Abstract Trophic plasticity is a distinctive feature of freshwater fishes, representing an essential strategy for fish living in resource-variable environments. We analyzed the stomach contents of individuals sampled in two Atlantic Forest streams to identify the primary food sources consumed by Psalidodon aff. fasciatus and verify the existence of spatial, seasonal, and ontogenetic variations. The diet was determined by analyzing the stomach contents using the Volume Method to quantify the importance of food items. In general, Psalidodon aff. fasciatus was classified as an omnivorous species, consuming mainly insects, plant material, and filamentous algae. The results also showed significant effects for all factors considered (spatial, seasonal, and ontogenetic). Finally, Psalidodon aff. fasciatus demonstrated considerable trophic plasticity, which can result in better use of available resources in the environment and improved resource partitioning, reducing intraspecific and interspecific competition. fishes resourcevariable variable environments aff spatial seasonal variations items general species insects material algae (spatial ontogenetic. . ontogenetic) Finally partitioning competition

RESUMO Estudos sobre a composição da dieta de peixes e de como as espécies exploram os recursos alimentares são especialmente relevantes em sistemas de água doce tropicais, onde a plasticidade alimentar é uma estratégia comumente utilizada. Neste trabalho, analisamos o espectro alimentar e as variações ambientais, sazonais e ontogenéticas na dieta de uma espécie de caracídeo [Hemigrammus marginatus Ellis, 1911 (Characidae)] na região do Alto Rio de Contas, na Chapada Diamantina, Brasil. Verificamos um hábito alimentar onívoro/invertívoro e oportunista, com predominância de insetos e microcrustáceos, seguido de algas filamentosas. A sazonalidade não alterou os recursos alimentares utilizados pela espécie. No entanto, foram detectadas diferenças nas categorias dominantes ao comparar os dois rios amostrados, bem como no consumo do alimento ao longo do desenvolvimento da espécie. Esses dados ampliam o conhecimento acerca da biologia de H. marginatus e permitem entender a influência das características dos rios e de seus entornos na ecologia trófica da espécie, visto que, embora o item insetos tenha sido expressivo em ambos os locais estudados, microcrustáceos exibiram maior importância relativa no ambiente mais lêntico, o que possivelmente está relacionado à variação na disponibilidade destes recursos no ambiente.

ABSTRACT Studies on the composition of fish diet and on how species exploit food resources are especially relevant in tropical freshwater systems, where dietary plasticity is a commonly used strategy. In this study, we analysed the food spectrum and the environmental, seasonal and ontogenetic variations in the diet of a characid species [Hemigrammus marginatus Ellis, 1911 (Characidae)] in the region of the Upper Contas River, in the Diamantina Plateau, Brazil. We verified an omnivorous/invertivorous and opportunistic food habit, with predominance of insects and microcrustaceans, followed by filamentous algae. Seasonality did not alter the food resources used by the species. However, differences were detected in the dominant categories when comparing the two sampled rivers, as well as in the food consumption throughout the development of the species. These data increase the knowledge about the biology of H. marginatus and allow understanding the influence of the characteristics of the rivers and their surroundings in the trophic ecology of the species, as our findings have shown that, although insects have been expressive in both studied localities, microcrustaceans exhibit a greater relative importance in the more lentic environment, which is probably related to the variation in the availability of these resources in the environment.