Dam Removal and Implications for Fish Health: Ceratomyxa shasta in the Williamson River, Oregon, USA

The removal of dams on a river is one potential tool for the ecological restoration of native salmonid fishes. However, the removal of barriers also introduces risks, such as the introduction of fish pathogens into previously isolated populations. The proposed removal of four dams on the Klamath River, Oregon-California, provides an opportunity for examining the disease risks associated with dam removal. A salmonid pathogen endemic to the region, Ceratomyxa shasta, is responsible for high mortality in juvenile Chinook salmon Oncorhynchus tshawytscha and coho salmon O. kisutch below the dams. Above the dams, parasite densities are lower and not implicated in salmonid mortality, except in the Williamson River tributary, where high parasite densities raise concerns over the restoration of anadromous fish that are likely to take advantage of spawning habitat in that river. In the current study, baseline information on parasite density, distribution, and genotype composition in the Williamson River was gathered to determine how salmonid reintroduction might be affected by parasite dynamics. Assay of water samples highlighted two areas of high parasite density: between the mouth of the Williamson River and the confluence of the Sprague River tributary, and above the Spring Creek confluence. Despite these high parasite densities, mortality did not occur in sentinel coho or Chinook salmon. Genetic analyses of parasites from water samples and infected fish demonstrated that C. shasta genotype II was dominant and was associated with stocked nonnative rainbow trout O. mykiss. The absence of pathogenicity of this parasite genotype for Chinook and coho salmon suggests that reintroduction plans will not initially be adversely affected by the high parasite densities in the Williamson River. However, following dam removal, returning adult salmon will transport parasite genotypes present below the dams upstream. These genotypes are likely to become established and may reach densities that could affect juvenile Chinook and coho salmon.