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138 KAEDING ET AL. TABLE 1.-Variable loadings for the principal com- ponent analysis of loge-transformed morphologic mea- 100 surements taken from 597 specimens of Gila spp. (> 250 mm in total length) from the Colorado River at Black Rocks, Colorado, 1983-1985. U so z Principal Principal w ~ component component w 0 Morphologic measurement I II LL Fork length 0.940 -0.112 z 100 Depth of nuchal depression 0.530 0.768 c Head length 0.851 -0.417 a Distance between insertion of 50 pectoral and pelvic fins 0.807 -0.398 Length of dorsal fin base 0.867 0.279 Length of anal fin base 0.866 0.367 Caudal peduncle length 0.906 0.209 0 Caudal peduncle de th 0 634 -0 591 p only 1(0.4%) of the humpback chub and 19 (7.1 %) of the roundtail chub failed to be supported by the principal component analysis (Figure 1). When the field taxonomic designations in Figure I were replaced by sex determinations, we saw no evi- dence of clustering. Our analysis based on eight morphologic measurements had a greater ability to assign specimens to species than an analysis of a similar number of largely different measure- ments taken from humpback and roundtail chubs in the Yampa River (Douglas et al. 1989). Forty-nine (86%) of the specimens designated as unclassified Gila sp. in the field clustered with humpback chub (Figure 1). Although this suggests that field personnel were conservative in their classification of humpback chub, pairwise chi- square analyses of the relative frequencies of the various dorsal-anal fin ray count combinations showed that the unclassified Gila sp. that clustered with humpback chub were not entirely humpback chub. Highly significant (P < 0.01) differences in ray-count frequencies occurred among all taxa (Table 2). The unclassified Gila sp. group appar- ently included some roundtail chub, hybrids, or backcross individuals with ray-count combina- tions that differed from those of humpback chub. 15 4 14 10 12 HUMPBACK 19 CHUB 3 6 9 10 17 4 4 0 01 7 5 11 7 ROUNDTAIL 29 CHUB 3 4 8 8 13 12 9 7 21 7 1 5 1 1 1 3 6 13 2 MAY JUNE JULY MONTH FIGURE 2.-Seasonal changes in the frequency of oc- currence of expressible gametes in female (closed bars) and male (open bars) humpback chub and roundtail chub of mature size in the Colorado River at Black Rocks, Colorado, 1983 and 1984. Numbers are sample sizes. Not included in the figure are 73 fish collected in April and May, none of which showed expressible gametes. Seasonal Changes in Reproductive Characters We found no important differences between years in either the occurrence of expressible ga- metes or gonadosomatic indices, and those data were pooled within species. Spawning of the humpback chub and roundtail chub temporally overlapped in 1983 and 1984. We captured female humpback chub with expressible ova between late June and late July, whereas female roundtail chub with such ova were captured between mid-June and late July (Figure 2). Although our infrequent sampling in 1985 did not allow us to estimate spawning time, we captured a female humpback chub with flowing ova on 28 June. As in most fishes, the period when male humpback and roundtail chubs expressed milt generally lasted longer than the period during which females ex- pressed ova (Figure 2). Declines in gonadosomatic indices were closely TABLE 2.-Frequency (percent) of combinations of dorsal and anal fin rays (dorsallanal) for humpback chub, unclassified Gila sp., and roundtail chub longer than 250 mm total length. Fish were collected from the Colorado River at Black Rocks, Colorado, 1983-1985. Number of dorsal fin rays/number of anal fin rays Taxon N 8/9 8/10 9/8 9/9 9/10 9/11 10/9 10/10 10/1] 10/12 t~ Humpback chub 269 0.0 3.3 0.4 11.2 65.4 0.7 0.7 17.8 0.4 0.0 Unclassified Gila sp.a 49 2.0 4.1 2.0 34.7 40.8 0.0 0.0 10.2 4.1 2.0 Roundtail chub 250 6.8 0.4 2.0 72.0 18.4 0.0 0.0 0.4 0.0 0.0 a Limited to specimens that clustered with humpback chub in the principal component analysis (see Figure 1).