Evidence for strong genetic structure in a regionally important, highly vagile cyprinid (Henicorhynchus lobatus) in the Mekong River Basin

For effective management of a freshwater species, it is necessary to have an understanding of the spatial scale of population differentiation. In many cases, common ecological methods such as mark-recapture are inappropriate for large freshwater systems. In this study, a molecular genetics approach was employed to determine the population structure of a small, but regionally very important, cyprinid species (Henicorhynchus lobatus) in the lower Mekong River Basin (MRB). A fragment of the mtDNA ATPase 6 and 8 genes was sequenced for 136 individuals sampled from the main Mekong River channel, from two major tributaries of the Mekong River and from a site in an adjacent river drainage (Chao Phraya). An exact test and SAMOVA analysis revealed considerable differentiation among sites from the Mun River (a tributary of the Mekong River in Thailand) and all other samples from the MRB. Furthermore, the most upstream site in Lao PDR (BP) was significantly differentiated from sites further downstream. It is argued here that the former pattern is a result of historical drainage rearrangement and the latter a function of current ecological differences expressed as discrete migration pathways. The results of the study clearly show the existence of discrete population units for H. lobatus within the MRB that may require separate management strategies.