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PROCEEDINGS OF THE DESERT FISHES COUNCIL, 1995 SY?PoSIuM VOU M XXVII-PUBUSHED 1996 DOUGLAS, M. E. '; MARSH, P. C. (MED . Dept Zodogy and m..= PCM - Center for Emdrm Sh-l", Arirom State Urdventty. Tempe. AZ) Seasonal and yearly population estimates for Catostomus latipinnis and Pantostem discobolus (Teleostei, Catostomidae) in the Grand Canyon region of Arizona KEYWORDS: flannelmouth sucker; bluehead sucker; population; estimation; little Colorado River ABSTRACT As a result of habitat disturbance by modem humans, four indigenous big-river fishes of western North America [Ptyd:odaeilus &dus (Colorado squawfish), Gila elegans (bonytail chub), Gila gpha (humpback chub), andXyraudfen tetanus (razorback sucker)] are federally listed as endangered. A fifth [Gila robusta (roundtail chub)] has been proposed as a candidate for such listing, and a sixth [Catostomus insignis (flannelmouth sucker)] is under scrutiny. A seventh [Pantosteus discobolus (bluehead sucker)] is believed secure. Life histories are enigmatic for the majority of these fishes, and their densities in most tributaries are relatively unknown. During 1991/95, adult flannelmouth and bluehead suckers were netted, PIT-tagged, and released unharmed on a monthly basis at three locations within the little Colorado River (LCR), a tributary of the Colorado in the Marble Canyon region of northern Grand Canyon. Tag/recapture data were used to generate seasonal and yearly population estimates based on an open-population model allowing for immigration, emigration, and ontogeny. Estimates were normalized by fishing effort and compared by season and year within- and between-species. Within-species variance in population estimates is a reflection of longevity, while between- species differences are related to life history. RESUMEN Como un resultado del disturbio de habitat por humanos modemos, cuatro peces indigenas de rios grandes del Oeste de Norteamerica [ Ptychocheilus lucius (salmon blanco), Gila elegans (charalito elegante), Gila cypha (charalito jorobado) Xyraudwn teranus (matalote jorobado)] estan enlistados a nivel federal como en peligro. Un quinto [(Gila robwta (charalito aleta redonda)] ha sido propuesto como candidato Para esta categoria, y un sexto [(Catostomus insignis (matalote boca de franela)] esta bajo revision. Un septimo [(Pantosteus diswbolus) (matalote cabeza azul)] se tree que esta seguro. Las etapas del cido de vida de la mayoria de estos peces se desconocen y sus densidades en la mayoria de los tributarios son relativamente desconocidas. Durante 1991/95, se capturaron y marcaron con PIT adultos de matalote boca de franela y matalote cabeza azul, y se liberaron sin daflo mensualmente en tres localidades dentro del little Colorado River (LCR), un tributario del Colorado en la region de Marble Canyon en el norte de Grand Canyon. Los datos de marcado/recaptura se utilizaron para generar estimaciones poblacionales estacionales y anuales basados en un modelo de poblacion-abierta permitiendo la inmigracion, migracion y ontogenia. Las estimaciones se normalizaron por medio del esfuerzo de pesca y se compararon por aflo y por estacion dentro y entre especies. La varianza intra-espedes en la estimacion poblacional es un reflejo de la longevidad, mientras que las diferencias entre especies se relaciona con los ciclos de vida. DOWLING, T. E."; TIBBETS, C. A.; NAYLOR, G. J. P. (Department ofzodoU,A&mwState Ur&cnlty.Tempe, AZ) Evolutionary genetics of Gila based on mitochondrial DNA restriction site and sequence data: a progress report KEYWORDS: Gila taxonomy, mtDNA; cytochrome b; restriction sites; evolutionary genetics ABSTRACT In recent years, researchers have invested considerable effort studying members of the genus Gila, especially those species situated in the mainstem Colorado River (Gila gpha, Gila elegans, Gila robusta). Here we discuss preliminary data from the third and final portion of the Gila Taxonomy Project [analysis of 33