4.3 Biomarkers that can distinguish the differences between fish communities
The LEfSe results showed that biomarkers selected by both eDNA and electrofishing exhibited similarity in headwaters and lower reaches but differences in middle reaches. eDNA could identify the fishes of Danioninae, Siluridae, Nemacheilidae, Oxudercidae, Acheilognathinae, and Barbinae, indicating that eDNA has a high degree of recognition for rheophilic fish living in headwater habitats. With widespread rapids and riffles, rushing waters in headwaters had a great perturbation effect on DNA remaining in the environment, especially for those deposited in the sediments. Thus, the eDNA samples collected in rapids (e.g., sites L1 – L2 in the Liuxi River and Z1 – Z3 in the Zeng River) contained more species OTU information than the eDNA samples collected in slow-flowing or limnetic waters of the middle and lower reaches.
An interesting finding was the difference between eDNA-based biomarkers and biomass-based biomarkers. The biomarkers selected by LEfSe through relative biomass preferred to screen out species with larger body sizes, such as Xenocypris, Squaliobarbus, and Pterygoplichthys in zone IV and Ictalurus and Rhabdosargus in zone VII. This is a problem that limits the promotion and application of eDNA in a wider research area. As suggested by Rourke et al. (2022), key influencing biotic factors on eDNA effects included the taxon examined as well as their body size, distribution, reproduction, and migration. Nevertheless, there is considerable evidence to support using eDNA as an ancillary tool for assessing fish population abundance and/or biomass across discrete spatiotemporal scales, following preliminary investigations to determine species and context-specific factors that influence the eDNA abundance/biomass relationship (Bylemans et al., 2019; Doi et al., 2017). Advantages of eDNA monitoring relative to other approaches include reduced costs, increased efficiencies, and nonlethal sampling (Stewart, 2019).