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22 Weighted Usable Area (IFG3) Method I feel that any practical application of the IFG3 model must eventually manifest itself in some relationship between weighted usable area and fish biomass. Thus, although it was not specified as a part of the contract, I collected data on species composition, numbers, and biomass per acre by species for every study stream outlined in the contract in order to evaluate the reliability of the IFG3 model in assessing this relationship. Good data on wild rainbow trout populations were collected only on the Fryingpan River. In all the other streams the rainbow trout present were known to originate from plants of creel size fish. Brook, brown, and cutthroat trout populations were known to be from wild stocks. The biomass (lbs/acre) of trout (by species) was plotted against the weighted usable area (hereafter WUA) measured at the average discharge (from U.S.G.S. gaging data). Regressions for brook trout and cutthroat trout showed no relationship between the two parameters at all. Problems with the reliability and ap- plicability of the data used to generate probability curves for these two species may explain the lack of any correlation in the biomass-WUA relationship.. Bovee (1978), on a classification scale of excellent, good, fair, or reconnaissance grade, rates the prob- ability curves for adult cutthroat trout at only fair or recon- naissance grade. He also recognizes that the probability curves for cutthroat were a composite from several sources and that in- dividual subspecies of cutthroat trout may show great variation from the curves as presently used. Likewise, the probability curves for brook trout (Bovee, ibid) are rated reconnaissance grade in three of four instances and fair in the fourth instance. Bovee (1978) rates the curves for adult brown trout as presently used either good or excellent. A plot of WUA for adult brown trout against brown trout biomass indicates a very good correlation (r=0.87) exists between these two parameters (Figure 1). The data on biomass and WUA for the study streams are given in Appendix A. The data in Table 9 below compares the species composition (per- cent) in the Colorado Phase II study streams and the ratios of WUA by species and life stage. In most cases the species with the greatest WUA advantage was the dominant species in the stream. The Fryingpan River is the best example of the comparison between wild rainbow and brown trout where the adult WUA showed no clear cut advantage for either species. However, the brown trout juvenile and fry WUA was significantly greater than for rainbow trout and population estimations have shown the brown trout to have a slight advantage in biomass at Fryingpan River, Station I and an enormous advantage in biomass at Fryingpan River, Station II. The only real discrepancies that occurred in the WUA-biomass correlation were when brook trout curves were run in conjunction with any other species curves. In these cases brook trout invariably