher in the Snake River than in Lower Granite Reservoir—the first reservoir juvenile fish encounter during their seaward migration—that is due to differences the prey community between the two systems. We documented recent changes to the food webs in lower Snake River reservoirs that included describing the ecology of nonnative Siberian prawns, opossum shrimp, and resurgence of the endemic sand roller. Each of these species influences juvenile fall Chinook salmon either directly or indirectly.

Beam trawling on Lower Granite Reservoir

Beam trawling in Lower Granite Reservoir on the Snake River. Photo credit, Ken Tiffan, USGS - Western Fisheries Research Center. (Public domain)

Recent research has focused on estimating the loss of juvenile fall Chinook salmon to smallmouth bass predation in the Snake River. Smallmouth bass are very abundant and are effective predators of juvenile salmon. We showed that bass consumption rate of juvenile fall Chinook salmon has increased 15-fold since the mid-1990s when the last predation study was conducted. This is largely due to increased numbers of juvenile salmon available as prey that has resulted from both increases in natural production and hatchery releases. Interestingly, bass abundance has not changed appreciably through time. Efforts are currently underway to develop a method to distinguish the origin (e.g., hatchery or natural) of juvenile salmon consumed by bass to determine if one prey is more vulnerable than the other.
A list of cooperator publications related to this study can be found here.