|Abstract or Summary
- Ceratomyxa shasta is a myxozoan parasite of salmonids and requires the freshwater polychaete, Manayunkia speciosa to complete its life cycle. The parasite’s distribution is currently limited to the Pacific Northwest region of North America and has been reported to cause substantial losses of both wild and hatchery salmonids. The spatial and seasonal distribution of C. shasta can vary considerably both within and between river systems. This variation was thought to be a result of specific habitat requirements limiting polychaete distribution and abundance. Field studies were conducted in the Klamath River basin where C. shasta is suspected to have caused high losses in migrating juvenile salmonids. The purpose of this study was to document the host-parasite distribution of C. shasta in the river, assess its ability to cause disease, and study aspects of the polychaetes habitat ecology. This is the first study to report the broad-scale distribution of M. speciosa in a river and the various characteristics of those populations.
The seasonal distribution of C. shasta in the Klamath River was investigated by exposing separate groups of C. shasta-susceptible rainbow trout (Oncorhynchus mykiss) at monthly intervals during the study. The spatial distribution was assessed by one basin-wide exposure. The ability of C. shasta to cause disease in native Klamath River salmonids was investigated by exposing fall Chinook salmon (Oncorhynchus tshawytscha; Iron Gate Hatchery strain), along with the known susceptible strains of rainbow trout. The distribution and habitat preferences of M. speciosa were investigated by collecting benthic habitat samples from a variety of habitat types. Where populations were identified, sampling was conducted at a finer scale to study aspects of those populations such as size, density, and age-structure. The distribution and prevalence of C. shasta infection in populations of M. speciosa were determined by using a pooled prevalence strategy on several polychaete populations throughout the Klamath River.
Ceratomyxa shasta, with few exceptions, was only detected in exposure groups from the main-stem Klamath. The parasite could be detected from April until November when water temperatures reached 6˚C. It is likely the parasite could be detected beyond the temporal limits of this study. Prevalence of infection was high with little mortality in rainbow trout exposures groups above Iron Gate dam (Upper Klamath River). Mortality was 100% for rainbow trout exposure groups below Iron Gate Dam (Lower Klamath River). Fall Chinook salmon demonstrated a high level of resistance to the parasite above Iron Gate Dam compared to the rainbow trout, but suffered nearly 50% mortality below the dam. This suggests that resistance of native stocks to the parasite can be overwhelmed in the Lower Klamath River and provides further evidence that infectious dose is high relative to the Upper Klamath River.
The polychaete host, M. speciosa, was found to occur throughout the Klamath River and was often located in slow flowing depositional habitats such as pools and reservoirs. River populations were highly aggregated into small areas whereas reservoir populations were large, widespread and centered at the inflow area. Sand-organic matter substrates and mat-forming epilithic algae were primary microhabitats. Flow velocity, habitat stability and life traits such as dispersal ability appear to be primary factors limiting distribution and abundance of the polychaete. Populations of M. speciosa tested for the prevalence of C. shasta infection demonstrate a low mean prevalence of 0.27% with areas of elevated infection (4.8 and 8.3%) located just downstream of Iron Gate Dam. This suggests that this area may be the primary source of infectious actinospores contributing to the high juvenile salmonid mortality observed in the Lower Klamath River.