Pattern of genetic structure of the common stream fish, Neolissochilus soroides (Pisces: Cyprinidae), addresses the importance of protected areas in eastern Thailand.

The well-known presence of mahseers, Neolissochilus soroides (Cyprinidae) in forested streams in Eastern Thailand has attracted many visitors to protected areas for several decades. Due to its historical high abundance, this species was never considered of conservation concern. In this study, we revealed that mahseers are very limited to upstream areas in mountains facing the Gulf of Thailand and are now solely confined to habitats within protected areas. The fish were sampled below seven waterfalls in four national parks. All individuals were analyzed using a fragment of the mitochondrial DNA cytochrome b gene and eight nuclear microsatellite DNA loci to uncover spatial genetic patterns. Cytochrome b data showed very low genetic diversity but indicated strong genetic differentiation between the most distant Klong Kaeo (KK) population and the other populations. The more variable microsatellite DNA markers showed moderate level of genetic diversity compared to other mahseers but revealed limited gene flow among all locations (overall FST = 0.25, p < 0.05). In addition, significant correlation was found between genetic and geographic distances (p < 0.05). Genetic bottleneck analyses provided an insight in evolutionary history in N. soroides that may have experienced past reduction in population size. As eastern Thailand is a relatively heavily populated region, habitat modification in lowland areas is inevitable and it will have consequences for species that prefer fast flowing and clear water. Protected areas therefore provide valuable sanctuaries for N. soroides in headwater streams.