Evidence for population genetic structure in two exploited Mekong River fishes across a natural riverine barrier.

Impacts of urban development on aquatic populations are often complex and difficult to ascertain, but population genetic analysis has allowed researchers to monitor and estimate gene flow in the context of existing and future hydroelectric projects. The Lower Mekong Basin is undergoing rapid hydroelectric development with around 50 completed and under-construction dams and 95 planned dams. The authors investigated the baseline genetic diversity of two exploited migratory fishes, the mud carp Henicorhynchus lobatus (five locations), and the rat-faced pangasiid catfish, Helicophagus leptorhynchus (two locations), in the Lower Mekong Basin using the genomic double digest restriction site-associated DNA (ddRAD) sequencing method. In both species, fish sampled upstream of Khone Falls were differentiated from those collected at other sites, and Ne estimates at the site above the falls were lower than those at other sites. This was the first study to utilize thousands of RAD-generated single nucleotide polymorphisms to indicate that the Mekong's Khone Falls are a potential barrier to gene flow for these two moderately migratory species. The recent completion of the Don Sahong dam across one of the only channels for migratory fishes through Khone Falls may further exacerbate signatures of isolation and continue to disrupt the migration patterns of regionally vital food fishes. In addition, H. lobatus populations downstream of Khone Falls, including the 3S Basin and Tonle Sap system, displayed robust connectivity. Potential obstruction of migration pathways between these river systems resulting from future dam construction may limit dispersal, which has led to elevated inbreeding rates and even local extirpation in other fragmented riverine species.

Cryptic Lineages and a Population Dammed to Incipient Extinction? Insights into the Genetic Structure of a Mekong River Catfish.

An understanding of the genetic composition of populations across management boundaries is vital to developing successful strategies for sustaining biodiversity and food resources. This is especially important in ecosystems where habitat fragmentation has altered baseline patterns of gene flow, dividing natural populations into smaller subpopulations and increasing potential loss of genetic variation through genetic drift. River systems can be highly fragmented by dams built for flow regulation and hydropower. We used reduced-representation sequencing to examine genomic patterns in an exploited catfish, Hemibagrus spilopterus, in a hotspot of biodiversity and hydropower development-the Mekong River basin. Our results revealed the presence of 2 highly divergent coexisting genetic lineages which may be cryptic species. Within the lineage with the greatest sample sizes, pairwise FST values, principal component analysis, and a STRUCTURE analysis all suggest that long-distance migration is not common across the Lower Mekong Basin, even in areas where flood-pulse hydrology has limited genetic divergence. In tributaries, effective population size estimates were at least an order of magnitude lower than in the Mekong mainstream indicating these populations may be more vulnerable to perturbations such as human-induced fragmentation. Fish isolated upstream of several dams in one tributary exhibited particularly low genetic diversity, high amounts of relatedness, and a level of inbreeding (GIS = 0.51) that has been associated with inbreeding depression in other outcrossing species. Our results highlight the importance of assessing genetic structure and diversity in riverine fisheries populations across proposed dam development sites for the preservation of these critically important resources.