The trophic structure of a fish assemblage in a first order stream in the upper Paraná River basin was investigated using standard methods of diet analysis and underwater observations utilizing snorkeling. Three stretches of the Córrego São Carlos were studied. Eighteen fish species belonging to five orders and ten families were captured. The stomach analysis of 299 fishes revealed that 70% of the food items are autochthonous, 24% allochthonous, and 6% material of unidentifiable origin. Eighteen pairs of species (33%) showed significant feeding overlap, though this overlap does not necessarily indicate competition given the temporal and spatial segregation during foraging. Three feeding guilds were found. The invertivores included Astyanax altiparanae, Moenkhausia sanctaefilomenae, and Oligosarcus pintoi, whose diet demonstrated a predominance of allochthonous items, and Rhamdia quelen, Trichomycterus sp., Corydoras aeneus, and Crenicichla britskii, that had a predominance of autochthonous items in their diets. In this group, A. altiparanae and M. sanctaefilomenae are drift feeders, R. quelen is a benthic opportunistic predator, Trichomycterus sp. and C. aeneus are grubbers, and O. pintoi and C. britskii are ambush predators. The omnivore with a tendency to herbivory is represented by Phalloceros caudimaculatus, which feeds mainly on algae. The periphitivores included Hisonotus sp., Hypostomus nigromaculatus, and H. ancistroides that are grazers with a diet composed mostly of diatoms, clorophyts, and organic matter. The results indicated that the fish assemblage in the Córrego São Carlos is structured at spatial, temporal, and trophic levels, and shows partitioning of the food resources. The addition of the fish species in each trophic guild along the stream is possibly due to the longitudinal increase of microhabitats that makes available more feeding sites.
stream fishes; feeding; spatial segregation; trophic segregation; resource partitioning