4.2 Appropriate threshold of taxonomic classification for evaluating fish diversity
Compared with aquatic invertebrates, fish have more comprehensive DNA information in the gene bank, and thus, using eDNA metabarcoding to distinguish fish diversity has broad application prospects. Wang et al. (2023) suggested that in complex habitats, such as fresh and brackish converging lakes, analysing the invertebrate composition or diversity at the phylum or class level (at least to family) is effective enough to reflect the environmental properties. The great difference between eDNA usefulness for fish and invertebrates is the structure of primary taxonomic units, which is much simpler for fish than for invertebrates. At the class or order level, different invertebrate assemblages had totally different ecological properties (e.g., freshwater vs. brackish or littoral vs. limnetic); however, for fishes, order-level identification could not distinguish the habitat-specific ecological properties. Thus, according to our results, we recommend that an appropriate threshold for eDNA-based fish monitoring is at the genus or species level, which could be further chosen according to the monitoring or research targets. For example, if users focus on the influence of environmental changes on fish community structure, genus-level identification with eDNA is effective; if users focus on the fish distribution, species-level identification might be favourable.