Nearshore ecosystem predictors of a bacterial infection in juvenile Chinook salmon

Disease epidemiology requires information about ecological and environmental condi- tions to identify factors that can influence disease progression. Bacterial kidney disease (BKD) is an endemic disease among Pacific Northwest salmonids that causes significant morbidity and mortality in artificially propagated stocks, but risk factors for infection among free-living salmon are unknown. We evaluated infection by the causative agent of BKD, Renibacterium salmoninarum, in 1752 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. Infection intensity was ≤10 cells per slide for 77% of the fish. Correlations between the density of Chinook salmon 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. Influential risk factors by logistic regression were temperature, densities of marked and unmarked Chinook salmon, and density of river lamprey Lampetra ayresis. Renibac- terium salmoninarum were 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 salmon density are consistent with horizontal transmission of R. salmoninarum during the juvenile neritic phase, posing a potential for infectious interaction between sympatric hatchery and wild fish.