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Transac[ions of the American Firheries Sxiety 118:131-137, 1989 ® Copyright by the Ameripn Fisheries Society 1989 Origin of Brood Stock and Allozyme Variation in Hatchery-Reared Bonytail, an Endangered North American Cyprinid. Fish W. L. HINCKLEY Department ojZoalogy, Arizona State University Tempe, Arizona 85287, USA DONALD G. BUTH Department of Biology, University of California Las Angeles, California 90024, USA RICHARD L. MAYDEN Department ojBiology, Posl Offue Box 870344, University ojAlabama Tuscaloosa, Alabama 35487. USA Abstract. -Brood stock oCcriticaliy endangered bonytail Gila elegans was obtained between 1976 and 1981 from Lake Mohave, Arizona-Nevada, and propagated by artificial means and by natural reproduction in ponds at Dexter National Fish Hatchery, New Mexico. Twenty-four naturally produced Fz individuals were examined electrophoretically for soluble gene products of 45 loci. Six polymorphic loci were identified: aspartate aminotransferase (sAa[-A), glucose-6-phosphate isomerase (Gpi-B), and phosphoglucomutase (Pgm-A) from skeletal muscle; esterase (Est-2) Crom brain; and catalase (Cat-1) and t.-iditol dehydrogenase (Iddh-A) from liver. Eiectromorph distri- butions at each locus agreed with Hardy-Weinberg expectations. Hatchery bonytail had a lower mean level of heterozygosity than arroyo chub G. orcu[ti. However, direct-count heterozygosity for bonytail was comparable to mean values reported for other western North American cyprinids. Aliozyme variation expressed byhatchery-produced bonyiails suggested a genetically variable stock suitable for reintroductions into appropriate wild habitats. The bonytail Gila elegans is one of the most critically endangered North American freshwater fishes now listed under the U.S. Endangered Species -Act of 1973. This streamlined, riverine cyprinid commonly inhabited larger streams of the entire Colorado River system from Wyoming and Colorado, USA, to northwestern Mexico (Jor- dan 1891; Jordan and Evermann 1896; Gilbert and Scofield 1898; Hinckley 1973; Holden 1980). Since 1976, only one individual has been identi- fied from the upper Colorado River (in 1984; Kaeding et al. 1986), fewer than l0 have been caught in the Green River subbasin (Tyus et al. 1982; H. M. Tyus, U.S. Fish and Wildlife Service, USFWS, personal communication), and no more than five individuals have been taken from Cat- aract Canyon of the Colorado River below its con- fluence with the Green River (Valdez and Wil- liams, in press). The largest number of bonytails, 34 specimens, was collected from a lower Colo- rado River mainstream reservoir, Lake Mohave, Arizona-Nevada, in the period 1976 through 1988 (Hinckley 1983, unpublished). Lake Mohave was impounded in 1954. ~~ a`~ Some specimens tentatively identified as bony- tails from the upper Colorado basin showed mor- phological evidence of hybridization with round- tail chub G. robusta or humpback chub G. cypha (Holden and Stalnaker 1970; Valdez and Clem- mer 1982; see, however, Smith et al. 1979). There- fore,questions ofgenetic contamination have been raised for the species (USFWS, Denver, unpub- lished). Specimens from Lake Mohave are rela- tively uniform in morphology with no external indications of interspecific hybridization (~inck- ley, unpublished). No other species of Gila are now known from Lake Mohave. Reasons for the decline of this species include destruction and alteration of habitat from water developments, introduction and establishment of nonnative fishes, and myriad other human-in- duced factors (Miller 196 i ;Hinckley and Deacon 1968; Tyus et al. 1982; Valdez and Clemmer 1982; Williams et al. 1985). Like razorback suckers Xy- rauchen [exanus in Lake Mohave and elsewhere (Hinckley et al., in press), large bonytails persist, but little recruitment into adult populations has been detected since about the 1950s. A precipitous 131