Fish Trophic Structure and Potential Competitive Interactions in a Typical Subtropical River

Invasive non-native fish species are key drivers of biodiversity decline and functional homogenization in freshwater ecosystems. As a typical subtropical coastal river, the Jiulong River Basin faces a high risk of non-native fish invasions, with tilapia species dominating non-native fish biomass. However, quantitative understanding of the nutritional interactions between tilapia species and native fishes remains limited. This study used carbon (δ 13 C) and nitrogen (δ 15 N) stable isotope analyses to assess the trophic structure, ecological niche width, and niche overlap of fish communities in the Jiulong River Basin, China. The results showed that fish δ 13 C and δ 15 N values ranged from −27.27‰ to −14.88‰ and 7.54‰ to 14.15‰, respectively, with trophic levels ranging from 2.33 to 4.27. Non-native fish species contributed nearly 50% of the total biomass, with tilapia species accounting for 39.16%. The average isotopic niche width of tilapia species (SEAc = 3.06‰ 2 ± 0.79) was significantly larger than that of native species (SEAc = 1.30‰ 2 ± 1.54). Moreover, the average niche overlap between tilapia species and native fishes was 50.78%, reaching 95%, 81%, and 66% for Pseudogobio vaillanti (Sauvage, 1878), Acrossocheilus fasciatus (Steindachner, 1892), and Gnathopogon argentatus (Sauvage & Dabry de Thiersant, 1874), respectively. The results indicate that non-native fishes, especially tilapia species, occupy broader ecological niches and exhibit substantial trophic overlap with native fishes, potentially affecting native fish populations. These findings provide a scientific reference for invasion risk assessment and management in subtropical river ecosystems.