Seasonal variation in the diet of Moenkhausia bonita (Characiformes: Characidae) in the upper Paraná river (Argentina)

The trophic ecology of fish provides insight into the relationships between species and their environment, as well as the effects of seasonal variations on resource availability. We analyzed the stomach contents of Moenkhausia bonita specimens collected during the fall and summer of 2025 in the Upper Paraná River (Corrientes, Argentina), with the aim of comparing dietary composition between the two seasons. The specimens were captured using electrofishing, and their stomach contents were analyzed in a Sedgewick-Rafter chamber under an optical microscope. A total of 20 stomachs (10 per season) were analyzed, yielding eight food items identified to the lowest possible taxonomic level and grouped into Arachnida, Coleoptera, Diptera, fish scales, eggs, insect fragments, Thysanoptera, and zooplankton. In addition, the presence of plastic fibers as ingested material was recorded. Insect fragments predominated at both stations, accounting for 49.7% of the stomach contents in the fall and 65.7% in the summer. Zooplankton decreased from 24.0% to 0.4%, while eggs increased from 5.7% to 11.6%. Plastic fibers constituted the third most abundant ingested material (13.3% in the fall and 19.0% in the summer), being found in 100% of the stomachs analyzed. Thysanoptera were recorded exclusively in summer (2.3%). Diet composition differed between seasons (PERMANOVA, Bray-Curtis; Pseudo-F = 8.58; p = 0.001). Previous studies describe M. bonita as an opportunistic species that tends to feed on invertebrates; therefore, the observed pattern would reflect prey availability and its feeding strategy. The results show greater prey diversity in the fall and greater prey abundance in the summer. The presence of plastic fibers in 100% of the stomachs represents the first record for this species in this section of the Paraná River and highlights the importance of monitoring plastic pollution in aquatic ecosystems.