Thesis (M.S.)--Montana State University--Bozeman, 2007. / Typescript. Chairperson, Graduate Committee: Christopher S. Guy. Includes bibliographical references (leaves 66-72).

The presence of nonnative trout and the recent introduction of Myxobolus cerebralis in the Logan River drainage pose a threat to the native Bonneville cutthroat trout population (Oncorhynchus clarki Utah). The variability in the response of susceptible trout populations to M. cerebralis, causing agent of whirling disease, suggests that environmental factors may influence the effects of the parasite in infected environments. I investigated the relationship between temperature, discharge, substrate size, nutrient concentration (nitrogen and phosphorus), periphyton (chlorophyll a), and the relative abundance of Tubifex tubifex to the distribution, and prevalence of M. cerebralis in wild salmonid populations and sentinel fish in the mainstem of the Logan River and two of its tributaries. In addition, I investigated the potential influence of biotic (e.g., food availability, M. cerebralis prevalence) and abiotic factors (e.g., temperature) on the distribution abundance, and condition of salmonid fishes. Differences in mean temperature and discharge across sites explained most (>70%) of the variability in prevalence of M. cerebralis observed along the Logan River. However, the prevalence of the parasite was not related to other factors that can influence its life cycle, such as productivity and substrate composition. The results also indicate that the fish fauna presents a longitudinal change reflected in a zonation pattern. Cutthroat trout dominates the headwaters and high-elevation reaches, while reaches at lower elevations of the mainstem and tributaries were dominated by brown trout. The transition between these species was consistent with changes in environmental characteristics. Cutthroat trout dominates the fish community in mainstream reaches with the lowest average minimum temperature and highest diel temperatures, and where small boulders and small cobbles are the predominant substrate. This study provides insights of the abiotic and biotic factors that affect the distribution, abundance, and condition of salmonid populations along the Logan River. Identifying these factors is crucial to effectively manage this and other trout streams, where ensuring the conservation of native cutthroat trout populations is a priority. Further, I present baseline information of the potential linkages between environmental factors and M. cerebralis distribution and prevelance, which could be used to develop plans to minimize the potential negative effects of this parasite on wild salmonid populations.

This study tracked the chronology and severity of Myxobolus cerebralis infection and related it to survival of age-0 kokanee to determine whether M. cerebralis represented a significant agent of morality in the population. Environmental conditions and losses to predation were identified and linked to age-0 kokanee survival to identify other sources of mortality in the population. We attempted to track survival of age-0 kokanee from the outmigration into the reservoir until the end of the first growing season in September of each year. The number of age-0 kokanee that entered the reservoir was calculated by obtaining estimates of total egg deposition and egg-to-fry survival for the 1999 brood year. Age-0 kokanee were primarily infected by the parasite after they had entered the reservoir in spring. Prevalence and severity increased rapidly throughout the summer and nearly all age-0 kokanee were infected by August of both years. In total, 495 age-0 kokanee were examined for clinical lesions associated with M. cerebralis in 1999 and 2000, but only one fish displayed a cranial abnormality in 2000. Low survival rates of age-0 kokanee from July through September were observed and coincided with increased prevalence and severity. However, because of high reservoir temperatures, low food levels, predation, and entrainment, evidence for M. cerebralis acting as a direct source of mortality on age-0 kokanee was elusive during our study. In age-1 and older kokanee we did not find any evidence that M. cerebralis had an effect on growth and overall condition of the fish. Infection severity and presence of clinical signs varied between years and were likely more related to limitations of the diagnostic techniques. Any effects that M. cerebralis may have had on kokanee are likely to occur during the first growing season and may impact the host's ability to cope with the other environmental stressors identified at Porcupine Reservoir. Despite high mortality rates, the kokanee population at Porcupine Reservoir was considered to be overabundant based on estimated densities and length distributions of spawning fish. M cerebralis did not appear to have significant effect on the abundance of the kokanee population but we were unable to determine whether or not the parasite was a source of direct mortality because of other potential sources of mortality.