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.. -29- The data in Table 15 summarizes the history (1984-1991) of water storage in and downstream releases from McPhee Reservoir compared to the response of the downstream trout population. The data for 1988 through 1991 provide insight into what are the minimal water requirements of the aquatic ecosystem in this stream. It is noteworthy that the trout population was still increasing in density and biomass in 1988 and 1989 when downstream water deliveries still exceeded 100,000 AF/water year. A severe decline in the trout population took place between October 1989 and 1990 when the annual downstream releases dropped to 37,700 AF. Only 14,300 AF were delivered to the river between March I and September 30, 1990. Included in this 7-month period was the 100- day period (March 6-June 14) when the flow was 20 cfs. In the 1991 water year, total downstream delivery was 50,800 AF (Table 15) and yet the trout population continued to decline through October 1991. Fifty-one thousand AF of water equates to 17.6% of the inflow during the 1991 water year, but hardly more than 10-12% of the average annual discharge (AAD) into McPhee Reservoir. Fishery biologists have studied the flow needs of fish in streams for several decades (in terms of AAD). Tennant (1975) determined the safe minimum flow for stream-dwelling salmonids in Montana was 30% of the average annual discharge (AAD) and the optimum flow was 60% of AAD. Nehring (1979) evaluated three different streamflow modeling methods on 18 different reaches of 15 trout streams in Colorado. He found the average minimum flow for the three different methodologies (across all streams) equated to 28.4% of AAD for a single transect-single calibratiQn flow method, 26.4% of AAD for a multiple transect-single calibration flow, and 27.9% for the IFIM (Instream Flow Incremental Methodology) system. The IFIM system, a multiple transect- multiple calibration flow method, is currently considered the state-of-the-art method, and is widely used in North America in formulating instream flow recommendations for fish (Reiser, et.al 1989). It is noteworthy how close all three methods compared to the recommendations of Tennant (1975). It is also noteworthy that the Dolores River trout fishery was continually in a state of expansion during the period from 1984 through 1989, when total annual releases to the river were more than 100,000 AF/year and between 40% and 70% of the inflow into McPhee Reservoir. Since the 1990 water year, flow releases to the river have ranged between 37,700 and 50,800 AF (17-26% of the annual inflow). This equates to 10.8% and 14.5% of AAD, assuming the AAD into McPhee Reservoir is 350,000 AF. These flows are significantly lower than the minimum flow requirements for maintenance of a steady-state stream ecosystem (Tennant 1975; Nehring 1979). It is not surprising that the Dolores River trout population has continued to decline through October 1991. A viable aquatic ecosystem is dependent upon its water supply over both the short-term (months) and long-term (years). When the water supply is inadequate, the stream ecosystem responds by supporting less fish. Based on information acquired in this study, two more years of 30 cfs flows from October through April and 65 cfs May through September may result in further reductions in the trout population. Such a year-round flow regime equates to less than 33,000 AF, or 4000 AF less than the river received in 1990. This is