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-27- Age 2 and 3 trout in the Dolores River make up the majority of the trout population in the 20-40 cm size range, while trout greater or equal to age 4 are for the most part 40 cm and larger (see Table SA, in the Appendix). In theory, habitat restrictions should increase competition between larger adult trout (~40 cm and ~age 4) and smaller and younger (20-40 cm, ages 2-3) adult trout, manifesting itself in higher mortality rates among the age 2 and 3 cohorts. Bachman (1984) clearly demonstrated preferred (energy efficient resting and feeding stations) habitats were occupied by the largest trout on a size-based dominance hierarchy. Furthermore, a limited amount of these preferred habitats existed in the stream. Life table data (Table 3A and 4A, Appendix) were used to calculate an average mortality rate by cohort (age) for both rainbow and brown trout for the Dolores River. These cohort mortality rates were compared to the average cohort mortality rates for all Gold Medal streams in Colorado that contained both rainbow and brown trout. These results are contained in Table 13. The age-specific mortality rates for Dolores River brown and rainbow trout (at age 2 and 3) are much higher than the mean mortality rates for the same age trout from Gold Medal streams (Table 13). To a lesser degree, the same relationship holds for age 4 and 5 brown trout as well. It is of little consequence that the mortality rates for the age 4 and 5 cohorts for Dolores River rainbow trout are slightly lower than those cohorts in Gold Medal streams. The unusually high mortality rates on the 2 and 3 year old cohorts are a much stronger determinant on the total density and biomass that these species can ever reach in the Dolores River under the current water management scenario. The net effect is a very depressed trout population for all cohorts older than age 1, when these unusually high mortality rates are applied to the cohorts in the early years on the life expectancy curve. Table 14. Comparison of total trout density (Njha) and biomass (kgjha) statistics for the Dolores River below McPhee Dam with four other Colorado tailrace trout fisheries. Brown Trout Rainbow Trout Total Trout" Stream Name N/Ha Kg/Ha N/Ha Kg/Ha N/Ha Kg/Ha Dolores Riverb 75 15.3 43 13.3 132 35.3 Gunnison Riverc 681 114.8 472 142.6 1153 257.4 Fryingpan Riverd 726 135.7 481 154.6 1207 290.3 Blue River" 693 106.1 - - -- - - -- 693 106.1 South Plattef 813 146.4 975 340.6 1604 481. 4 AverageS 728 125.8 643 212.6 1164 283.8 " Includes cutthroat trout in b Average of 6 years of data. d Average of 8 years of data. f Average of 8 years of data. the Dolores River estimate. C Average of 10 years of data. " Average of 4 years of data. S Exclusive of the Dolores data.