3.2 The composition of fish communities at different taxonomic
levels
The relative percentage of eDNA OTU richness showed that, at the order
level (Fig. 3A), Cypriniformes, Cichliformes, Siluriformes, and
Gobiiformes made up > 83% of the fish communities in the
three rivers. At the family level (Fig. 3B), Cichlidae, Cultrinae,
Cyprinidae, and Labeoninae made up > 56% of the fish
communities in Zones II − VIII, whereas Cyprinidae, Oxudercidae, and
Heteropteridae made up > 50% of the fish communities in
Zone I. At the genus (Fig. 3C) and species levels (Fig. 3D), the
composition of fish communities was dispersed and determined by
different genera and species. Notably, except for zones I and II, where
native Oryzias curvinotusand Hemiculter leucisculus had the highest OTU richness,
respectively, exotic Coptodon zillii and Oreochromis
aureus had the highest OTU richness in the other zones, indicating that
the structure of fish communities in the middle and lower reaches was
dominated by alien species. This result indicated that the eDNA
metabarcoding protocol had advantages in discovering dominant species
and exotic species.
The cluster analysis between the
eight spatial zones showed that the
order- and family-level grouping
(Figs. 3A – 3B) was insufficient to distinguish the longitudinal
differences in fish community composition from headwaters to the
estuary. Compared with the order- and family-level grouping, the
genus-level (Fig. 3C) grouping could more clearly distinguish the
differences between upstream zones I − IV and downstream zones V − VIII.
Moreover, the species-level grouping (Fig. 3D) was the most effective in
distinguishing upstream zones I − III, midstream zones IV − V, and
downstream zones VI – VIII. An interesting finding was that at the
family level, upstream zone II and downstream zone VIII were clustered
in the same group, which was caused by the widespread species of
Cichlidae, Cultrinae, and Oxudercidae that could not be distinguished by
family-level identification. In contrast, compared with the family
level, the higher resolution at the genus and species levels could
distinguish Coptodon and Oreochromis of Cichlidae as well
as Hemiculter and Parabramis of Cultrinae, which were the
key genera that distinguished fish distribution in the upper and lower
reaches.