Application of Environmental DNA for Assessing the Distribution and Biomass of Brachymystax lenok Tsinlingensis in the Zhouzhi Heihe River

In recent years, the population of <i>Brachymystax lenok tsinlingensis</i> has experienced a rapid decline due to habitat degradation caused by overfishing and dam construction. In the current study, water samples were collected from four tanks containing <i>B. lenok tsinlingensis</i> individuals with wet weights of 5.8 g, 12.3 g, 34.7 g, and 81.7 g, respectively, and the cycle threshold (Ct) values of <i>B. lenok tsinlingensis</i> in each tank were measured using quantitative PCR. Additionally, water samples were collected from six sites along the Zhouzhi Heihe River (Yu Dongquan, Ban Fangzi, Ba Mugou, two sites along Wang Jia, and Qingshui Rivers), and the Ct values of <i>B. lenok tsinlingensis</i> at these sites were quantified. The results show that the relationship between the Ct values measured by the quantitative test and the fish biomass in the corresponding tanks followed an exponential function, with a regression coefficient of 0.9987. The biomass for the six sites was determined using the established relationship, and the results indicate that the biomass of <i>B. lenok tsinlingensis</i> was highest in Yu Dongquan, followed by Ban Fangzi, Ba Mugou, Wang Jia River 2, and Qingshui, with the lowest biomass observed in Wang Jia River 1. These findings are consistent with the species' adaptive survival distribution across the six sites. This study presents a standardized approach for using an eDNA-based quantitative PCR assay to monitor <i>B. lenok tsinlingensis</i>. It provides an advanced technical method applicable to the protection and monitoring of other aquatic species in future studies.