RENIBACTERIUM-SALMONINARUM IN SPRING-SUMMER CHINOOK SALMON SMELTS AT DAMS ON THE COLUMBIA AND SNAKE RIVERS

We evaluated Renibacterium salmoninarum infection in smelts of hatcher y and wild spring-summer chinook salmon Oncorhynchus tshawytscha sampl ed during most of the out-migration at Little Goose (1988) and Lower G ranite dams (1988-1991) on the Snake River and at Priest Rapids and Mc Nary dams on the Columbia River(1988-1990). We sampled 860-2,178 fish per dam each year. Homogenates of kidney-spleen tissue from all fish w ere tested for the presence of R. salmoninarum antigens by the enzyme- linked immunosorbent assay (ELISA), and homogenates from 10%; of the f ish were examined by the fluorescent antibody technique (FAT). Althoug h only 1-11% of fish sampled at a given dam during any 1 year exhibite d lesions characteristic of bacterial kidney disease, 86-100% of the f ish tested positive for R. salmoninarum antigen by ELISA, whereas 4-17 % of the fish tested positive by the FAT. During most years, a majorit y (68-87%) of fish testing positive by the ELISA had low R. salmoninar um antigen levels, but in 1989. 53% of positive fish from Lower Granit e Dam and 52% from McNary Dam showed medium-to-high antigen levels. Fo r most years, the highest mean antigen levels were measured in fish sa mpled after 75% of the total out-migrants had passed a given dam. When the largest numbers of fish were being collected for bypass or downri ver transportation, mean antigen levels were relatively low.