The declines of the Canadian and T^rryall populations were 

 followed by a rebuilding of the population. In the Canadian popu- 

 lations, Stelfox (1976) found after a 25 year period the population 

 had achieved their previous levels. Spencer (1943) reported the 

 T^rryall herd had exceeded its previous estimated populations only 

 16 breeding years after its catastrophic decline. Line C (Figure 

 2) illustrates a population trend for the Ural-Tweed herd approxi- 

 mating a recovery rate similar to these populations. This popula- 

 tion trend shows a dramatic decline leveling off at 25 percent of 

 the original level (42 animals) and then rebuilding to the original 

 population level after a 25 year period (1990). The Ural-Tweed 

 herd should have experienced a population trend similar to this. 

 Since this herd has not started to rebuild (Brown 1983, pers. 

 commun.), a factor which is suppressing the population growth is 

 indicated. 



EkxJlogical successicxi has advanced, largely due to active fire 

 suppression, causing a further loss of suitable bighorn sheep habi- 

 tat. However, the bighorn sheep population should have responded 

 after the decline of the mid-1960's, rebuilding to a population 

 level somewhere below the peak. To determine the level to which the 

 population should have rebuilt, Line D (Figure 2) was produced to 

 illustrate a natural population fluctuation based on the availabil- 

 ity of suitable habitat. This is a normal fluctuation occurring 

 naturally in most populations. Using this reasoning, a 1990 popula- 

 tion of 115 sheep was estimated, the same number determined to be 

 present 25 years before the peak (1940). The population trend 

 illustrated by Line E approximates the population recovery that 

 should have occurred following the decline, if only the advancement 

 of ecological succession was suppressing the Ural-Tweed population. 

 The population would have stabilized at approximately 115 sheep. 



The remaining factor that could be suppressing the expected 

 population recovery is the Libby Dam project with the resulting 

 loss of habitat and increased human disturbance. The impact of the 

 project and associated facilities was assumed to be the difference 

 in population levels between the existing population (25 animals) 

 and what would be expected if only the advancement of ecological 

 succession was acting on the population (115 animals). Using this 

 assumption, 90 bighorn sheep (115-25) were lost from the Ural-Tweed 

 population as a direct result of the construction of the Libby Dam 

 project and associated impacts. A plus or minus 10 percent range 

 for the estimated population level yielded a range of 78 to 102 

 sheep impacted by the project. 



A qualitative loss estimate of high was assessed based on 

 criteria (a) through (d) on page 9. Inundation of winter and 

 spring ranges, displacement of seasonal ranges (i.e. lambing areas) 

 and additional human disturbance were considered in the development 

 of this assessment. 



36 



