Ontogenetic niche expansion and seasonal trophic plasticity in a critically endangered floodplain catfish species.

The feeding ecology of the critically endangered catfish Eutropiichthys vacha was examined across ontogenetic stages and seasonal hydrological phases in the Chalan Beel floodplain wetland of Bangladesh. A total of 300 individuals were sampled across three hydrological seasons (monsoon, post-monsoon and pre-monsoon). Diet composition was analysed using stomach content examination, feeding morphology was quantified from mouth dimensions and trophic structure was assessed using Levin's niche breadth, Morisita-Horn dietary overlap, multivariate ordination, ratio of within-individual component to total niche width (WIC/TNW) ratios and Amundsen plots. Six prey categories were identified from non-empty stomachs, with zooplankton and crustaceans dominating diets across size classes and seasons. Mouth area increased significantly with body size, indicating ontogenetic scaling of feeding capacity. However, dietary breadth and prey richness showed limited variation across size classes, indicating no clear ontogenetic expansion of niche width. Feeding incidence varied seasonally, with the highest proportion of empty stomachs recorded during the pre-monsoon period and substantially higher feeding activity during the monsoon and post-monsoon periods. Dietary overlap remained consistently high across size classes (C = 0.987-0.998) and seasons (C = 0.985-0.996), indicating strong conservation of population-level diet composition. High WIC/TNW values further indicated limited individual specialization. These findings demonstrate that E. vacha exhibits a tropically flexible but strongly generalist feeding strategy, driven by consistent reliance on abundant prey resources in a dynamic floodplain ecosystem.