Resource Characteristics of Six Rare and Endemic Fish Species in the Dam-Regulated Hongshui River and Their Relationships with Environmental Factors

To assess the impacts of cascade dam development on riverine fish, this study investigated the population structure and spatiotemporal distribution of six rare and endemic fish species in the Hongshui River mainstream from 2022 to 2023. Results indicated a significant resource decline, with the proportion of rare species dropping to 18.33%. Populations exhibited pronounced characteristics of age rejuvenation and miniaturization, with mean body lengths failing to reach growth inflection points. Although growth parameter analysis indicated rapid growth patterns (k > 0.2), stock assessment revealed that Semilabeo obscurus and Onychostoma gerlachi were overexploited (E > Emax), while Ptychidio jordani maintained the highest biomass. Redundancy analysis (RDA) identified water chemistry (e.g., conductivity, TN) as the primary driver of seasonal distribution, whereas the proportion of natural free-flowing river segments significantly influenced community variation under dam regulation (corresponding 59.44% of variation). Conservation strategies should prioritize the protection of remnant natural river segments, implementation of ecological flow regulation to simulate natural hydrological rhythms, and strict enforcement of minimum capture size limits to ensure the sustainable utilization of these rare and endemic resources.