Abstract: The reproductive traits of bluegill Lepomis macrochirus in the Otsumo River, a small river in western Japan, were investigated by measuring gonad sizes (gonadosomatic index (GSI)) during the pre-spawning season. The bluegill population in the river was divided into two groups based on the water flows in their habitats. Fish that inhabited the lotic upstream area exhibited significantly higher δ 13C values than those in the lentic downstream area. The δ 15N values of females in the upstream area significantly exceeded those in the downstream area, whereas those of males did not differ significantly between the two areas. Females in the downstream area had significantly larger ovaries than those in the upstream area. Males exhibited no such difference in gonad size between the areas. Possible reasons for the differences in carbon sources, gonad sizes, and GSI between the two areas are discussed.