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q u!JI t{3 &(Y! i /iA(?rsh Copeia, 1998(4), pp. 915-925 (f'e .r1 /) , 1; tA /) (p Population and Survival Estimates of Catostomus latipinnis in Northern Grand Canyon, with Distribution and Abundance of Hybrids with Xyrauchen texanus . MICHAEL E. DOUGLAS AND PAUL C. MARsH Population sizes, movements, and potential hybridization were examined for two indigenous Colorado River fishes, Catostomus latipinnis (flannelmouth sucker) and Xyrauchen texanus (razorback sucker) in the Little Colorado River (LCR) of Grand Canyon National Park and the Navajo Nation (Coconino County, AZ). Catostomus latipinnis is a "species of concern," and X. texanus is federally listed as endangered. Within Grand Canyon, both occur in greatest abundance in the LCR and its conflu- ence with the mainstem Colorado River. During a 50-trip period (I July 1991-27 June 1995), 2619 unique individuals (> 150 mm TL) were evaluated, consisting of 2578 C. latipinnis and 41 putative X. texanus/C. latipinnis hybrids. Cormack-Jolly- Seber estimates (adjnsted for effort) were calculated by trip for C. latiPinnis and ranged from 1591-5214 (average 2507). Seasonal estimates indicated peak move- ments in spring and autwnn (especially the former). During 1991-1993, survival estimates of C. latipinnis were stable over all adult size classes (defined by overall body size) but decreased within larger size classes during 1994. Population estimates remained stable during the four years of the study. It is unknown whether C. lati- pinnis is at carrying capacity within Grand Canyon. A population estimate of putative X texanus/ C. latipinnis hybrids ranged from 8-136 (average 30) for 26 (of 48) trips. Xyrauchen texanus is a transitory member of Grand Canyon's indigenous fish com- munity, moving through the canyon to more viable habitat up- or downriver. This aspect of its life history is now curtailed by Glen Canyon Dam, at the northern terminus of Grand Canyon. ,. THE Colorado River basin comprises nearly 650,000 km2 of the most arid terrain in western North America. It is composed of sev- eral divisions (Minckley et al., 1986): the Colo- rado and Green Rivers (and tributaries) form a distinct zoogeographic segment in the upper basin because they terminated in closed basins. prior to Pliocene. A "contemporary middle seg- ment" straddles both upper and lower basins of the Colorado River [demarcated at Lee's Ferry by the Colorado River Compact of 1922 (Mar- tin, 1989:25)]; it is composed of the White, Vir- gin, and Little Colorado Rivers (and parts of the Bill Williams drainage). The lowermost segment consists of the Gila River and the remaining portion of the Colorado River. Major changes have occurred in these segments as a result of dam construction and chronic dewatering for agriculture and urban development (Fradkin, 1984). The 2400 km of riverine habitat formerly occupied by large-river fishes has been frag- mented into 965 km (Miller, 1982). A corollary to this urbanization and agricultural develop- ment is a significant reduction of fish biodiver- sity in southwestern United States (fig. lc of Dobson et aI., 1997). Minckley et aI. (1986:580) grouped endemic Colorado River fishes into three major catego- ries. His "big-river forms" range throughout the system in larger streams and are composed of seven species: four cyprinids (Gila robusta, G. cy- pha, G. elegans, Ptychocheilus lucius) and three ca- tostomids [Catostomus latipinnis, C. (Pantosteus) discobolus, Xyrauchen texanus]. The first of the cyprinids and the first two catostomids are "spe- cies of concern" (U.S. Fish and Wildlife Service, 1996); the remainder are endangered (U.S. Fish and Wildlife Service, 1994). Catostomus la- tipinnis and X. texanus are the subject of this re- port. Catostomus latipinnis was historically distribut- ed in all moderate-to-Iarge rivers throughout the Colorado River basin (Minckley and Hol- den, 1980). Today, it is essentially extirpated from the lower basin; reintroduction has been attempted by Arizona Game and Fish Depart- ment below Lake Mead (W. L. Minckley, pers. comm.), but C. latiPinnis does poorly in im- poundments (Minckley, 1973). The ecology of C. latipinnis is relatively un- known (McAda and Wydoski, 1985). It typically inhabits pools and deeper runs of rivers and of- ten enters mouths of small tributaries (Minckley and Holden, 1980). In the Yampa River, ripe @ 1998 by the American Society of Ichthyologists and Herpetologists