Anguillids in the upper Nu-Salween River, South-East Asia: species composition, distributions, natal sources and conservation implications

Context. Anguillids are ecologically and commercially important fishes in the Indo-Pacific, but river fragmentation threatens their populations. The free-flowing large river, Nu-Salween River, may be their last remaining habitat within China. Aims. This study investigated the species composition, distributions and natal sources of anguillids from the Nu-Salween River. Methods. We adopted a convolutional neural network (i.e. morphology-molecule network, MMNet) integrating morphological and molecular data to identify eel specimens collected in the upper reach from 2017 to 2021. Key results. Three anguillid species (187 individuals of Anguilla bengalensis, 34 of Anguilla bicolor and 25 of Anguilla marmorata) were identified using integrative taxonomy. The MMNet method, with accuracies exceeding 99.90%, outperformed methods using morphological or molecular data alone for each of the three eel species. A. bengalensis has the widest distribution range, whereas A. bicolor and A. marmorata have narrower ranges and may be new to this region. These riverine eels are parts of widespread Indo-Pacific metapopulations. Conclusions. The upstream distribution expansions of eels in the Nu-Salween are likely to be due to climate change and anthropogenic disturbances. Implications. We argue that the Nu-Salween River is a key habitat for migratory fishes surrounding the Indo-Pacific and should be maintained as free-flowing because neighbouring large rivers are increasingly dammed.