Predicted redistribution of Ceratomyxa shasta genotypes with salmonid passage in the Deschutes River, Oregon.

A series of dams on the Deschutes River, Oregon, act as migration barriers that segregate the river system into upper and lower basins. Proposed fish passage between basins would reunite populations of native potamodromous fish and allow anadromous fish of Deschutes River origin access to the upper basin. We assessed the potential redistribution of host-species-specific genotypes (O, I, II, III) of the myxozoan parasite Ceratomyxa shasta that could occur with fish passage and examined the influence of nonnative fish on genotype composition. To determine the present distribution of the parasite genotypes, we exposed eight salmonid species-three native and five stocked for sport fishing-in present and predicted anadromous salmonid habitats. We monitored fish for infection by C. shasta and sequenced a section of the parasite ribosomal DNA gene from fish and water samples to determine parasite genotype. Genotype O was present in both upper and lower basins and detected only in steelhead Oncorhynchus mykiss. Genotype I was spatially limited to the lower basin, isolated predominantly from Chinook salmon O. tshawytscha, and lethal for this species only. Genotype II was detected in both basins and in multiple species, but only as a minor component of the infection. Genotype III was also present in both basins, had a wide host range, and caused mortality in native steelhead and multiple nonnative species. Atlantic salmon Salmo salar and kokanee O. nerka were the least susceptible to infection by any genotype of C. shasta. Our findings confirmed the host-specific patterns of C. shasta infections and indicated that passage of Chinook salmon would probably spread genotype I into the upper Deschutes River basin, but with little risk to native salmonid populations.