Fish disease dynamics in changing rivers: Salmonid Ceratomyxosis in the Klamath River

The myxozoan parasite Ceratomyxa shasta has been identified as the main contributor of mortality in salmon in the Klamath River, California. The life cycle of the parasite is complex, involving a polychaete and a salmonid host. Infection dynamics are greatly influenced by environmental factors, such as temperature and water velocity. If we are to control the disease it is important to predict the impact of environmental scenarios on spore concentration and infection prevalence. Here, we introduce a model based on partial differential equations to study the spore concentration in the river and the infection prevalence of returning salmon. The analysis of the model shows that for current climate conditions, additional dam release can reduce the actinospore concentration up to 48% and the prevalence up to 40% thus providing a potential disease management option. However, the infection risk is likely to increase for future climate conditions by 10–54% and this will lead to an infection level comparable to that of a recent high-disease year. Our simulations show that dam removal cannot be assumed to mitigate the effect of climate change or to have influence on infection prevalence. We show that our detailed model system can be reduced to a simpler exponential dose-response function, which predicts infection levels based on transmission rates, travel time and mean spore concentration, quantities that can be measured in-situ or given empirically. The dose-response function may therefore be a useful tool in disease management.