Effects of Ceratomyxosis on Population Dynamics of Klamath Fall-Run Chinook Salmon

Animal populations are frequently infected by pathogens, but it is not always easy to determine the importance of pathogens to overall population dynamics. It is especially difficult to detect the effects of disease in population time series data because the effects are often local while overall population dynamics are also affected by larger-scale environmental factors. We overcame this difficulty by applying multivariate time series analysis to extract local effects from spawning abundance data and by comparing the survival rate of juvenile fall-run Chinook salmon Oncorhynchus tshawytscha from two locations in the Klamath River basin of California, one of which is affected by a high concentration of the myxozoan parasite Ceratomyxa shasta. To assess the effect of the disease (ceratomyxosis) caused by C. shasta on the population dynamics, we analyzed spatially structured abundance data for naturally spawning salmon and survival data for hatchery-released salmon for associations with exposure to C. shasta and stream discharge, another important factor with respect to ceratomyxosis in juvenile salmon. The results suggest that ceratomyxosis reduces the survival of the Chinook salmon that migrate through the location where parasite densities are highest and that this effect is also detectable in spawning abundance estimates.