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002J28 Part I IllTRODUC'rION The greenback cutthroat trout, (Oncorhvnchus clarki stomias, formerly Salmo clarki stomias), is one of the most colorful subspecies of cutthroats (Figure I), and was cne of the rarest. At the time of the enactment of the Endangered Species Act in 1973, cnly two small historic populations of greenback cutthrcat trcut were known to exist - Como Creek and South Fork, Cache La Poudre River - that conformed to the meristics of the type specimens. These two small headwater streams of the South Platte River drainage collectively represented 4.6 kilometers of stream habitat and supported less than 2,000 greenbacks. Since then, seven additional historic populations have been identified, five pcpulations in the South Platte River drainage and two, populations in the Arkansas River drainage. The historic populations are listed in Table 1. Contrary to the common name of the fish, the back of the greenback is not especially green in cclor. In older age classes (4 years or more), mature males display crimson red,colors:along the ventral region during the spring spawning season, especially in laks environments. Historic Distribution The greenback is native to the headwaters of the South Platte and Arkansas river drainages within Colorado and a small segment of the South Platte drainage within Wyoming. The greenback and the Rio Grande cutthroat trout (Oncorhvnchus clarki virainalis), represent the easternmost limits of native trout distribution in the western United States, (Behnke, 1984). The greenback declined so rapidly in the 1800's that the original distribution of the subspecies is not precisely known. Behnke and Zarn (1976) assumed the original distribution included all mountain and foothill habitats of the Arkansas and South Platte drainages (Figure 2). The greenback was known to "cccur within these drainages at lower elevations than it occupies today, however, little is known of its exact historic lake and stream distributicn and the range in elevation it once occupied. The cnly other trout thought to have occurred within the greenback's native range was the yellowfin cutthroat (Oncorhvnchus clarki macdonaldi) collected from Twin Lakes (Arkansas 'River drainage) in 1889 (Behnke 1979). The yellowfin cutthroat became extinct in the early 1900's. Tvoe SDecimeuB According to Behnke (1979), "There is considerable confusicn concerning the name stomias in regard to where the original type specimens actually came from. It is possible that the specimens on which the name is based were not greenback trout taken from the South Platte drainage. Cope (1872), in the same publication in which he names ~. nleuriticus, named ~ stomias from specimens collected ~r9m: "The South Platte River at Fort Riley, Kansas." The South Platte River drainage does not enter the State of Kansas. In later 1