Relation of Spawning and Rearing Life History of Rainbow Trout and Susceptibility to Myxobolus cerebralis Infection in the Madison River, Montana

The Madison River, Montana, is the site of a whirling disease epizootic among rainbow trout Oncorhynchus mykiss caused by the parasite Myxobolus cerebralis . We investigated how the timing and location of the spawning and rearing of rainbow trout influences the risk of exposure to the parasite. Sentinel fish exposures were used to assess the spatial and temporal variation in disease severity in 1998 and 1999. Redd counts, radiotelemetry of spawning rainbow trout, and electrofishing estimates of fry abundance indicated that during the critical 2–3‐month posthatch period, when rainbow trout are most susceptible to whirling disease, the majority of fry in the Madison River population are concentrated within a relatively small portion of the upper drainage. Disease severity ranged from low to very high in this area of the river. Large temporal variation in disease severity was also evident: severity was significantly higher in the spring than in the fall and in 1999 than in 1998. These data indicate that the effects of whirling disease on rainbow trout populations are governed by a complex interaction between the timing and location of key life history events (spawning, emergence, and early rearing) and the spatial and temporal variation in the presence of the infectious stages of M. cerebralis . Life history diversity within rainbow trout populations may therefore help buffer the effects of whirling disease and help explain why population‐level effects are so variable among infected populations.