Resource utilization pattern and guild structure of fish communities in low order rivers

Resource utilization and guild structure in fish communities breeding in low order rivers from transitional areas between chaco serrano and yungas, in spring and summer 1995-1996 seasons, are studied. Fish communities are composed of a minimum of 9 species and a maximum of 15 species. Densities were among 2 to 12 ind/m2. Astyanax eigenmanniorum is the dominant species in rivers with high macrophytes growth. The siluroids Ixinandria steinbachi, Corydoras paleatus and Trichomycterus spegazzini were the most abundant species in bedrock rivers with high water velocity. Resource utilization is analized in terms of feeding habitats, stomach contents data and asociated fauna. Cluster analysis is used to classified species into guilds, based on interespecific similarities. Three main guilds are recognized: detritivores, carnivores (with the subguilds macrocarnivores and microcarnivores) and omnivores. The first guild are characterized by narrow niche breadths along the feeding habitats and food items. The macrocarnivores subguild present species with wide niche breadths along the feeding habitats and by a narrow niche breadths along the food items. They eat vertebrates, like fishes, larval anurans and aquatic and aereal insects, that they catch at the water-air interfase. Microcarnivores shows similar features, but the prey are smaller, forming part of zooplankton and microbenthos. Generalized species conform the omnivores guild. They have wide niche breadths along food items and feeding habitats. The analysis of the interrelationships among the communities components shows that food competition does not play an important role in structuring the communities in this high energy environments.Original Abstract: Se estudia la utilizacion de recursos y estructura en gremios de poblaciones de peces que se reproducen en rios de bajo orden en ambientes de transicion entre chaco serrano y yungas, en el periodo de primavera-verano de los anos 1995 y 1996. Las comunidades de peces en estos ambientes estan compuestas por un minimo de 9 especies y un maximo de 15 especies, con densidades variables entre 2 a 12 individuos/m2; siendo Astyanax eigenmanniorum la especie dominante en los cursos de agua con gran desarrollo de macrofitas, y los siluridos Ixinandria steinbachi, Corydoras paleatus y Trichomycterus spegazzini los mas abundantes en los rios con mayor velocidad de agua y lechos pedregosos. Se analiza la utilizacion de recursos mediante la caracterizacion de habitats, anal