Abstract. — We computed corre- 

 lations between various population 

 estimates for Central California 

 chinook salmon Oncorhynchus 

 tshawytscha and both freshwater and 

 maiine environmental variables using 

 methods that account for intraseries 

 correlation in a more accurate and 

 conservative way than those used 

 previously. These indicated a nega- 

 tive influence of ENSO (El Nino- 

 Southern Oscillation) conditions in 

 the year during which most of these 

 fish are caught or leave the ocean to 

 spawn. Although freshwater envi- 

 ronmental influences have been pre- 

 viously proposed on the basis of 

 correlation analysis, and have been 

 demonstrated using direct survival 

 estimates based on marked fish, they 

 were not detectable using correlation 

 techniques that accurately account 

 for intraseries correlation. There 

 was also weak evidence for an influ- 

 ence of conditions associated with a 

 negative upwelling index at the time 

 chinook salmon enter the ocean. 

 However, because these conditions 

 are associated with high river flows 

 in addition to oceanographic effects, 

 these correlations may merely result 

 from the influence of freshwater 

 flows. To further describe oceano- 

 graphic influences we computed the 

 principal components of upwelling 

 index, sea level height, and sea sur- 

 face temperature. The first principle 

 component, which reflected the 

 effects of ENSO conditions in the 

 equatorial Pacific during the previ- 

 ous winter, was significantly corre- 

 lated with chinook salmon abundance 

 in their final year, and marginally 

 correlated with abundance during 

 the first ocean summer. This work 

 demonstrates new techniques for 

 reducing spurious correlations and' 

 the practical difficulties involved in 

 sorting out the multivariate influ- 

 ences on populations subject to re- 

 mote forcing through oceanographic 

 and meteorological conditions. 



Determination of Factors Affecting 

 Recruitment of Clninool< Salmon 

 Oncorhynchus tshawytscha 

 in Central California 



Robert G. Kope 



Department of Wildlife and Fisheries Biology 



University of California, Davis, California 95616 



Present address: Tiburon Laboratory, Southwest Fisheries Center 



National Marine Fisheries Service, NOAA, 3150 Paradise Drive 



Tiburon, California 94920 



Louis W. Botsford 



Department of Wildlife and Fisheries Biology 

 University of California, Davis, California 95616 



Manuscript accepted 23 January 1990. 

 Fishery Bulletin, U.S. 88:257-269. 



Understanding the influences of envi- 

 ronment on survival and abundance 

 of chinook salmon Oyicorhynchus 

 tshaurytscha is necessary for manage- 

 ment of both fisheries and surface 

 water in central California. The 

 salmon fisheries are managed by 

 allocating predicted preseason abun- 

 dance between catch and spawning 

 escapement, and flow rates in central 

 California rivers are currently man- 

 aged to allow adequate flows for 

 spawning and rearing of chinook 

 salmon. Better understanding of en- 

 vironmental influences on salmon 

 populations could improve the ability 

 to forecast preseason abundance 

 and more clearly define the impacts 

 of water management on salmon 

 abundance. 



Chinook salmon spawn in fresh- 

 water, but spend most of their lives 

 in the ocean. In California's Central 

 Valley, there are four distinct runs of 

 chinook salmon named for the timing 

 of their spawning migrations: fall, 

 late-fall, winter, and spring. Typically 

 over 90% of the spawning salmon are 

 from the fall run, and over 80% 

 spawn in the Sacramento River and 

 its tributaries (USFWS 1987). Fall 

 run chinook salmon migrate up- 

 stream July through November, and 

 spawn October through January. 



Eggs incubate in the gravel October 

 through March, and young fish rear 

 in the streams and migrate down- 

 stream December through June 

 (USFWS 1987). Most fish spawn at 

 age 3, with some spawning at ages 2 

 and 4, and a few fish at age 5 (Dett- 

 man et al. 1986, Reisenbichler 1986, 

 Kope 1987). The precocious 2-year- 

 old spawners are primarily males and 

 are called "jacks." While in the 

 ocean. Central Valley chinook salmon 

 are harvested by commercial and 

 recreational troll fisheries. Catches in 

 both fisheries are dominated by 

 3-year-old fish (Denega 1973, Kope 

 1987). 



In addition to natural production, 

 several hatcheries and spawning 

 facilities produce fish to enhance the 

 fisheries and mitigate for losses of 

 spawning and rearing habitat result- 

 ing from water development projects 

 (Fig. 1). The contribution of hatchery 

 production to spawning runs has in- 

 creased in recent years, and has been 

 estimated to be approximately 15% 

 of the total run in the upper Sacra- 

 mento River (Reisenbichler 1986), 

 78% of the run in the Feather River, 

 and 87% of the total in the American 

 River (Dettman and Kelley 1986). 



Numerous studies have examined 

 relationships between environmental 



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