Renibacterium salmoninarum AS A CAUSE OF MORTALITY AMONG CHINOOK SALMON IN SALT WATER1

A study to investigate the contribution of Renibacterium salmoninarum to salt water mortality of chinook salmon, Oncorhynchus tshawytscha, was conducted. Smolts were obtained from each of three Oregon Department of Fish and Wildlife hatcheries. Using the direct fluorescent antibody test (DFAT), a determination of incidence of infection by R. salmoninarum was made for groups of fish from the three hatcheries. Samples of fish from each site were placed in separate salt water tanks at the Oregon State University Marine Science Center. Mortalities occurring in salt water were examined for the presence of R. salmoninarum using DFAT, and for other bacterial fish pathogens by preparation of microbiological cultures of kidney tissue on tryptic soy agar. The study included examinations of both the fall and spring release groups from each fish population. Results of the investigation demonstrated the capacity of R. salmoninarum to cause mortality of chinook salmon in salt water and there were indications that horizontal transmission occurred. Detectable incidence of R. salmoninarum before introduction into salt water was higher among populations of chinook released in the spring than in the fall release groups.