Location: May, Idaho

The parasite Myxobolus cerebralis has been implicated in the declines of salmonid species in the west. We observed a high degree of fluvial disruption in the Pahsimeroi River drainage. We found evidence that fluvial disruptions did exist before the settlement of the valley by Europeans and exacerbated since then. We used data from sentinel exposures conducted from 2001-2003 to evaluate factors hypothesized to affect the distribution and intensity of M. cerebralis, the causative agent of salmonid whirling disease. We observed no patterns in infection severity consistently relating to abiotic variables. Over time we observed a variable infection in the middle Pahsimeroi River reach ranging from none to severe M. cerebralis pathology. On the lower Pahsimeroi River reach, we observed a consistent detectable pathology. We compared information gathered on the fish distributions and relative abundance to sentinel exposure results to evaluate the possible role of the salmonid host on infectivity of M. cerebralis. We observed differences in fish compositions in stream reaches that may be linked to M. cerebralis infection dynamics.