3.1 Spatial distribution pattern of fish communities along the river
There were 131 fish species belonging to 22 orders, 59 families, and 103 genera detected by the eDNA sampling protocol. There were 92 fish species belonging to 16 orders, 48 families, and 79 genera detected by electrofishing in situ. 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. NMDS results showed that, regardless of the data types (i.e., novel eDNA OTU richness and traditional biological counting), the relative percentage (%) of OTU richness (Fig. 2A), individual number (Fig. 2B), and biomass (Fig. 2C) could group the 38 sampling sites into eight spatial zones, including I (L1 − L2 and Z1 – Z3), II (L3 − L6), III (L7 − L11 and Z4), IV (L12 − L14 and P1 − P2), V (Z6 – Z7 and P3 − P4), VI (Z8 − Z11), VII (P5 − P8), and VIII (Z5 and P9 − P12). Along the river continuum, these zones were distributed in sections with heterogeneous habitats, including headwaters, upper stream, middle river, lower reaches, and river mouth. Our results suggested that eDNA could indicate the spatial distribution pattern of fish communities as well as the pattern reflected by electrofishing.