VARIATIONS ON SPECIES COMPOSITION AND FISH COMMUNITY STRUCTURE IN THE MAINSTREAM AND TRIBUTARIES OF THE MIDDLE AND LOWER XIANGJIANG RIVER

Fish communities are integral components of aquatic ecosystem and are highly sensitive to environmental alterations. The construction of cascade dams in the Xiangjiang River has posed significant threats to its fishery resources. In order to assess spatial and temporal variations in fish community structure among mainstream and tributaries of the Xiangjiang River basin, fish resources were investigated in four mainstream sections (Yongzhou, Hengyang, Zhuzhou, and Changsha) and three main tributaries (Mishui, Leishui, and Liuyanghe River) of the Xiangjiang River in Summer and Autumn from 2022 to 2024. Fish were caught by traditional fishing gear. The widespread presence of dams and barrages in the mainstream and tributaries has created predominantly slow-flowing and lentic habitats at all seven sampling sites. Fish species richness was the highest in Liuyanghe, Yongzhou, Zhuzhou, and the lowest in Hengyang, Chang, and Leishui River. The similarity index in fish species composition among the four mainstream sections and between Liuyanghe River and Mishui River comfirmed a moderate similarity. Shannon-Wiener and Margalef indices were the highest in the Yongzhou section and Liuyanghe River, and were the lowest in the Leishui River, with intermediate values recorded at the other sections. The ecological compositions of species at the seven sampling sites were dominated by eurytopic fish, lower mesopelagic species, adhesive egg laying fish, and fish feeding on zooplankton, aquatic insects, mollusks and fish. The common dominant species of the fishing catches included <italic>Xenocypris microlepis</italic> and <italic>X. davidi</italic>, with <italic>Hypophthalmichthys molitrix</italic> and <italic>Aristichthys nobilis</italic> also contributing notably. Thees dominant species exhibited eurytopic traits, periodic strategy, and produce drifting eggs. The fish community structure differed markedly among the seven river sections. Cluster analysis distinguished four main groupings: (1) mainstream sites, characterized by <italic>S. argentatus</italic>, <italic>S. wui</italic>, <italic>Saurogobio dabryi</italic>, <italic>X. microlepis</italic>, and <italic>Pseudolaubuca sinensis</italic>. (2) tributary sites, dominated by <italic>X. microlepis</italic>, <italic>Distoechodon tumirostris</italic>, <italic>Carassius auratus</italic>, <italic>S. wui</italic>, and <italic>Culter alburnus</italic>. (3) the Dongjiang Lake site in the Leishui River, featuring <italic>H. leucisculus</italic>, <italic>C. alburnus</italic>, <italic>C. auratus</italic>, <italic>X. microlepis</italic>, and <italic>Abbottina rivularis</italic>. (4) the Changsha section during autumn and winter. Compared to record species, fish diversity has sharply decreased, and species compositions and fish community structure showed high similarity among the seven sections of the middle and lower Xiangjiang River. The dominant species included eurytopic, periodic fish and potamodromous, opportunistic species. This composition may reflect the homogenization of habitats caused by the construction of dams and barrages. These findings provide valuable insights into fishery conservation and management in the Xiangjiang River.