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In this study, we analyzed the trophic structure and carbon sources supporting fish assemblages of five low-productivity rivers with different dimensions. Using carbon (δ¹³C) and nitrogen (δ¹⁵N) isotope ratios, we identified the major carbon sources to fish species and estimated fish trophic positions in each river. We observed a similar trophic structure in almost all low-productivity rivers. Detritus (allochthonous resources derived from riparian zone—CPOM and processed organic matter from upstream sources—FPOM) were important to sustain the fish assemblages, especially in smaller rivers. We also observed a growing consumption of autochthonous primary producers, algae and periphyton, with the increase in river dimensions and especially for primary consumers. Fishes from the smallest river occupied lower mean trophic positions; however, the assemblage exhibited wider food-chain length than those from …