Kg. Thompson et al., Field exposure of seven species or subspecies of salmonids to Myxobolus cerebralis in the Colorado River, Middle Park, Colorado, J AQUAT A H, 11(4), 1999, pp. 312-329
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 wh
irling disease, on the wild trout fishery in the river. During 1995-1996 an
d 1996-1997, we conducted field exposures of sentinel fish to examine the v
ulnerability of seven species or subspecies of salmonids exposed to the par
asite in the Colorado River. During 1995-1996, brook trout Salvelinus fonti
nalis and Colorado River cutthroat trout O. clarki pleuriticus experienced
85% or higher mortality within 4 months of exposure. Groups of fish introdu
ced 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 s
econd summer. By April 1996, dead fish or sacrificed fish representing seve
n groups were shown by histology to contain mature M. cerebralis spores. Du
ring 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 i
n July survived well and developed fewer M. cerebralis spores (prevalence a
nd mean concentration) than any group except brown trout. Rainbow trout par
ented by fish that recruited after M. cerebralis was established in the dra
inage developed significantly fewer myxospores than these parented by fish
that recruited before establishment of M. cerebralis (P less than or equal
to 0.0001), but survival was not different between these groups (87% versus
89%, respectively). The poor performance of parasitized brook trout and cu
tthroat trout suggests that establishment of M. cerebralis in high-country
streams may have serious consequences for these species.