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<br />c;-; <br />co <br /> <br />establishment of white suckers in that system, Some of this data suggests that the <br />frequency of hybridization among suckers may be increasing, Results of recent <br />collections from the La Plata River indicate that the fathead minnow may be the most <br />abundant non-native species in that system, <br /> <br />('"'".:) <br />c,") <br />f') <br /> <br />I"."' <br /> <br />DISCUSSION <br /> <br />Historical fish fauna surveys provide a temporal account of native and non-native <br />species, and their abundance and composition. A brief discussion of distribution, <br />abundance and habitat use for each species collected during historical fish surveys with in <br />the study area is provided, <br /> <br />Historical Fish Distribution <br /> <br />Native Fishes <br /> <br />Gila robusta (roundtail chub): Roundtail chub are native to the San Juan drainage. The <br />earliest reports of this species in tributaries of the San Juan River came from collections <br />made in the Animas River during 1934. Subsequent sampling efforts in the Animas <br />River through 1963 indicated that a larger population than occurs today may have been <br />present. Since 1963 more than 60 sampling attempts in the Animas River have resulted <br />in the capture of only two specimens of round tail chub. Apparently this species has <br />become rare in the Animas River, and established populations that were found <br />historically no longer occur. Since the early 1970s roundtail chubs have been reported in <br />collections from the San Juan River above and below Navajo Reservoir, and from most <br />tributaries to the San Juan River below Navajo Reservoir, Some of the largest <br />populations were reported from the Mancos River and Navajo Wash. Representatives of <br />this species have also been found in the Florida 10ver (a tributary to the Animas), which <br />may account for the occasional occurrence ofthis species in the Animas River. <br /> <br />A habitat study to document seasonal habitat use by native species was conducted during <br />1994 (Miller et al. 1995). The results of that study provide the habitat associations listed <br />below. During the 1994 habitat study representatives of all size classes were present in <br />the data, The young-of-year (yay) size class was encountered primarily in low velocity <br />habitats (glide and particularly pool habitat), Most ofthe roundtail chubs encountered <br />during this study were within the juvenile size class, Juvenile and adult roundtail chubs <br />in this study were mostly found inhabiting areas of low velocity. Roundtail chubs were <br />typically found in pool habitat and were often associated with woody debris. Although <br />few round tail chub adults were collected during this study the trends in habitat use were <br />similar to those observed by the juvenile size class, Most adult roundtail chubs were <br />collected from pool habitat. Glide habitat was occasionally occupied, but no adults were <br />collected from riffle habitat. <br /> <br />Rhinichthys osculus (speckled dace): The speckled dace is a native fish in the San Juan <br />River and its tributaries. Historically, this species has been one of the most frequently <br />encountered fish in collections from the San Juan drainage. The earliest report of this <br /> <br />Draft Final Tributary Report <br />Miller Ecological Consultants, Inc. <br /> <br />January 31, 2000 <br />Page 20 <br />