SURVEY OF UPPER RATTLESNAKE CREEK, A TROUT-SPAWNING TRIBUTARY OF THE CLARK FORK RIVER (MT), FOR TUBIFEX TUBIFEX

Rattlesnake Creek is a tributary of the Clark Fork River that flows through parts of western Montana and through Missoula. Currently, an effort is underway to increase spawning habitat for bull and westslope cutthroat trout in the Clark Fork drainage. One proposal in this effort includes the modification or removal of the Mountain Water Company dam on lower Rattlesnake Creek that currently blocks fish passage. The removal or modification of this dam would open miles of prime spawning habitat. However, the Clark Fork River is known to be contaminated with Myxobolus cerebralis, the causative agent of salmonid whirling disease. The presence of this parasite in the Clark Fork River has lead to concerns that removal/modification of the dam would allow the parasite to spread into the upper reaches of Rattlesnake Creek, endangering the wild trout populations already established there. Therefore, the objective of this study was to survey the upper portions of Rattlesnake Creek for the presence of the aquatic oligochaete, Tubifex tubifex. T. tubifex is an obligatory host for M. cerebralis and the parasite cannot be transmitted to trout in its absence. An earlier, cursory study by another investigator did not detect T. tubifex in the Rattlesnake, so we conducted a more comprehensive survey of the creek for this oligochaete. Further, these worms were screened for the presence of M. cerebralis triactinomyxons (TAMs; stage of parasite that infects trout)

Epizootiology of the Causative Agent of Salmonid Whirling Disease in the Rock Creek Drainage: 1998-2008

Whirling disease, caused by the myxozoan parasite Myxobolus cerebralis, remains a serious health threat to salmonid fish in the western U.S. Although various aspects of this host-parasite system have been studied, investigations examining the overall epizootiology of whirling disease in an ecosystem are lacking. Therefore, in June 1998, such a study was initiated in the Rock Creek watershed of west central Montana. Parameters examined included assessing the intensity of infection in trout using sentinel cages stationed throughout the drainage and determining the percentage of Tubifex tubifex releasing M. cerebralis at sentinel cage and other localities. Also, habitat and water quality parameters were measured. Overall, the intensity of M. cerebralis infections in sentinel trout increased significantly throughout the drainage between June of 1998 and 2006 with the biggest jump occurring between 1998 and 1999. In addition, the range of M. cerebralis expanded considerably over this period. There was no strict correlation between habitat condition and the occurrence of the parasite as fish became heavily infected in optimal and marginal habitats. During this period, the parasite apparently caused a dramatic decline in wild rainbow trout densities but the brown trout population numbers increased. However, it now appears that disease intensity peaked in 2006 and is on the decline in this watershed. The decline cannot be directly attributed to a change in the prevalence of M. cerebralis-infected T. tubifex as these numbers remained statistically the same from 1998 to 2008. Similarly, changes in water temperature and water flow do not account for the decrease in disease intensity. However, it is possible that wild rainbow trout are developing some resistance to the parasite, a phenomenon recently documented to be occurring in the Willow Creek Reservoir of southwest Montana