Abstract approved:

The myxozoan parasite of salmonids, Ceratomyxa shasta, is established throughout the Klamath River, CA-OR, with high parasite densities below the series of dams and above the dams (upper basin) in the northernmost tributary, the Williamson River (WMR). Two objectives were designed to address concerns about C. shasta effects on reintroduced anadromous salmonids in the upper basin. The first objective investigated parasite dynamics in the WMR and the second determined the virulence of parasite genotypes in allopatric and sympatric salmonid strains. Parasite density, distribution and genotype composition were assessed from water samples collected throughout the WMR and its tributaries. High parasite densities occurred in two sections of the WMR and the genotype associated with stocking of an allopatric strain of rainbow trout predominated. In laboratory cross-infection experiments, the stage of the parasite infective for fish of genotypes I and II (biotype IIR and IIC) were used in three separate trials to challenge four strains of salmonid; sympatric strains of Chinook (Oncorhynchus tswytshcha) and coho (O. kisutch) salmon and allopatric strains of Chinook salmon and rainbow trout (O. mykiss). Genotype I caused mortality in both Chinook strains, whil