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1 1 far greenbacks, appears to be allowing for the downstream expansion of cutthroats into brook trout populations in some streams within RMNP (North Fork of the Big Thompson River, North Inlet and North St. Vrain). However, in other areas, (Ouzel, Hidden Valley, George and Cornelius Creeks) brook trout continue to expand into populations of greenbacks despite no legal angler harvest of greenbacks. Habitat Requirements Habitat requirements of greenback cutthroat trout appear little different from other species of trout. Bulkley (1959) gathered information on age, growth, food habits, and movement of a slightly hybridized population in the headwaters (3,200 m) of the Big Thompson River, Rocky Mountain National Park (RMNP). Nelson {1972) provided data on age, growth, and fecundity of a dense, unexplaited, and slightly hybridized greenback population in Island Lake, Boulder Creek watershed. Restoration efforts should be directed to habitats that are capable of supporting a minimum of 20 kg/ha of fish. Habitats occupied by non-native fish will require their removal prior to the introduction of greenbacks to ' prevent hybridization and competition. Stable reproducing populations of greenbacks in Colorado are rarely found above timberline since cold water temperatures do not allow for sufficient time for spring spawning, hatching and establishment of fry during the short ice-free period. Currently, the highest known elevation of a long-term reproducing population is the Upper Hutcheson Lake population at 3,402 m. Due to the availability of surplus greenbacks, experimental introductions are being conducted in high elevation fishless waters to document the effect of elevation as a limiting factor on greenbacks. Two timberline lakes that were stocked with non-native cutthroats in RMNP, but became fishless after the termination of the non-native fish stocking, were subsequently stocked with native greenbacks. In one of these lakes (Lake Odessa at 3,048 m), greenbacks spawned from late June to early July and established a reproducing population. Greenbacks stocked in the other lake, Crystal Lake (3,511 m) spawned by mid- July, but survival past the egg stage was not documented through 1992. f The lower elevation limit of greenback survival is not known. However, greenbacks stocked in a low elevation lake (1,889 m) on Fort Carson, Colorado, have survived and attained a size of 2.0 kg. Future experimental stocking should involve lower elevation projects to determine greenback survival in low elevation habitats, and in association with native non-salmonid forage species in low elevation habit ate. Reproduction Spawning is generally initiated in the spring when water temperatures reach 5C-8C. Due to the influence of elevation on water temperatures, greenbacks in Lytle Pond on Ft. Carson (1,889 m) spawn by early April, greenbacks in Hunters Creek (2,896 m) spawn in mid-June, and greenbacks in Upper Hutcheson Lake (3,402 m) spawn in mid-July. Although greenbacks are spring spawners, older greenback males in high elevation streams (Hunters Creek and the headwaters of the North Fork of the Big Thompson River within RMNP), were observed to be in spawning colors and running milt in mid-September. Although Como Creek greenbacks can produce eggs at age Z in the hatchery, females in small subalpine streams within Colorado appear to mature after their third to fourth summer of life when they reach lengths of approximately 180 mm.