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-21- Table 10. Brown trout fry numbers (5 - 15 cm) captured while electrofishing the Dolores River below McPhee Dam, 1988 - 1991. Sampling Period Station Description 10/88 9/89 10/90 10/91 (CDOW I.D. & River Mile) IFIM Site (3B-20 River mile 0.5) 6 4 10 15 Ferris Canyon Site (3B-15 @ River Mile 6) 3 0 19 80 Rock Stockpile Site (3B-lO @ River Mile 10) 12 36 26 45 Total All Stations 21 40 55 140 Proper flow management for trout reproduction implies maintenance of stable or slowly increasing flows from the onset of spawning, through egg incubation, hatching, and fry emergence. Reductions in flow of 50-75% or more after spawning (but prior to hatching and/or fry emergence) results in little or no fry survival (Becker et al 1982). Rapid or widely varying (two to ten fold) flow fluctuations have devastated rainbow and brown trout recruitment in other Colorado streams (Nehring and Miller 1987; Nehring 1988). Trout fry (~50 mm total length) are very poor swimmers during the first 30 days following emergence and are unable to adapt to rapid changes in depth and velocity associated with rapidly varying streamflow. Examination of the spring-earl~ summer flow regime for the river from 1984 through 1991 (Table 1) reveals why spawning success of rainbow and Snake River cutthroat (SRC) trout has been minimal since the gates closed on the reservoir. Discharge levels exceeded 3000 cfs during the spring-early summer period from 1984 through 1987. Variations in flow during April, May, and June (the rainbow and cutthroat trout spawning, egg incubation, and hatching period) ranged from 125 cfs to more than 4400 cfs between years. Within-year variations ranged up to 20 fold, as occurred in 1985 when the flow varied from 187 cfs to 3700 cfs (Table 1). While flow variations have not been as wide nor peak spring flows as high during the 1988 to 1991 period, the high flow periods have ended as much as a month earlier (May l4-June 7) in 1988 through 1991 compared to the 1984-1987 period when peak flows lasted into late-June or early July (Table 1). Flow reductions to 20-80 cfs (April 4-June 7) during the 1988-1991 period (after spawning flows had been in the 400-1200 cfs range) probably resulted in the loss of either incubating eggs or pre-emergent rainbow and cutthroat trout larvae in the gravel (Becker et al. 1982). In contrast, while prolonged periods (i.e., April through June) of high peak flow are detrimental to fry survival immediately after fry emergence, there is much less chance that flow reductions would be low enough to dry out deposited eggs or pre-emergent trout larvae. Therefore, if there has been any rainbow and/or cutthroat trout spawning success, it is more likely that it occurred during 1984 through 1987 than 1988 through 1991.