Chinook Salmon in the California Central Valley: An Assessment

Abstract

This paper summarizes information on recent historical distribution and abundance of chinook salmon in the California Central Valley drainage, focusing on the period from the 1950s to today. Most of the principal Central Valley streams that historically supported salmon runs still do so, but nearly half of them have lost at least one seasonal salmon run and several major streams have had all their salmon runs extirpated. Overall abundance of chinook salmon in the Central Valley system has decreased to less than 75% of their number in the 1950s. Fall‐run chinook salmon in the Sacramento River basin compose by far the most abundant Central Valley stocks, but they substantially declined between 1953–1966 and 1967–1991. Fall‐run chinook salmon stocks in the San Joaquin River basin and Sacramento‐San Joaquin Delta tributaries showed various changes between 1953–1966 and 1967–1991 but altogether constitute only a minor portion (now 4%) of the total Central Valley spawning escapements. Three other chinook salmon runs (winter, spring, late‐fall) have shown much more pronounced reductions in recent decades. Central Valley salmon have been heavily supported by hatchery production, but the effects of hatcheries on natural stocks remain poorly understood. Major efforts are underway to restore regional chinook salmon and steelhead stocks, several of which are listed under both California and U.S. endangered species statutes.

Juvenile Chinook salmon (Oncorhynchus tshawytscha) growth in off-channel and main-channel habitats on the Sacramento River, CA using otolith increment widths

“…We also observed stranding of juvenile salmon in both 2001 and 2002 as off-channel ponds were disconnected from the main-channel, and water temperatures increased to lethal limits (≈24°; Rich 1987). The extensive alteration of the Sacramento River's natural flow regime has greatly decreased connectivity to offchannel habitats in the spring (Yoshiyama et al 2000). If salmon life history strategies are adapted to utilizing off-channel habitats, the mortality costs associated with high temperatures and stranding may be significant (Higgins and Bradford 1996).…”

“…The Sacramento River is the largest river system in California and is also one of the most disrupted in the world (Yoshiyama et al 2000). The river originates near Mt.…”

“…Fall-run Chinook salmon have an "ocean-type" life history (Healey 1991) and are currently the largest of the four runs in the Sacramento River (Yoshiyama et al 2000). Fall-run adult migration peaks during September and October and spawning occurs soon after adults reach their natal stream.…”

“…Declines are particularly severe in California, where subspecies, and/or populations of three anadromous salmonid species, Chinook salmon (Oncorhynchus tshawytscha), coho salmon (O. kisutch) and steelhead trout (O. mykiss), currently have state or federal protection (Ruckelshaus et al 2002). Yoshiyama et al (2000) have documented a 75% decrease in the numbers of Chinook salmon in California's Central Valley since 1950. Much of this decline in the Central Valley is attributed to the reduction in spawning and rearing habitats, due to dams and diversions.…”

“…Much of this decline in the Central Valley is attributed to the reduction in spawning and rearing habitats, due to dams and diversions. (Yoshiyama et al 2000).…”