The trophic ecology of fishes in the genus Rhamdia remains poorly reported. Here, we aim to describe the diet composition and infer the trophic levels, niche breadth, and niche overlap between Rhamdia guatemalensis and Rhamdia laticauda in northeast Oaxaca, Mexico. Fishes were sampled using an electrofishing device between 2016 and 2017. We calculated the Quotient index and the diet composition was analyzed using the percentage of the Index of Relative Importance (%IRI) to analyze possible ontogenetic and sexual differences on feeding ecology. We then calculated the trophic niche breadth using the Levins´ standardized index (BA). Trophic levels (TLk) of each species were calculated using the trophic index and trophic niche overlap using Pianka´s index (Ojk). The results indicate that both species are carnivorous and tend to prey on aquatic insects, with R. guatemalensis feeding on nine different orders of animals and R. laticauda displaying a broader diet spectrum, encompassing 19 orders. Our data showed ontogeny and sex differences in the trophic ecology of R. guatemalensis. They were secondary consumers (TLk > 3) and showed a specialized diet (BA < 0.4). We found evidence of trophic niche partitioning between adults (Ojk = 0.40) and females (Ojk = 0.42). Our results highlight the fundamental role of this species in the area as a carnivorous/insectivorous, predatory fishes, secondary consumers, and biological control for Diptera, Ephemeroptera, and Megaloptera. Both species minimize the trophic overlap through food resource partitioning mediated by fish body size. Additionally, adults and females consume different prey, which allows the coexistence of closely related species. Our findings regarding trophic ecology could be crucial for devising conservation and management plans for these particular Rhamdia species.
