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PROCEEDINGS OF THE DESERT FISHES COUNCIL 1992 - VOLUME XXIV <br />CLARKSON, R. W. (Research Branch, Arizona Game and Fish Department, Phoenix, AZ) <br />Foods of young-of-year native fishes in the Little Colorado River, Arizona, and infestation <br />patterns by Asian fish tapeworm, Bothriocephalus acheilognathi / Alimentos de peces nativos <br />j6venes en el Rio Colorado, Arizona, y patrones de infestation por el pez vermiforme <br />asiatico, Bothriocephalus acheilognathi <br />KEYWORDS: Gila cypha; Rhinichthys osculus; Pantosteus discobolus; Bothriocephalus acheilognathi; food habits; <br />parasites; Little Colorado River <br />ABSTRACT <br />Foods and tapeworm infestation rates were examined from stomachs of larvae, early post-larvae, and juvenile <br />speckled dace (Rhinichthys osculus), bluehead sucker (Pantosteus discobolus), and humpback chub (Gila cypha) from <br />the Little Colorado River in 1990-1991. Chironomidae was an important food item for all three species, with the two <br />cyprinids reducing their utilization of this component with age, and bluehead sucker increasing its use with age. Ostracods <br />were used most often by larval and early post-larval dace, and by juvenile chub. Vascular plant material was noteworthy <br />in stomachs of younger dace and chub, while bluehead sucker frequently contained inorganic materials. Asian fish <br />tapeworm suddenly appeared in chub stomachs in 1990. Three larvae, 11 early post-larvae, and 47 adult and sub-adult <br />chub examined from 1989 did not contain tapeworms, while chub larvae collected from May-June 1990 (n =24) exhibited <br />a 91.7% infestation rate, older 1990 young-of-year specimens (n=15) exhibited a 100% infestation rate, and 44% of 18 <br />adults examined from 1990 were infested. In contrast, tapeworms did not appear in 1991 young-of-year chub stomachs <br />until they exceeded 50 mm in length beginning in September. A low incidence in speckled dace was noted in 1991. <br />Effects of the tapeworm on its hosts are not well studied, but may include reduced growth, depressed swimming ability <br />via elevated muscle fatigue, other debilitating effects, and elevated mortality. <br />RESUMEN <br />Las tasas de alimentaci6n a infestaci6n por gusanos fueron examinadas a partir de est6magos de larvas, postlarvas <br />tempranas y juveniles del charal moteado (Rhinichthys osculus), matalote cabeza azul (Pantosteus discobolus), y el charal <br />jorobado (Gila cypher) del Rio Colorado Pequefio en 1990-1991. Chironomidae fue un grupo tr6fico importante Para las <br />tres especies, con una utilizacion reducida de este componente por parte de los dos ciprinidos conforme a la edad, y el <br />matalote cabeza azul incrementando su use conforme a la edad. Los ostracodos fueron mas frecuentemente utilizados por <br />larvas y postlarvas del charalito, y por charales juveniles. El material de plantas vasculares se presentd notablemente en <br />est6magos de los charal y charales mas jovenes, mientras que los de matalotes cabeza azul frecuentemente contenian <br />materiales inorgdnicos. De repente, en 1990 apareci6 el gusano asiatico en est6magos de charales. Tres larvas, once <br />postlarvas tempranas y cuarenta y siete adultos y subadultos de charales fueron examinados desde 1989 y no contenian <br />vermes , mientras que larvas de charal durante mayo y junio de 1990 (n = 24) presentaron una tasa de infestaci6n del <br />91.7 %, los especimenes de juveniles del aiio mas viejos de 1990 (n = 15) exhibieron una tasa de infestation del 100 <br />%, y 44% de 18 adultos examinados de 1990 estuvieron infestados. En contraste, los vermes no aparecieron en <br />est6magos de juveniles del afio de charales hasta que excedian los 50 mm en longitud empezando en septiembre. Una <br />incidencia baja en el charalito moteado se not6 en 1991. Los efectos de los vermes en sus hospederos no ester bien <br />estudiado, pero posiblemente incluye la reducci6n en crecimiento, disminuye la habilidad de nado mediante una elevada <br />fatiga del mdsculo, otros efectos debilitativos, y una elevada mortalidad. <br />HIMBS, CLARK (Department of Zoology, University of Texas at Austin, Austin, TX 78712) <br />Does birth weight correlate with predation rate? Yes and no / ?Esta correlacionada el peso <br />al nacer con la tasa de depredacidn? Si y no <br />KEYWORDS: Poeciliidae; predation; birth weight; cannibalism; Texas; trophic dynamics <br />ABSTRACT <br />Gambusia birth weights vary between species and correlate negatively with prey susceptibility and positively with <br />parental predation rates. Gambusia affinis birth weights vary extensively between populations and do not correlate with <br />parental predation rates. Those birth weights tend to be negatively associated with prey susceptibility. Similar to <br />Gambusia affinis population variations in predation rates, the birth weight data do not correlate with known geographic <br />or environmental patterns. These two data sets (and others now being reported) suggest caution when applying a limited <br />number of populations to the trophic dynamics of a species. <br />RESUMEN <br />El peso al nacer de Gambusia varia entre especies y se correlaciona negativamente con susceptibilidad de la presa <br />y positivamente con las tasas de depredaci6n de los padres. El peso al nacer de Gambusia affinis varia extensivamente <br />entre poblaciones y no se correlaciona con las tasas de depredaci6n de los padres. Estos pesos al nacer tienden a ser <br />asociados negativamente con la susceptibilidad de la presa. Similar a las variations de la poblaci6n de Gambusia affinis <br />en las tasas de depredacidn, los datos de peso al nacer no se correlacionan con el conocimiento geografico o los patrones <br />27