Abstract: To explore the resource utilization characteristics and interspecific relationships between invasive alien fish and native fishes under reservoir formation, we conducted fish surveys in river reaches upstream and downstream of the Maling Water Project dam on the Mabiehe River within the Pearl River Basin from 2023 to 2024. We calculated the spatial niches of Coptodon zillii and six native fish species, and analysed their trophic niches using carbon and nitrogen stable isotopes. The results showed that C. zillii was dominant in the reservoir area and downstream reaches of the dam. It preferred lentic or slow-flowing habitats and possessed a narrow spatial niche width. Native fishes including Onychostoma gerlachi and Semilabeo obscurus exhibited wider spatial niches and balanced habitat utilization. The spatial niche overlap index between C. zillii and Carassius auratus, as well as S. obscurus, was high (0.89–0.99), whereas the overlap values with other native fishes were low (0.13–0.31). Significant differences were observed in the two-dimensional δ¹³C-δ¹⁵N space among different river reaches and species (p<0.05), while no significant temporal differences were detected (p>0.05). The trophic niche width of C. zillii was at a moderate level, and the probability of trophic niche overlap with O. gerlachi, Acrossocheilus yunnanensis and C. auratus exceeded 50%. Our study indicates that lentic and slow-flowing habitats created by reservoir impoundment may facilitate the colonization and expansion of C. zillii. High overlap exists in spatial and food resource use between this species and several native fishes, implying potential interspecific competition risks.