Impacts of Whirling Disease Infections on Mountain Whitefish: A Pilot Study
Ron Pierce, Lisa Eby and Richard Vincent
Montana Fish, Wildlife and Parks
There has been little research regarding the impacts of whirling disease on wild mountain whitefish (Prosopium williamsoni) populations. Laboratory exposures of young-of-the-year mountain whitefish to known numbers of Myxobolus cerebralis triactinomyxons demonstrated that infections tended to concentrate in the caudal region and documented significant mortalities during the first 24 hrs following exposure (Beth MacConnell, U.S. Fish and Wildlife Service). Fish that survived the initial exposure exhibited clinical signs of the disease (MacConnell et al. 2002). There have been only a few documented whirling disease infections in wild mountain whitefish populations. Gelwick et al. (2000) reported that M. cerebralis-infected mountain whitefish were found in the main stem Salt River, Wyoming. More recently in 2005, M. cerebralis-infected mountain whitefish have been captured in Mission Creek, a tributary to the Flathead River, Montana (personal communication Craig Barfoot fisheries biologist for the CSKT Fisheries Program). In the Mission Creek out-migration screw trap up 25% of the captured young mountain whitefish had severely deformed caudal peduncles (Figure 1). These same effects are likely occurring in the Blackfoot River Basin in streams where other fall spawners (e.g., brook trout Salvelinus fontinalis) are impacted. For example in Kleinschimdt Creek, MT, brook trout have declined >90% in the presence of high whirling disease infection (Pierce et al. 2004). The potential impacts of the disease on mountain whitefish in these systems has not been evaluated.
Not only is little is known regarding the vulnerability of mountain whitefish to whirling disease, but there is not much information on the distribution and life history of mountain whitefish. This is because it has historically been widely distributed and is not a targeted game fish. Thus, initial infections and population declines may not be detected until the problem is large. Mountain whitefish do not appear to compete with native trout for food, but serve as a very important forage fish for bull trout (Salvelinus confluentus) (Brown 1971). Infected populations may see decreased recruitment and population declines as a result of direct mortality, decreased foraging ability, and increased vulnerability to predation. Population declines of an important forage fish such as mountain whitefish could have large impacts on Montana’s stream fish communities.