Effects of environmental stressors on juvenile trout exposed to Myxobolus cerebralis

Abstract

Declines in survival of young-of-the-year rainbow trout in some Colorado rivers have been attributed to the presence of Myxobolus cerebralis, the causative agent of salmonid whirling disease. A study was initiated to determine if environmental stress factors could be playing a role in year-class disappearances in the Upper Colorado River. Gas saturation levels were monitored throughout a 40 kilometer reach of the Upper Colorado River during the summer and fall of 1995 to identify possible sources of gas supersaturation. Water discharged from the spillway of Windy Gap Reservoir was found to be the main source of man-made supersaturation, while photosynthetic activity of aquatic plants was determined to be the natural source of supersaturation in the study area. Clinical signs of whirling disease and gas bubble trauma were recorded for 1656 rainbow (Oncorhynchus mykiss) and 3060 brown trout (Salmo trutta) young-of-the-year collected from 1994 through 1996 in the Upper Colorado River. Higher proportions of both rainbow and brown trout were moribund in late summer and fall than in spring and early summer months. Proportions of moribund fish of both species decreased with distance downstream of Windy Gap Reservoir. A controlled laboratory experiment was conducted to test the hypothesis that chronic low levels of gas supersaturation affect growth, morbidity, and mortality of fingerling rainbow trout infected with M cerebralis. Fingerling rainbow trout, exposed and unexposed to M. cerebralis were held in gas saturations of 110, 107.5, 105, and 100 - 102.5% for 22 weeks. Significant effects on growth, morbidity and survival were found due to M. cerebralis infection. Gas saturation levels tested did not significantly affect growth, morbidity or survival of fish in this experiment A second laboratory experiment was conducted to test the effects of elevated water temperature, bacterial pathogens, and gas supersaturation on M. cerebralis-infected rainbow trout Fingerling rainbow trout were exposed to all combinations of these stressors for six months. Mortality was significantly affected by exposure to M. cerebralis (P = 0.0002) and elevated water temperature (P = 0.0002). Lesser effects were observed due to elevated gas saturation and exposure to Flavobacterium psychrophilum, the causative agent of bacterial coldwater disease. Morbidity was significantly affected by exposure to M. cerebralis (P = 0.0001), and M. cerebralis -F. psychrophilum interactions (P = 0.0556). Mortality of fish was significantly increased (P = 0.0001) with addition of stress factors.