Ceratomyxa shasta: Epizootiology in Chinook Salmon of Central Oregon

Abstract

Abstract Infection frequencies were high (47% to 77%) in all groups of juvenile spring chinook salmon Oncorhynchus tshawytscha exposed to Ceratomyxa shasta in water from the Deschutes River (Oregon) during 1973 and 1974. Higher survival after low-level exposures (early spring and short prior exposures) indicates an immune-type resistance in this stock. Infection frequencies for age-0 hatchery smolts of fall chinook salmon increased with exposure time to 50% after 20 days in June 1976, and to 70% after 25 days in June 1977. Mean times until death were inversely related to exposure times through 25 days of exposure in 1977. Juvenile fall chinook salmon periodically captured from the wild populations in the Deschutes River (km 150 to 160) were not infected prior to May in 1978 and June in 1979. After these times, infection frequencies increased to 56% on July 7 in 1978, and to 90% on July 16 in 1979. Infection frequencies then generally decreased with time both years, indicating the wild populations consiste...