Population subdivision in Siamese mud carp Henicorhynchus siamensis in the Mekong River basin: implications for management

Abstract

A molecular approach was employed to investigate stock structure in Siamese mud carp Henicorhynchus siamensis populations collected from 14 sites across mainland south-east Asia, with the major focus being the lower Mekong River basin. Spatial analysis of a mitochondrial DNA fragment (ATPase 6 and 8) identified four stocks in the Mekong River basin that were all significantly differentiated from a population in the nearby Khlong River, Thailand. In the Mekong River basin, populations in northern Lao People's Democratic Republic and northern Thailand represent two independent stocks, and samples from Thai tributaries group with those from adjacent Mekong sites above the Khone Falls to form a third stock. All sites below the Khone Falls constituted a single vast stock that includes Cambodia and the Mekong Delta in Vietnam. While H. siamensis is considered currently to undertake extensive annual migrations across the Mekong River basin, the data presented here suggest that natural gene flow may occur over much more restricted geographical scales within the basin, and hence populations may need to be managed at finer spatial scales than at the whole-of-drainage-basin level.

Potential Effects of Hydroelectric Dam Development in the Mekong River Basin on the Migration of Siamese Mud Carp (Henicorhynchus siamensis and H. lobatus) Elucidated by Otolith Microchemistry

“…Therefore, the larger otolith Ba variability does not necessarily translate into longer fish migrations but reflects a greater spatial complexity in water elements encountered by the fish. Due to the unique otolith elemental profiles, H. siamensis from the Thai North likely did not share migration routes with individuals from the other regions, corroborating a finding by Adamson et al [23] who identified a genetically distinct population of the species in this region.…”

“…Elemental profiles largely overlapped between Mun, Tonle Sap and MM with no significant differences in Sr or Ba at both otolith core and surface. However, because the genetic analysis identified a single stock of H. siamensis below the Khone Falls [23], H. siamensis in the Mun River, which connects with MM above the Khone Falls, likely have a different natal origin from fish in Tonle Sap or MM. During the peak flood season, the Mekong River water causes the Tonle Sap River to reverse its flow [50], likely mixing the water and homogenizing elemental signatures throughout the vast geographical extent.…”

“…Adamson et al [23] considered H. siamensis from these regions as a single genetically homogeneous stock. Although both the mean otolith chemical signatures and the genetic analysis could not differentiate between the two regions, the shape of the otolith elemental profiles was considerably different with much more diverse core signatures for the Songkhram fish in the dry season than the Mun fish, suggesting that multiple H. siamensis stocks utilize the Songkhram River in this season.…”

“…Multiple genetically discrete populations or stocks have been identified in both species of Siamese mud carp within the Mekong basin [22], [23]. However, ecological evidence that supports the genetic differentiation other than the single natural barrier of the Khone Falls has been lacking, thus hampering the development of effective fisheries management strategies.…”

Genetic differentiation and distribution routes of the bronze featherback Notopterus notopterus (Osteoglossiformes: Notopteridae) in Indochina

“…One possibility is the existence of a physical barrier. The Khone Falls lie at the national border between Lao PDRand Cambodia, and these falls definitely represent a significant barrier to upstream dispersal of a common cyprinid fish (Adamson et al, 2009). However, this physical barrier is most likely not responsible for the genetic differences we observed because other fish species are able to migrate to the upper side of the river across the falls (Baird, Flaherty & Phylavanh, 2003;Mekong River Commission, 2004).…”