Effects of Multiple Stressors on Morbidity and Mortality of Fingerling Rainbow Trout Infected with

Myxobolus cerebralis, the causative agent of salmonid whirling disease, has been implicated in year-class losses of rainbow trout Oncorhynchus mykiss in several rivers in Colorado. The hypothesis that other factors, such as elevated water temperature, bacterial pathogens, and gas supersaturation, are contributing to these year-class losses was tested in a laboratory setting. Fingerling rainbow trout were exposed to all combinations of these stressors for 6 months. Mortality and morbidity were evaluated for each of the test groups using analysis of variance (ANOVA). Mortality was significantly affected by exposure to M. cerebralis (P = 0.0002) and elevated water temperature (P = 0.0002). Morbidity was significantly affected by exposure to M. cerebralis (P = 0.0001). A significant linear increase (P = 0.0020) in mortality was observed with M. cerebralis infection and addition of all combinations of one, two, and three stress factors.