Climate variability and marine survival of coho salmon (Oncorhynchus kisutch) in the Oregon production area J. ANTHONY KOSLOW, 1,2,* ALISTAIR J. HOBDAY 1,2 AND GEORGE W. BOEHLERT 1 1 Paci®c Fisheries Environmental Laboratory, 1352 Lighthouse Avenue, Paci®c Grove, CA 93950 USA 2 CSIRO Marine Research, GPO Box 1538, Hobart, Tasmania 7001 Australia ABSTRACT Time series of adult recruitment for natural runs of coho salmon from the Oregon coastal region (1970± 94) and marine survival of hatchery-reared coho salmon from California to Washington (1960±94) are signi®cantly correlated with a suite of meteorological and oceanographic variables related to the biological productivity of the local coastal region. These vari- ables include strong upwelling, cool sea surface temperature (SST), strong wind mixing, a deep and weakly strati®ed mixed layer, and low coastal sea level, indicating strong transport of the California Current. Principal component analysis indicates that these variables work in concert to de®ne the dom- inant modes of physical variability, which appear to regulate nutrient availability and biological produc- tivity. Multiple regression analysis suggests that coho marine survival is signi®cantly and independently related to the dominant modes acting over this region in the periods when the coho ®rst enter the ocean and during the overwintering/spring period prior to their spawning migration. Linear relation- ships provided good ®ts to the data and were robust, capable of predicting randomly removed portions of the data set. Key words: California Current, climate, coho, Oncorhynchus kisutch, recruitment, salmon, survival INTRODUCTION Salmon of the genus Oncorhynchus have comprised major commercial ®sheries from California to Alaska for over the past 100 years, and provided a mainstay for coastal aboriginal peoples for millennia (Magnuson et al., 1996). Recent years, however, have seen a decline in salmon abundance from the Strait of Georgia to California, whereas Alaskan salmon runs have been generally at their highest historical levels. Native stocks of coho salmon (Oncorhynchus kisutch) in the southern part of its range have displayed a particularly worrisome trend, having disappeared from approximately half the streams that historically con- tained this species; abundance is presently estimated to be less than 6% that in 1940 (Brown et al., 1994). Coho salmon are presently listed as threatened in California and part of Oregon (NMFS, 1999). Several factors contribute to the decline in salmon abundance off the Paci®c coast of North America: over®shing, freshwater habitat degradation, loss of genetic integrity due to interactions of native and hatchery ®sh, and climatic in¯uences, the latter par- ticularly during the marine phase of the life cycle (Moyle, 1994). The critical importance of marine cli- mate on Paci®c salmon production is indicated by: (i) the continued decline in adult coho and chinook (O. tshawytscha) salmon returns despite massive increa- ses in hatchery smolt releases of these species (Brown et al., 1994; Beamish et al., 1995); (ii) synchronous trends in salmon production affecting several species over large geographical areas (i.e. the northern Alaskan extent of the range or from the Strait of Georgia south) with northern and southern salmon populations often appearing to be out of phase with each other (Beamish and Bouillon, 1993; Francis and Hare, 1994; Hare and Francis, 1995; Gargett, 1997; Mantua et al., 1997; Hare et al., 1999; Welch et al., 2000); and (iii) correlations of salmon production with indices of marine climate, such as upwelling (Nickelson, 1986), sea-surface temperature (Lawson, 1997; Cole, 2000) and the position and intensity of the Aleutian Low pressure system in winter and spring (Beamish, 1993; Beamish and Bouillon, 1993; Hare and Francis, 1995). *Correspondence. e-mail: tony.koslow@marine.csiro.au Received 17 April 2000 Revised version accepted 11 May 2001 FISHERIES OCEANOGRAPHY Fish. Oceanogr. 11:2, 65±77, 2002 Ó 2002 Blackwell Science Ltd. 65