We evaluated Renibaeterium salmoninarum infection in smolts of hatchery and wild spring-summer Chinook salmon Oncorhynchus tshawytscha sampled during most of the outmigration at Little Goose (1988) and Lower Granite dams (1988)(1989)(1990)(1991) on the Snake River and at Priest Rapids and McNary dams on the Columbia River (1988)(1989)(1990) . We sampled 860-2,178 fish per dam each year. Homogenates of kidney-spleen tissue from all fish were tested for the presence of R. salmoninarum antigens by the enzyme-linked immunosorbent assay (ELISA), and homogenates from 10% of the fish were examined by the fluorescent antibody technique (FAT) . Although only 1-11% of fish sampled at a given dam during any l year exhibited lesions characteristic of bacterial kidney disease, 86-100% of the fish tested positive for R . salmoninarum antigen by ELISA, whereas 4-17% of the fish tested positive by the FAT. During most years, a majority (68-87%) of fish testing positive by the ELISA had low R. salmoninarum antigen levels, but in 1989, 53% of positive fish from Lower Granite Dam and 52% from McNary Dam showed medium-to-high antigen levels . For most years, the highest mean antigen levels were measured in fish sampled after 75% of the total out-migrants had passed a given dam . When the largest numbers of fish were being collected for bypass or downriver transportation, mean antigen levels were relatively low .
During the winter of 1995 and spring of 1996, the trapping facility in the fish ladder at Lower Granite Dam on the Snake River was modified to create a system that permits processing large numbers of adult fish while minimizing stress caused by handling. This was accomplished primarily by incorporating a gravity‐flow dewatering system into the trap. Adults passing up the fish ladder can be sampled randomly or with the use of diversion systems that sample for coded wire or passive integrated transponder tags. About 13% of the 1,060,059 adult Pacific salmon Oncorhynchus spp. and steelhead O. mykiss that passed through the facility from 1996 through 2001 were diverted for various research collections and observations.
During 1990, a relatively high percentage of adult anadromous salmonids returning to the Snake River had marine‐mammal tooth and claw abrasions. From 1990 through 1993, annual incidence of these injuries ranged between 14.0 and 19.2% for spring–summer chinook salmon Oncorhynchus tshawytscha and between 5.4 and 14.2% for steelhead O. mykiss. Open wounds were observed on about one‐third of the fish with abrasions. Although these results suggest that predation by pinnipeds may be an important mortality factor for Snake River spring–summer chinook salmon, further research is needed to accurately define the magnitude of the predation.
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