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montane stream in 1967 prior to restocking with 50 pure greenback cutthroat ' trout, which later established a reproducing population. However, in 1973, two brook trout were found above the barrier, and by 1977, electrofishing for more than one mile above the barrier produced only brook trout (Behnke 1976, 1979). ' The mechanism by which brook trout displace greenbacks is not thoroughly understood. However, in colder habitats, it probably includes an advantage gained through a one year earlier sexual maturation by brook trout and through larger brook trout young-of-the-year (YOY). Brook trout spawn in the fall. Their fry emerge from the redds much earlier in the year than do the spring spawning greenbacks, and the YOY brook trout can be 30 mm longer than YOY greenbacks by their first October. In Hidden Valley Creek, Rocky Mountain ' National Park (RMNP), YOX brook trout (65 mm) and YOY greenbacks (35 mm) are usually found in the shallow stream habitat by October and appear to compete for food and space during winter minimum flows. Fausch and Cummings (1986), found brook trout juveniles to occupy more energetically favorable positions ' than greenbacks in stream habitats when the two were found in sympatry within Hidden Valley Creek, RMNP, and indicated that brook trout juveniles were dominant over juvenile greenbacks (probably due to their larger size). . However, Fausch and Cummings found aggression between adult (>150 mm) brook ' trout and greenbacks to be minimal. Although brook trout dominate greenbacks and represent 60~-90$ of the fish population in Black Hollow and Hidden Valley Creeks, greenback hybrids and Colorado River cutthroats have successfully co-existed for over 40 years and/or dominate (50~ to 90$ of fish numbers) over brook trout within Lake-of- Glass, Thunder Lake and Willow Creek, Rocky Mountain National Park (RMNP). Greenbacks have also demonstrated that they can invade dense brook trout populations in some circumstances, such as in the North Fork of the Big ' Thompson River. Greenbacks were introduced into a fishless habitat above an un-named falls on the upper North Fork of the Big Thompson River, RMNP in 1970, and established a reproducing population. By 1986, greenbacks had drifted downstream, and represented 14.5 percent of the fish over 50 mm in length in the stream section from Lost Falls to the unnamed falls. In this section, brook trout did not exceed 280 mm in length, though greenbacks reached 304 mm. Arkansas River greenbacks in Lytle Pond (U.S. Army, Ft. Carson) successfully coexist with brook trout, with brook trout numbers declining. However, spawning habitat at Lytle pond is less favorable in the fall than in the spring, and may provide greenbacks with a competitive advantage over brook trout at this location. Brown trout. Wang (1989) observed the behavior and competition of yearling South Platte greenbacks and brown trout in an indoor stream aquarium. Brawn trout were found to be more aggressive than equal-sized greenbacks. Brown trout even outcompeted greenbacks that were 1.27 times longer and 1.69 times heavier. Slow current combined with dim light significantly increased ' attack frequency of brown trout on greenbacks. Few greenback restoration projects involve former brown trout habitat. However, the dominance of brown trout over greenbacks (as indicated by Wang's study) is evident in George and Cornelius Creeks, where brown trout appear to be displacing greenbacks (Steve Puttmann, Colorado Division of Wildlife, pers. comm., 1991). Angler Harvest. The removal of adult greenbacks by anglers, may have had a negative impact upon greenbacks, especially when exotic salmonids were , present. Cutthroat trout are more easily caught than other species. Removal of the older, larger greenbacks might favor brook trout, which reproduce at smaller sizes and younger ages. Changes in fishing regulations in effect since 1982 within RMNP that limited the harvest of non-native cutthroats and , Colorado River cutthroat to two fish over 250 mm, and catch-and-release only