4.1 The eDNA-based distribution and composition of fish communities in rivers
The fish species detected by eDNA across the 38 sampling sites contained all the fish species sampled by electrofishing, indicating that the capacity of eDNA to find species was stronger than that of electrofishing (also see Balasingham et al. (2018; Beng and Corlett (2020). Generally, the extra species detected by eDNA were the species that have been reported in the Pearl River system but were not distributed in the designed sampling sites of this study. This indicated that on the one hand, eDNA could trace the DNA released by fish species that were difficult to sample by traditional methods; on the other hand, the DNA of these hard-catch or rare fish species might be carried by upstream or tributary waters and then collected at downstream or mainstream sites. Therefore, eDNA in water reflected more information on species distribution than the traditional method. However, eDNA contains more extraneous species information than traditional in situ field monitoring, which confuses site-specific assessments of ecological health or biological integrity. In fact, the similar spatial distribution pattern shown by NMDS and the high regression coefficient between eDNA- and number-based diversity indicated that eDNA could reveal the core information on the composition and diversity of fish communities as comprehensively as those reflected by individual number and biomass collected by traditional in situ investigation. Therefore, we suggest that researchers and managers use eDNA as an ecological tool to identify fish community structure and associated environmental factors.