20 



This could be interpreted in several ways. The HQI model may not be applicable 

 to the trout/habitat relationships in the kinds of streams included in our study. 

 The poor fit of the data might also be due to deficient accuracy or precision 

 in our measurement of some habitat variables or of standing crop of trout. Some 

 of the habitat measurements called for in the HQI method seem more subjective than 

 would be desirable (e.g., vegetative substrate) or anotherwise difficult to 

 accurately determine (e.g. annual streamflow variation), but this may be 

 compensated for at least partially by the transformation of measurements to class 

 ratings involved in the procedure (Binus 1979). 



Comparison of the results of the HQI method against results of the multiple- 

 regression analysis in the following section of the report would seem to indicate 

 that much of the unpredictiveness of the HQI in our streams lay in the procedure 

 of combining all forms of cover, i.e., failure to distinguish between pool-and- 

 turbulence cover and solid overhead cover. Comparison of the predictiveness of 

 the HQI method in our streams against the better predictiveness of the multiple- 

 regression analyses desribed below may also point to an inappropriateness of using 

 biomass per unit of stream surface area as an expression of trout abundance. 



It is far more important to consider the possible inference from the 

 lower-than-predicted distribution of actual trout abundances that some 

 unmeasured influence is preventing trout in these streams form being as abundant 

 as the HQI predicts they should be, i.e., as the physical habitat would allow. 

 Actual abundance is lower than HQI-predicted abundance particularly in the 

 stations with highest predicted values, i.e., highest habitat rating (Fig. 3). 

 Candidate variables for such abundance-depressing effect would be water pollution 

 and intense angling harvest. Neither were measured in this study, however, urban 

 water pollution problems were revealed in a previous study on Bozeman Creek (Blue 



