Ecosystem Predictors for a Bacterial Infection in Puget Sound Juvenile Chinook Salmon

Ecological and environmental factors are significant influences on disease dynamics.   Although evaluating these factors for wild fish populations can be difficult due to reduced accessibility and sampling biases, this information is crucial for epidemiological assessments.  Bacterial kidney disease (BKD) is endemic among Pacific Northwest salmonids, and it causes significant morbidity and mortality in artificially propagated stocks.  However, risk factors for infection among free-living salmon are poorly characterized. We evaluated infection by the causative agent of BKD, Renibacterium salmoninarum, in 1,752 fish across 52 sampling sites monthly from May to November, 2003 as a component of a broader study of neritic habitat use in Puget Sound by juvenile Chinook salmon (Oncorhynchus tshawytscha). Overall infection prevalence was 31.8%, and exhibited significant temporal and spatial variations.  Not surprisingly, infection intensity was low (i.e., ≤ 10 cells per slide) for 77% of the fish. Positive correlations between the density of Chinook salmon in the catch with infection prevalence and with infection intensity were observed across multiple spatial scales. Capture location was a stronger predictor of infection than fish origin (based on coded wire tags) or genetic stock (based on microsatellite analysis). Influential risk factors by logistic regression were temperature, densities of Chinook salmon, and density of river lamprey (Lampetra ayresis). R. salmoninarum was found in gut contents and kidney of river lamprey, suggesting this species may be a transmission vector. The low infection intensity, lack of an effect of fish origin, effect of capture bay, and strong associations with Chinook density are consistent with horizontal transmission of R. salmoninarum during the juvenile neritic phase.  These potential infectious interactions between sympatric hatchery and wild fish have implications for the management of fish health in hatcheries.