The most useful data of the 1987 study were the fish 

 population estimates for the spring-early summer period. 

 Figure 2-1 displays estimates of the numbers of rainbow and 

 brown trout 7.5 inches or more in total length in 31 sections 

 covering 135 river miles (RM) . Exact comparison with 

 estimates generated in previous years is not possible because 

 section lengths vary due to the improved mapping and 

 measuring techniques in 1987. Older estimates were based on 

 numbers of trout 6 inches or more in most cases. Despite 

 these computational differences, estimates from 1987 are very 

 similar to those from previous years. 



Data presented in Figure 2-1 show that fish population 

 distribution varies considerably from the headwaters to 

 Milltown. In the uppermost sections from the Warm Springs 

 Pond 2 outflow to the end of the pH shack section (RM 501- 

 498), brown trout densities were between 1,500-2,000 fish per 

 mile. A precipitous drop in trout numbers to a level of 

 about 500 per mile, occurred between the end of the pH shack 

 section and the Galen Bridge (RM 498-491) . From the Galen 

 Bridge to below Drummond (RM 491-409) populations slowly 

 declined in density from about 250 per mile to 150 per mile. 

 A more abrupt change occurred from about Bear Creek to 

 Beavertail (RM 409-385) where populations of trout were about 

 50 per mile. Rainbow trout numbers became significant in 

 this section, presumably reflecting the influence of 

 recruitment from Rock Creek. Trout population numbers 

 increased substantially to about 250 per mile in the segment 

 from about the mouth of Rock Creek to Milltown Dam (RM 385- 

 366) . Rainbows were the most abundant trout, with brown 

 trout the other dominant species in this segment. 



Trout Spawning and Rearing Habitat 



Throughout the Clark Fork above Milltown, with the 

 exception of the Warm Springs section, trout populations 

 appear to be of lower density than the habitat might support. 

 The factors that determine trout abundance over much of the 

 upper river are not well known nor easily discernable. If 

 physical habitat in the most basic sense is present in excess 

 of population levels, then some other factor (s) must be 

 limiting population density. Either the number of trout 

 available from reproductive efforts is inadequate to fill the 

 available habitat, or something kills a significant fraction 

 of the population on a regular or, at least, frequent basis. 



Conditions for trout reproduction in the river are 

 poor. Most of the upper river seems unsuitable for trout 

 reproduction due to siltation and other substrate deficien- 

 cies. Successful reproduction may occur in the uppermost 

 reaches of the river near Warm Springs, at least in some 



2-29 



