21 



Ribbons of the Big Sky Country APO 1979). 



It could also be that streamflow variability was not adequately estimated. 

 The summer low flow may often be less due to irrigation withdrawal than it was 

 when measured in 1982, a fairly wet summer. Also, there may be withdrawals 

 for municipal water supply, making winter low flow severe in the Bozeman Creek 

 system. This was not analysed in our study. 



Insufficient reproduction of trout could be another factor preventing 

 standing crop from reaching the potential indicated by the HQI. Although 

 age-structure analyses of the trout populations have not yet been done, the size 

 structures of the Bozeman Creek rainbow trout population, the main fish in that 

 stream, appear to indicate fewer fish of age I and II than of age III or IV. This 

 is the reverse of the situation that must exist in a population being replenished 

 by local reproduction. It is likely that the rainbow trout population of our 

 study area on Bozeman Creek consists largely of immigrants from upstream or from 

 the East Gallatin River, downstream. The combination of immigration, body growth 

 and whatever reproduction exists within the study may not be creating enough 

 biomass to saturate the habitat. If low reproductive rate is also an influence 

 for brook trout in Bozeman Creek and in the other study streams (it is the 

 predominant fish in the other streams), it is not as strongly the case. Size 

 distributions of brook trout in most study stations indicate that a more normal 

 age structure probably exists. 

 Multiple-regression Modelling of Trout and Habitat Relationships 



When logarithms of the habitat variables involved in the HQI (but not ~^ 

 transformed to class ratings) were entered as independent variables into stepwise 

 multiple regression against logarithms of each of the four trout abundance expression; 

 used separately as dependent variables, stronger correlations resulted than in the 



