Field Exposure of Seven Species or Subspecies of Salmonids to Myxobolus cerebralis in the Colorado River, Middle Park, Colorado

Recent failures in recruitment of rainbow trout Oncorhynchus mykiss in the Colorado River in Middle Park, Colorado, USA, led to studies of the effect of the myxosporean parasite Myxobolus cerebralis , the causative agent of whirling disease, on the wild trout fishery in the river. During 1995–1996 and 1996–1997, we conducted field exposures of sentinel fish to examine the vulnerability of seven species or subspecies of salmonids exposed to the parasite in the Colorado River. During 1995–1996, brook trout Salvelinus fontinalis and Colorado River cutthroat trout O. clarki pleuriticus experienced 85% or higher mortality within 4 months of exposure. Groups of fish introduced at greater mean weight or later in the summer tended to survive better than others, but mortality often was severe among these groups during the second summer. By April 1996, dead fish or sacrificed fish representing seven groups were shown by histology to contain mature M. cerebralis spores. During 1996–1997, three groups of cutthroat trout O. clarki spp. experienced poorer survival than brown trout Salmo trutta and rainbow trout introduced at similar weights. Snake River cutthroat trout O. c. bouvieri introduced in July survived well and developed fewer M. cerebralis spores (prevalence and mean concentration) than any group except brown trout. Rainbow trout parented by fish that recruited after M. cerebralis was established in the drainage developed significantly fewer myxospores than those parented by fish that recruited before establishment of M. cerebralis ( P ≤ 0.0001), but survival was not different between these groups (87% versus 89%, respectively). The poor performance of parasitized brook trout and cutthroat trout suggests that establishment of M. cerebralis in high‐country streams may have serious consequences for these species.