نبذة مختصرة : Physical barriers within streams and rivers can lead to population fragmentation of fish species, and results in some undesirable genetic consequences. This study evaluated the impacts of three physical barriers (one large waterfall and two small concrete weirs) on genetic variation of Garra cambodgiensis, a small-bodied (4-10 cm) cyprinid native to Southeast Asia, and widely distributed in headwater streams of the Chao Phraya and Mekong river systems. In the upper Nan River drainage basin, especially in the Wa River, the populations are threatened by high fishing pressure and rapid habitat change. We analyzed five polymorphic microsatellite loci for 191 samples, which were collected during November-December 2016, from six locations: above and below Sapun Waterfall (SPA and SPB) in Pun Stream, Nakham Dam (NKA and NKB) in Mang Stream and Sawanua Dam (SWA and SWB) in Wa River. The sample above Sapan Waterfall (SPA) was most genetically distinct from other samples (FST = 0.097 - 0.307) and had the lowest genetic diversity. For the remaining samples, samples above and below the weirs were genetically similar (FST = 0.051 for SWA-SWB; 0.024 for NKA-NKB). STRUCTURE analysis suggested unequal admixture between the Nakham Dam and Sawanua Dam samples, suggesting some restrictions to downstream migration. The results suggest that a large barrier (>10 m high) has a significant impact on genetic differentiation, while smaller concrete weirs (< 5 m high) cause few genetic differences among populations. The barrier size should be an important consideration for the design of fish friendly structures.
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