Introduction:

Generalist trophic strategies and opportunistic feeding habits of nektonic fish species inhabiting oligotrophic streams in the Amazon Rainforest suggest that minor morphological and niche differences can mediate the occurrence of closely related species in sympatry, alleviating interspecific competition for resources.

Objective:

To analyze the ecomorphology, diet composition, vertical and horizontal habitat use, and foraging behavior of four Characiform species in syntopy, to understand resource partitioning and species coexistence.

Methods:

From August to October 2011 (dry season), up to 30 specimens of each species were collected from each of eight sampled streams in the Adolpho Ducke Forest Reserve, Amazonas, Brazil, for ecomorphological analyses, with up to 10 of these used for stomach content analysis. Foraging behavior was quantified through underwater observation of vertical and horizontal space use and foraging frequency in the water column. The dietary importance of food items was determined using the Feeding Index (FI i ), and ecomorphological attributes were used to characterize body shape and fin morphology.

Results:

Differences were detected in foraging behavior and habitat use. Hyphessobrycon. aff. melazonatus predominantly occupied the stream margins, and the other species utilized the channel. Additionally, only H. aff. melazonatus exhibited a difference in stomach content composition. Ecomorphological characteristics showed divergence among species, particularly in body shape, mouth size, and orientation.

Conclusions:

The combined analysis demonstrated that differences observed here may mediate syntopic coexistence by alleviating interspecific competition through resource partitioning. The system’s sensitivity to anthropogenic impacts and climate change were highlighted on food availability and trophic relations of Amazon stream fishes and underscore the need for headwater stream conservation.