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PROCEEDINGS OF THE DESERT FISHES COUNCIL 1993 - VOLUME XXV CABRERA FEREGRINO, JOSE A. Facultad de Ciencias Riologicas, Universidad Juarez Autonomy de Tabasco Contribution to the reproduction and ontogeny of the Poecilid Poecilia petenense Contributi6n a la ontogenia y reproducti6n del Poecilido Poecilia petenense KEYWORDS: Heterogonia; Superfetal development; Gonadosomatic Index; Fertility index; sailfin molly ABSTRACT Poeciliids belong to the neotropical fauna showing a distribution northbound up to the Rio Bravo. Poecilia petenense (the sailfin molly) is distributed predominantly in the region of Peten in Guatemala and the Usamacinta River basin in Mexico. This species has a great worldwide aquarium trade demand mainly because of its great dorsal fin, from which its common name sailfin molly was derived. In the state of Tabasco it is know as "topota" and has great demand as live bait for artesanal and sport fisheries. This study was carried on to learn about the gonadosomatic and fertility index, the presence of superfetal development, and to describe the ontogenic development and the heterogenic index for the relationships TL/SL, TL/CL and CL/OD. The fertility index of 62.78 (upper-lower limits of 22 and 105) and gonadosomatic index of 3.22% (limits of 0.02 and 14.35) were obtained. Superfetal development is only occasional. Ontogenic development for all characters showed positive allometry. RESUMEN La familia Poecilidae pertenece a la fauna neotropical, presentando distribuc16n hacia el norte, hasty la cuenea del rio Bravo. Poecilia petenense se distribuye principalmente en la reg16n del Peten, Guatemala y en los drenajes del rio Usumacinta en Mexico; esta especie posee una importancia acuariofilica donde tiene una gran demanda por to llamativo del desarrollo de su aleta dorsal, por to cual tambi6n se le conoce como aleta de velo. En el Estado de Tabasco se le conoce como topota y es utilizado principalmente como carnada en pesca artesanal y ocasionalmente la deportiva. Los objetivos de este trabajo fueron establecer el IGS e IFe, la presencia de superfetaci6n, caracterizaci6n de algunos aspectos ontogen6ticos y establecer el fndice de heterogonia para las relaciones LT/LP, LT/LC y LC/DO. Encontrandose un IFe absoluto medio de 62.78 (con limite inferior de 22 y superior de 105), un IGS medio de 3.22% (con lfmites de 0.02 y 14.35%), encontrandose que la superfetaci6n es ocasional. En la caracterizact6n de ]as etapas del desarrollo ontogenetico se encontr6 que ]as relaciones LT/LP, LT/LC y LC/DO presentanon todas alometrfas positivas. [BOTH STUDENT PAPER COMPETITOR] i HOFFNAGLE, T. L. ; PERSONS, W.R.; DOSTER, G. TLH and GD - Research Branch, Arizona Gatnc and Fish Dkpanment, Flagstaff, Az; WRP - Research Branch, Arizona Game and Fish Depart nt, Phoenix, AZ Use of backwater areas by juvenile native fishes in the Colorado River, Grand Canyon Uso de las areas de remanso por los juveniles de peces nativos en el Rio Colorado del Gran Can6n KEYWORDS: Colorado River; Grand Canyon; native fish; speckled dace; humpback chub; tlannelmouth sucker; bluehead sucker; backwater CLAVES: Gran Can6n; peces nativos; pez moteado; charal jorobado; matalote boca de franela; matalote cabeza azul; remanso ABSTRACT Use of backwater habitats in the Colorado River, Grand Canyon, by juvenile native fishes was studied over the past year. Comparisons of relative abundances within reaches revealed that speckled dace (Rhinichthys osculus) (48.5%) and flannelmouth sucker (Catostomus latipinnis) (32.5%) were the more common species in Reach 20 [Lee's Ferry (River Mile 0) to the Little Colorado River (LCR; RM 61.5)]. Humpback chub (Gila cypha) (33.3 %) was the most common species in Reach 30 [LCR to Bright Angel Creek (RM 87.6)]. In Reach 40 [Bright Angel Creek to National Canyon (RM 166.4)], bluehead sucker (Catostomus discobolus) (35.7 %) was most abundant. Speckled dace (33.8 %) and bluehead sucker (24.6 %) were the more common species in Reach 50 [National Canyon to Diamond Creek (RM 225.6)]. Comparisons among reaches showed that most bluehead suckers (66.5 %), flannelmouth suckers (52.4%) and speckled dace (67.9%) were collected in Reach 50, whereas most humpback chubs (87.6%) were taken in Reach 30. Humpback chubs caught in backwaters at night were significantly larger than those taken during the day. No significant relationships were found for comparisons of catch rates with water temperature, substrate, depth, turbidity, or ambient light. Juvenile native fishes were more common in areas near known spawning areas for these species, and it appears that the measured environmental variables had little effect on their use of backwaters. 33