2002). With regard to whirling disease status in the Blackfoot Watershed, many of these 

 factors are discussed below. 



Sentinel cages provide an indirect measure of TAM abundance in tested waters, 

 and were first deployed in the Blackfoot Watershed in 1998 (see methods). Sentinel cage 



Mean grade 

 category 



0.0-2.0 

 2.01-2.74 

 2.75-3.7 

 3.71-5.0 



Infection Level 

 Description 



Low 

 Medium 



High 

 Very High 



Table 15. Mean grade category 

 descriptions 



monitoring has continued through 2003 at 



established Blackfoot River sites and 



throughout tributaries in order to assess 



disease expansion. A mean grade is 



determined from histology results from 



sentinel fish exposed in each cage to 



determine infection severity at individual 



locations (Table 1 5). 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. Results of recent 



Blackfoot cages show progressive increases in the disease in the up-and downriver 



directions, and a corresponding expansion in the lower reaches of many tributaries 



(Figure 55). During the last two years, whirling disease in the mainstem Blackfoot River 



has 1) expanded (-20 river miles) to the upper mainstem above Lincoln, 2) increased 



infection levels in the lower Blackfoot River (below the mouth of the Clearwater River), 



and 3) appears to have 



stabilized in the middle 



Blackfoot River at moderate 



to high levels. 



The recent escalation 

 of the disease is also 

 expressing itself at two long- 

 term Blackfoot River 

 (Johnsrud and Scotty Brown 

 Bridge) population 



20 



18- 



16- 



14 



12 



10 



8-1 



6 



4 



2-1 







1993 



1996 



■ Johnsrud 

 I Scotty Brawn 



Figure 56. Rainbow trout showing clinical signs of WD in 

 two population monitorins sections in the Blackfoot River. 



monitoring sites, where 

 clinical signs of whirling 

 disease (cranial and skeletal 

 deformifies), first noticed in 

 1998. have dramatically 

 increased in 2000 and 2002 

 (Figure 56). 



Previous studies have classified salmonid 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 

 whitefish) can be infected by the parasite, but rainbow trout are reported to be the most 

 susceptible, and brown trout and bull trout more resistant (Table 16). This species 

 susceptibility description coincides with population (species) changes in Blackfoot 



93 



