Spatial and Temporal Occurrence of the Salmonid ParasiteCeratomyxa shastain the Oregon–California Klamath River Basin

The parasite Ceratomyxa shasta has been implicated as a significant source of salmonid mortality in the lower Klamath River, California (i.e., below Iron Gate dam). A study of the prevalence of C. shasta and its geographic and temporal distribution throughout the Klamath River basin was conducted to determine when and where juvenile salmonids encounter lethal parasite doses. Susceptible rainbow trout Oncorhynchus mykiss were exposed to C. shasta 3–4 d at seven locations in the Klamath River between Beaver Creek and Keno Reservoir in April, June, July, September, and November 2003. Individuals from a Klamath River strain of fall Chinook salmon O. tshawytscha were held in three locations in the upper Klamath River in April, June, and July. In June 2004, rainbow trout were exposed to the parasite for 4 d at 18 locations from Klamath Lake to the mouth of the Klamath River, including several major spawning tributaries; one exposure occurred in the lower Klamath River. Rainbow trout mortality due to in...