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136 KAEDING ET AL. though the precise nature of the mechanisms im- portant to these species is unclear. The principal objective of our study was to determine the im- portance of two common reproductive isolating mechanisms, temporal and spatial isolation, to the humpback chub and roundtail chub in the upper Colorado River. We also describe habitat use by humpback chub and discuss its possible impor- tance to the recovery of this endangered species. Methods The study area was the Black Rocks region (39°N, 109°W) of the upper Colorado River, Mesa County, western Colorado. Black Rocks is a nar- row, turbulent, deepwater reach formed by pas- sage of the river through a relatively erosion-re- sistant zone of metamorphic rock. River depths in this 1.8-km reach are several times greater than in adjacent reaches; mean depth is about 5 m, and depths of 17 m have been recorded (Valdez and Clemmer 1982). Collections of humpback chub and roundtail chub were made about weekly from April to July in 1983 and 1984, and during a 2-week period in April-May 1985. Trammel nets were the principal capture gear, but angling was used when debris in the water in spring and during the early-summer runoff precluded the use of nets. Captured fish were visually inspected in the field and initially classified as humpback chub or roundtail chub. Although mature fish of both species could almost always be reliably classified by such visual in- spection, afew individuals showed gross mor- phologies seemingly intermediate between these species. We therefore assigned these questionable specimens to a third category, unclassified Gila sp. Morphologic measurements and meristic counts used to distinguish between these species (Minek- ley 1973; Smith et al. 1979) and taken from each anesthetized fish included caudal peduncle depth, caudal peduncle length, depth of the nuchal depression, distance between insertions of pecto- ral and pelvic fins, head length, length of the anal fin base, length of the dorsal fin base, and number of anal fin and dorsal fin rays. Measurements were taken from the left side with a vernier caliper- depth micrometer, accurate to 0.1 mm. Principal component analysis (Wilkinson 1988) of the loge transformed morphologic measurements was later used to support objectively the taxonomic iden- tifications made in the field. Only fish whose taxo- nomic assignment in the field was supported by the subsequent analysis were considered hump- back or roundtail chubs. In these and all of our later analyses, we used data only from fish longer than 250 mm in total length, a size that represents mature fish (Vanicek and Kramer 1969; Kaeding and Zimmerman 1983). We recorded weight (g) and total and fork lengths (TL and. FL, mm) for each specimen captured. Ova or Holt that could be expressed when pressure was applied to the abdomen were noted. Sex de- terminations were based on external urogenital characteristics (Suttkus and Clemmer 1977) or on the occurrence of expressible gametes for speci- mens later released, and on inspection of excised gonads for fish kept for dissection. In addition to estimates based on the occur- rence of expressible gametes, spawning times for both chubs in 1983 and for roundtail chub in 1984 were estimated on the basis of gonadosomatic in- dices (100 x gonad weight whole-body weight). The small number (eight) of humpback chub sac- rificed for these analyses were females, whereas both female and male roundtail chub were ex- amined. Gonads were removed and preserved in 10% formalin in the field. In the laboratory, most connective tissue was removed, and the gonads were weighed to the nearest 0. I g. Humpback chub not used for gonad studies were tagged (Carlin dangler tag) before release. A11 sacrificed speci- mens have been deposited at the U.S. Fish and Wildlife Service National Fish and Wildlife Lab- oratory in Fort Collins, Colorado. Radiotelemetry was used to monitor the move- ments of fish captured during April and May in 1983 (humpback chub only} and 1984 and 1985 (humpback and roundtail chubs), before the pre- sumed spawning season. Transmitters coated with beeswax and weighing 9 g were surgically implant- ed intraperitoneally. The transmitters were pow- ered by a mercury battery and had a theoretical life of about 90 d. Fish were captured from throughout Black Rocks, radio-tagged, and re- leased at the capture sites 4-24 h after implanta- tion. Weekly searches for radio-tagged fish in Black Rocks and in the adjacent upstream (29-km-long) and downstream (14-km-long) reaches were made from boats. Infrequent searches for missing fish were made outside these areas. Fish locations were estimated by use of three- or five-element Yagi antennas, then refined by use of a bidirectional loop antenna. River discharge and temperature data were recorded at the U.S. Geological Survey gauging station 6 km downstream from Black Rocks.