Infection in the Early Marine Phase of Juvenile Chinook and Coho

Ocean conditions have been shown to influence the growth, distribution, and survival of salmonids in the northeast Pacific Ocean from 1999-2004.  We investigated whether infection by Renibacterium salmoninarum, a common pathogen of Pacific salmon, differentially affected the growth and survival of yearling Chinook and coho salmon in years with different ocean conditions.  The prevalence of R. salmoninarum in juvenile coho salmon (detected by nested PCR) was positively correlated with increased survival .  The correlation was also strong for yearling Chinook salmon, but was not significant unless 1999 (an anomalous year) was excluded from analyses.  For both species, the prevalence (by nested PCR) and severity (by quantitative PCR) of R. salmoninarum infection was highest in 2000 and lowest in 2004.  To better understand the apparent paradox that higher prevalence correlates with increased survival, we examined the ocean conditions during our study years.  Years were categorized as either “cold ocean” (1999-2002) or “warm ocean” (2003-2004) years based on the summer values of the Pacific Decadal Oscillation (PDO) index.  The prevalence and severity of infection were both significantly higher in cold ocean years.  The weight residuals of infected salmon were not significantly different from uninfected salmon in the summers of cold ocean years.  However, in the summers of warm years, infected coho salmon had significantly reduced weight residuals compared to uninfected fish; the same trend was seen in yearling Chinook salmon, although the differences were not significant.  These data indicate that the infection is less detrimental in years with favorable ocean conditions, and suggest that R. salmoninarum infection may be one factor linking climate variability and ocean indices to salmon survival.