Abstract:

Aims: The lower reaches of the Lancang River boast a unique habitat and abundant fish resources, making the conservation of its freshwater fish diversity a subject of great concern. Affected by human activities such as hydropower development, the fish community structure in the lower mainstream Lancang River has been undergoing significant changes. In particular, the invasion of alien fish species has increased the ecological risks of the river. 

Methods: From 2023 to 2024, seasonal (dry season and wet season) fish surveys and sampling were conducted in different reaches of the lower mainstream Lancang River, including reservoir reaches, downstream reaches of dams, and natural reaches. The spatiotemporal variation characteristics of the fish community structure, as well as the species, distribution, and invasion risks of alien fish, were analyzed. 

Results: A total of 56 fish species were collected in the lower mainstream Lancang River, belonging to 8 orders, 22 families, and 47 genera, with Cypriniformes being the dominant order. Among the collected species, 4 are endemic to the Lancang River, mainly distributed in the downstream reaches of dams and natural river sections; 25 are alien species, predominantly found in reservoir areas and downstream reaches of dams. Index of relative importance (IRI) analysis revealed that the dominant fish species in reservoir areas and downstream reaches of dams are alien species such as Hemiculter leucisculus, Parachromis managuensis, Coptodon zillii, and Oreochromis niloticus, while the dominant species in natural river sections are native fish including Mystacoleucus marginatus, Sikukia gudgeri, and Poropuntius huangchuchieni. Nonmetric multidimensional scaling (NMDS) analysis revealed significant spatial differences in the fish community structure of the lower mainstream Lancang River. This is mainly attributed to the higher abundance of alien fish in reservoir reaches and the dominance of native fish in natural river sections, while the seasonal differences between the wet season and dry season were relatively small. A separate analysis focusing on alien fish indicated that in terms of catch quantity, Hemiculter leucisculus, Parachromis managuensis, and tilapia (Oreochromis spp.) were the main alien fish species. In terms of spatial distribution, alien fish in the lower mainstream Lancang River were more abundant in reservoir reaches and less abundant in natural reaches. The proportion of their species and quantity in the catch showed a decreasing trend along the longitudinal gradient of the river. The risk assessment of alien fish demonstrated that species such as Coptodon zillii, Oreochromis niloticus, Parachromis managuensis, Hypostomus plecostomus, Oreochromis mossambicus, and Hemiculter leucisculus are high-risk invasive fish species in this region. 

Conclusion: In conclusion, while hydropower development has altered the fish community structure in the lower mainstream Lancang River, it has also created favorable conditions for the invasion of alien fish. To protect the fish resources in this region, differentiated strategies are proposed: priority should be given to the prevention and control of alien fish in reservoir reaches, with a focus on dominant species in terms of quantity and high-risk invasive species; in natural reaches, protected areas for native fish should be established to maintain the living and breeding environments of native fish species.

Key words: lower reaches of the Lancang River, fish community, alien fish, invasion risk assessment