infection severity at individual locations (Table 6). An important criterion for 

 determining cage deployment dates is based on water temperatures. Previous studies 

 have shown the highest infection levels coincide with a specific water temperature range 

 of 50 to 61 °F (Baldwin et al. 2000; Downing et al. 2002; Vincent 2002). In the 

 Blackfoot River, these temperatures coincide with an early summer (mid-June through 

 early July) sampling period for many basin- fed streams. 



The recent escalation of the disease appears to be expressing itself through 

 population declines in rainbow trout in the middle Blackfoot River (Results Part II). 

 Likewise, clinical signs of whirling disease (cranial and skeletal deformities), first 

 noticed in 1998, continue 



0) 



Q. 



E 



a 



(0 

 



c 

 tt 



0) 

 Q. 



30 

 25 

 20 

 15 

 10 

 5 

 



1993 1996 



1998 2000 

 Year 



2002 2004 



Figure 48. Percent rainbow trout with observed clinical signs of 

 whirling disease at two survey sites of the Blackfoot River. 



to increase in rainbow 

 trout at both long-term 

 monitoring sections of the 

 Blackfoot River (Figure 

 48). Observations of 

 opercular deformities, 

 thought to be a clinical 

 signs specific to brook 

 trout, are also present in 

 highly infected waters 

 (FWP unpublished data). 



Previous studies 

 have classified salmonids 

 based on susceptibility to 

 the disease, which varies 

 considerably by species 

 (MacConnell and Vincent 2002). All salmonids in the Blackfoot Watershed (WSCT, bull 

 trout, rainbow trout, brown trout, brook trout, and mountain whitefish) can be infected by 

 the parasite. Rainbow trout are reported to be the most susceptible and brown trout and 

 bull trout more resistant (Table 7). 

 The susceptibility of mountain 

 whitefish is unclear but a concern 

 as unpublished research suggests 

 high susceptibility (MacConnell 

 2005). Caudal deformities of 

 mountain whitefish are also 

 increasing in some waters of 

 western Montana (Craig Barfoot, 

 personal communication). 



Unfortunately, populations of 

 mountain whitefish are difficult to 

 sample, making assessments of 

 WD risk problematic. Research 

 into the ecology, life history within 

 a context of WD susceptibility is 

 needed and a topic of research 



Com m on Name 



Susceptibility 



Table 7. Susceptibility to whirling disease among 

 species of salmonids in the Blackfoot River. Scale of 

 to 3 or S: = resistant; 1= partial resistance; 2 = 

 susceptible; 3 = highly susceptible; S = susceptibility 

 is unclear (conflicting reports), (adapted from 

 MacConnell and Vincent 2002). " " " 



S8 



