Fish composition in the Chishui River
A total of 96,031 valid fish sequences were read, resulting in the identification of 77 species from six orders, 62 genera, and 18 families (Table 1 and Fig 2a). The dominant orders were Cypriniformes, Siluriformes, and Perciformes, accounting for 87.87%, 8.23%, and 3.44% of the total sequence abundance, respectively. Cyprinidae and Botiidae were the dominant families, representing 71% and 13.52% of the total sequence abundance, respectively, while Bagridae accounted for 7.34% of the total sequence abundance.
The upstream fish consisted of 38 species belonging to 33 genera in 14 families from two orders. Mesomigratory fish included 45 species from 36 genera in 10 families from five orders. Downstream fish consisted of 54 species from 45 genera in 15 families from seven orders (Fig 2a and b, and Fig 3a– c). An increasing gradient was observed from upstream to downstream. Notably, 17 species were found in the upper, middle, and lower reaches: Pseudorasbora parva , Xenocypris davidi ,Schizothorax prenanti , Zacco platypus ,Sinogastromyzon sichangensis , Misgurnus anguillicaudatus ,Carassius auratus , Ctenopharyngodon idella , Homatula potanini , Rhinogobius giurinus , Platysmacheilus exiguous ,Pseudobagrus ussuriensis , Rhodeus ocellatus , Rhodeus sinensis , Gambusia affinis , Acrossocheilus monticola , andHemiculterella sauvagei .
The Shannon-Wiener index of the Chishui River differed significantly between upstream and downstream regions (Fig 4a), with the downstream index being higher than the upstream index (P < 0.05). However, there was no statistically significant difference in the richness index. Community composition also varied between the upstream and downstream regions (PerMANOVA=0.86, P < 0.05; Fig 4b), but the difference between the middle and adjacent reaches was not significant.
A variance analysis of the diversity index and community analyses of the main streams and tributaries was also conducted. There were no significant differences between the groups. We further analyzed the differences in diversity in the upper, middle, and downstream tributaries. The Shannon-Wiener and richness indices of the upstream tributaries were significantly higher than those of the downstream tributaries (P < 0.05; Fig 5a). We also observed significant differences in the community composition between the upstream and downstream tributaries (PerMANOVA=0.72, P< 0.05; Fig 5b). The midstream tributaries show partial similarities to both the upstream and downstream tributaries.
We also explored the specific diversity and community differences among the identified tributaries by conducting a grouping ANOVA and community analysis. The results showed that the richness index of the Datong River was significantly higher than that of the other tributaries (P< 0.05; Fig 6a), indicating that it had the highest level of species diversity among all tributaries. Fish community analysis showed no significant differences between the secondary and fifth tributaries (Fig. 6b).