Reducing River Flows to Control a Parasitic Salmonid Disease in the Klamath River: Simulations Question the Efficacy of Desiccation as a Management Tool

Aquatic parasites cause disease in economically and culturally important Klamath River salmonids Oncorhynchus spp., and the myxozoan parasite Ceratonova shasta has been linked to high juvenile salmonid mortality. Because C. shasta requires the freshwater annelid Manayunkia occidentalis to complete its life cycle, management approaches have focused on disrupting annelid populations to reduce C. shasta and disease risk for salmonids. Previous studies have demonstrated that manipulating river discharge to achieve short‐term, high‐flow events is effective for disturbing annelids. There is also interest in the efficacy of manipulating river discharge to desiccate annelids to reduce disease risk. To investigate the efficacy of reducing streamflow to desiccate annelids, we simulated the amount of annelid habitat, categorized by quality, that would be dewatered as discharge was incrementally reduced. Simulations were accomplished by coupling two‐dimensional hydrodynamic models with an annelid presence/absence model. Simulations were conducted for three study sites located on the Klamath River main stem downstream of Iron Gate Dam, where discharge can be manipulated. Reductions in discharge from June base flow (31.2 m³/s or 1,100 ft³/s) to the lowest simulated discharge rate (17.0 m³/s; approximately half of base flow) resulted in an 11.8% reduction of the total wetted area of the river across all three study sites combined. For the same discharge simulation, reductions in annelid habitat differed based on the quality of habitat, with low‐ quality (<0.20 probability of annelids) to moderate‐quality (0.20–0.80 probability of annelids) habitat reduced by 8.3% (95% credible interval = 7.5–10.3) and 23.7% (10.7–27.5), respectively, while high‐quality habitat (>0.80 probability of annelids) was never desiccated. These results suggest that desiccation is unlikely to offer an effective solution for managing myxozoan parasites of salmonids that are transmitted by annelids because reducing streamflow had minimal effects on the amount of annelid habitat.