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<br />' drainages as a result of stocking in the late 1800's and early 1900's. The occurrence of brook and <br />brown trout in Rito Seco creek is not unexpected and is a pattern exhibited in similar Colorado <br />' streams. <br />The cutthroat trout collected in Rito Seco creek were native Rio Grande cutthroat trout <br />(Salmo clarkr virginalrs). Thirty cutthroat trout collected from Rito Seco creek and Indian Creek <br />by Colorado Division of Wildlife biologists in 1958 showed no signs of hybridization and this <br />' creek apparently holds a pure strain of native Rio Grande cutthroat [rout (R.J Behnke, CSU, <br />pers. comm.). The Rio Grande cutthroat is not listed as Threatened or Endangered by either <br />Federal or State agencies, but is listed as a Species o/ Special Concern in Colorado (Jim Bennett, <br />' CDOW, pers. comm.). <br />Fathead minnows are native to the Rio Grande drainage (Woodling 1985) and are common <br />throughout Colorado. <br />' Population Estimates <br />' Trout biomass ranged from 39.5 kg/ha (35.2 lbs/ac) at Site 1 to 101.4 kg/ha (90.3 Ibs/ac) <br />at Site 2 (Table 2). The variation in trout biomass appears to be related to the amount of <br />available habitat at the three sites. At Site 1 the channel was much straighter, with predominantly <br />' riffle habitat and a few plunge pools. At Sites 2 and 3 the greater development of pools together <br />with the greater variety of habitats associated with the meandering channel resulted in higher <br />' fish biomass. <br />The size distribution of brook trout at the study sites (Fig. 2) indicates that all life stages <br />' from Fry through adults are present. This suggests that self-sustaining populations of brook trout <br />are present in these reaches of the stream. The length-frequency distribution for brown trout also <br />' indicates that all life stages are present at all three sampling sites (Fig. 2). Brown trout have not <br />been stocked since 1980 and the populations appear to be self-sustaining. <br />' Only five native cutthroat trout were collected, four at Site 1 and one at Site 3. The sizes <br />ranged from 80 mm to 210 mm (3-8 inches), indicating the presence of juveniles and adults. <br />' Combined with the fact that this species has not been stocked, this suggests that natural <br />reproduction is occurring for this species, as would be expected on the basis of the 1958 <br />t collections. However, only five fish were collected in our study area. It appears probable that <br />these fish migrated downstream from reproducing populations in the upper reaches of Rito Seco <br />creek. <br /> <br />' S <br />1 <br />