Monitoring Biomass and Habitat Drivers of the Endangered <em>Brachymystax lenok tsinlingensis</em> in the Zhouzhi Heihe River Using eDNAqPCR and Bioindicator Assessment

The Brachymystax lenok tsinlingensis is an endangered endemic salmonid species facing severe threats from habitat fragmentation. Effective conservation requires non-invasive monitoring techniques and the identification of key habitat drivers. This study utilized environmental DNA (eDNA) quantitative PCR to estimate the spatial biomass distri-bution of the Brachymystax lenok tsinlingensis in the Zhouzhi Heihe River, integrating surveys of plankton, benthic macroinvertebrates, and multiple physicochemical factors (e.g., water depth, flow velocity, water temperature) to identify critical environmental influences. The results revealed significant spatial heterogeneity in fish biomass, with the highest values observed at river confluences and high-elevation sites. For instance, the confluence of the Heihe River and the Da Manggou River recorded the maximum biomass (1.5 × 10⁻² g/cm²), followed by the Hua Erping site (5.8 × 10⁻³ g/cm²). Among all measured variables, only water depth showed a significant positive correlation with Brachymystax lenok tsinlingensis biomass (r = 0.57). Spatial variations in plankton and benthic community structures reflected a water quality gradient from oligotrophic to eutrophic conditions, but their diversity was not directly linked to fish abundance. These findings indicate that the complexity of in-stream physical habitats—particularly the availability of deep pools—currently plays a more decisive role in the distribution of the Brachymystax lenok tsinlingensis than prey resource diversity. Therefore, conservation efforts should prioritize the restoration of structurally complex river habitats while maintaining high water quality to ensure the long-term survival of this species.