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DOUGLAS AND MARSH-GRAND CANYON CATOSTOMIDS 919 100 80 ; III > > .. :J en 40 60 "<f!. 20 o 1 2 3 4 5 6 7 8 9 10 Size Classes Fig. 3. Yearly survival probability by size-class for adult Catostomus latipinnis (> 150 mm TL) in the Lit- tle Colorado River (Navajo Nation, Coconino County, AZ). Probability values are for years 1991-1994. reported, based upon morphological identifi- cation at different capture dates. The 41 indi- viduals were recaptured a total of 60 times: a single individual was recaptured seven times; two individuals were recaptured five times; five individuals four times; eight three times; eight two times; and 17 but once. Additionally, 85% (35/41) were male, and 12% (5/41) were fe- male (one individual was not sexed). This sug- gests males may be more vagile or more nu- merous than females. Also, males were express- ing gametes in 46% (38/82) of captures, whereas 79% (65/82) were tuberculate. Twenty- four individuals had multiple (Le., > 1) cap- tures and, thus, at least two chances for sex de- termination. Only two contradictions were not- ed out of 76 opportunities (3%). Mitochondrial DNA was evaluated in 12 of 41 putative hybrids. Seven of these (58%) had X. texanus mtDNA (T. E. Dowling, pers. comm.). Five of the seven were electrophoretically iden- tified as hybrids, whereas two were not evaluat- ed (D. G. Buth, pers. comm.). Of five individ- uals with C. latipinnis mtDNA phenotypes, three were of hybrid origin, one was pure C. latipinnis, and one was electrophoretically unscorable. Thus, in synopsis, nine (of 12) individuals with morphologies suggesting hybrid origin were evaluated electrophoretically and with RFLP analysis of mtDNA. Eight of the nine (89%) were of hybrid origin, whereas one was pure C. latipinnis. None of the eight was an F] hybrid. Instead, they were backcrossed to C. latiPinnis in varying degrees (ranging from 60-90%). The one individual judged 60% C. latipinnis ap- peared to be an F] X F] hybrid (D. G. Buth, pers. comm.). Population estimates for X. texanus/C. latipin- nis hybrids over 26 of 49 trips (Appendix) were unadjusted for effort and consistently small 91 (08-136; mean = 30). Captures varied across seasons: 46% (43/94) occurred in spring; 32% (30/94) in autumn; 12% (11/94) in summer; and 10% (10/94) in winter. Clearly, the fish were more vulnerable to capture in spring and autumn, presumably due to greater movement or use of shallower habitats. Captures were occasionally numerous. Six tu- berculate males were captured with C. latipinnis in November 1991, 3.1 kIn above the LCR con- fluence. None was expressing gametes; an in- dividual captured at the confluence two days earlier was. Eight tuberculate males were simi- larly captured with C. latipinnis in April 1992, 3.01 kIn above the confluence (one was express- ing gametes). Likewise, 11 tuberculate males were captured with C. latipinnis April 1995, 2.98 kIn above the confluence. All were expressing gametes. Overall, 56% (53/94) of captures oc- curred approximately 3 kIn (or greater) above the confluence (one occurred almost 11 kIn above). Average capture distance above conflu- ence was 2.2 kIn. 92 94 93 DISCUSSION Catostomids are primarily benthic, and the basal stock is a deep-bodied fish of large, low- gradient rivers. A major adaptive event in the radiation of this family was gradual diversifica- tion of mountain suckers (Smith and Koehn, 1971; Smith, 1992a). The two study species of this report are members of the subfamily Catos- tomini and are part of an indigenous but de- clining Colorado River fish community. Catostomus latipinnis.-In Marble and Grand Canyons, Carothers and Minckley [So W. Ca- rothers and C. O. Minckley, U.S. Bureau of Rec- lamation, Final Rpt., 1981, unpubl. (hereafter USBR, 1981, unpubl.)] caught ripe C. latipinnis from March-May (= spring) at the mouth of the Paria River and other low-gradient streams. Postreproductive adults remained in these hab- itats through summer but returned to mainstem in winter when temperature equilibrated be- tween tributary and mainstem (Suttkus and Clemmer, 1979). Our results concur with and extend these observations. In the LCR, greatest numbers of C. latiPinnis occurred either in mid- summer (1993, 1994) or early autumn (1992). Populations gradually increased in number be- fore peaking significantly in late spring and then declining significantly into autumn/win- ter. A smaller population peak often occurred in late summer/early autumn as an apparent reproductive response to late summer rains. Our consistent annual population estimates