Report on the distribution,population, and ecology of the yunnan snub-nosed monkey (Rhinopithecus bieti)

The Yunnan snub-nosed monkey (Rhinopithecus bieti), an endangered species in China, has received more protection in theory than in practice. Therefore it is on the very verge of extinction. The population of the species was estimated less than 2,000 individuals spread in 19 distinct groups. It was confirmed that the monkey was confined to the Yunling Mountain System, the area between the Yangtze River (Changjiang, aka Jinshajiang) to the east and the Mekong River (Lancangjiang) to the west. We further concluded that a lowland belt to the east, about 100 km long and 20 – 30 km wide was not suitable habitat for the monkeys, and appeared to serve as the natural ecogeologic barrier for the species. Our results indicated that the southern limit of the distribution was at Longma (26°14′N), and that the northern limit of the distribution was at Xiaochangdu (29°20′N). The distribution area of the species was substantially smaller than previously estimated. There were substantial ecological differences between the southern and northern parts of the species range. The monkey was found only in fir-larch forest.     

Fish and fisheries in the Upper Mekong: current assessment of the fish community, threats and conservation

The Mekong flows north to south, through six countries in south–east Asia. Many studies have concentrated on fish and fisheries in the Lower Mekong, which has been identified as one of the largest inland fisheries in the world with an incredibly rich diversity of species. In contrast, fish and fisheries in the Upper Mekong (Lancang River) have remained relatively undocumented. In this paper, we synthesized information on freshwater fish biodiversity and fisheries in the Upper Mekong and documented 173 species and subspecies (including 87 endemic species) among 7 orders, 23 families and 100 genera. We divided the Upper Mekong into 17 sub-basins based on Digital Elevation Model (DEM) and then used fish species data to cluster the sub-basins. Four parts (the headwater, the upper reach, the middle reach and the lower reach) and one lake have distinct fish species communities associated with them. There was a linear relationship between fish species (x) and endemic species (y) as y = 0.5464x − 3.2926. Relationship between species number or endemic species number (y) and mean altitude (x) can be described as y = −54.352 ln(x) + 460.79 or y = −30.381 ln(x) + 253.85, respectively. Fisheries kept as about 6,000 t from 1989 to 1998, and then steadily increased to 10,000 t in 2004. We reviewed the overall threats to the Upper Mekong fish and fisheries, and found that hydrological alteration is the largest threat in the basin, followed by over fishing and the introduction of exotic species. In terms of specific river sections, water pollution was the most serious threat to fishes in the upper reach of the Upper Mekong, whilst migratory fishes in the lower reach of the Upper Mekong are seriously threatened by the construction of cascade dams. The Buyuan River and the Nanla River were identified as important feeding and spawning habitats for upstream migrant species and should be considered as a priority for conservation.     

Degradation of Tributyltin in Microcosm Using Mekong River Sediment

The degradation of tributyltin (TBT) and changes of bacterial number and community structures were investigated in microcosms using the sediment collected from the Mekong River, Vietnam. Concentrations of TBT in sediments were less than 0.62 ng/g (dry wt), lower than those reported from other areas. TBT-resistant bacteria were found in the three sampling sites, and the occurrence rates were 11–16% out of the total viable count. In this microcosm experiment, initial concentration of TBT [1.0–1.4 μg/g (dry wt)] decreased to 0.6 μg/g (dry wt) during 150 days, whereas that in the control microcosm with autoclaved sediment did not change, indicating that Mekong River sediment contains high TBT-degrading activity by microorganisms. The occurrence of TBT-resistant bacteria and the bacterial community structures monitored by denaturing gradient gel electrophoresis were almost the same between test and control groups, indicating that the addition of TBT had little influence on microbial community structure. Mekong River sediment seems to have a stable microbial community against TBT pollution.     

A biogeographic analysis and review of the far eastern Pacific coral reef region

New information on the presence and relative abundances of 41 reef-building (zooxanthellate) coral species at 11 eastern Pacific and 3 central Pacific localities is examined in a biogeographic analysis and review of the eastern Pacific coral reef region. The composition and origin of the coral fauna and other reef-associated taxa are assessed in the context of dispersal and vicariance hypotheses. A minimum variance cluster analysis using coral species presence–absence classification data at the 14 localities revealed three eastern Pacific reef-coral provinces: (1) equatorial– mainland Ecuador to Costa Rica, including the Galápagos and Cocos Islands; (2) northern– mainland México and the Revillagigedo Islands; (3) island group– eastern Pacific Malpelo Island and Clipperton Atoll, and central Pacific Hawaiian, Johnston and Fanning Islands. Coral species richness is relatively high in the equatorial (17–26 species per locality) and northern (18–24 species) provinces, and low at two small offshore island localities (7–10 species). A high proportion (36.6%, 15 species) of eastern Pacific coral species occurs at only one or two localities; of these, three disappeared following the 1982–83 ENSO event, three occur as death assemblages at several localities, and five are endangered with known populations of ten or fewer colonies. Principal component analysis using ordinal relative density data for the 41 species at the 14 localities indicated three main species groupings, i.e., those with high, mid, and narrow spatial distributions. These groupings correlated with species population-dynamic characteristics. These results were compared with data for riverine discharges, ocean circulation patterns, shoreline habitat characteristics, and regional sea surface temperature data to help clarify the analyses as these measures of environmental variability affect coral community composition. Local richness was highest at localities with the highest environmental variability. Recent information regarding the strong affinity between eastern and central Pacific coral faunas, abundance of teleplanic larvae in oceanic currents, high genetic similarity of numerous reef-associated species, and appearances of numerous Indo-west Pacific species in the east Pacific following ENSO activity, suggest the bridging of the east Pacific filter bridge (formerly east Pacific barrier). Accepted: 20 September 1999     

A new subspecies, Polynemus melanochir dulcis, from Tonle Sap Lake, Cambodia, and redescription of P. m. melanochir Valenciennes in Cuvier and Valenciennes, 1831 with designation of a neotype

 A poorly known threadfin, Polynemus melanochir Valenciennes in Cuvier and Valenciennes, 1831, is redescribed (as P. melanochir melanochir) on the basis of a newly designated neotype and a large number of specimens. Polynemus melanopus Sauvage, 1881 and 3 species commonly regarded as valid, Galeoides microps Steindachner, 1869, P. borneensis Bleeker, 1857, and Trichidion hilleri Fowler, 1905, are all considered junior synonyms of P. m. melanochir. In addition, a new subspecies, P. melanochir dulcis, is described on the basis of 3 specimens. Polynemus m. dulcis differs from P. m. melanochir in having a greater snout length [7% of SL vs. mean 6% (range 5–6%) of SL in the latter] and shorter upper caudal-fin lobe length [35% (34–35%) of SL vs. 39% (35–44%) of SL]. The former is currently known only from Tonle Sap Lake, Cambodia, whereas the latter is known from the Mekong River (Cambodia and southern Vietnam) and Kalimantan (=Borneo; Malaysia and Indonesia). Received: September 27, 2001 / Revised: December 20, 2001 / Accepted: December 29, 2001     

Subspecies validity for genus Culter (Teleostei: Cyprinidae) based on morphometrics analysis

The genus Culter, found only in East Asia (China, Vietnam, Korea and Russia), is a primary group of freshwater fishes in the subfamily Cultrinae (Cyprinidae). Of a total of 517 specimens studied, 235 were Culter alburnus from different localities and were used for morphometric analysis in this study. 17 landmarks were used in geometric morphometrics (GM) analysis of shapes. 13 traditional morphometrics (TM) and 21 truss-based morphometrics (TBM) characteristics were measured. The results of discriminant analysis and cluster analysis showed that four species-groups could be divided into separate groups. They were (1) C. alburnus + C. recurviceps, (2) C. dabryi dabryi + C. oxycephalus + C. dabryi shinkainensis, (3) C. oxycephaloides, (4) C. mongolicus mongolicus + C. mongolicus elongates + C. mongolicus qionghaiensis. The six populations of C. alburnus (Basilewsky, Nouv Mém Soc Imp Nat Moscou, 10:215–263, 1855), from Lake Poyanghu, River Songhuajiang, River Liaohe, River Jialingjiang, River Lijiang and Lake Taihu, should be classified as different geographical populations, but not subspecies. C. mongolicus elongates (He and Liu, Zool Res, 1(4):483–485, 1980) from Lake Chenghaihu and C. mongolicus qionghaiensis (Ding, Acta Zootaxon Sin, 15(2):246–250, 1990) from Lake Qionghaihu should be classified as subspecies. C. dabryi shinkainensis differed greatly from C. dabryi dabryi in body forms, and we suggest that C. dabryi shinkainensis should be changed to species, instead of subspecies, namely Culter shinkainensis (Yih and Chu, Acta Hydrobiol Sin, 2:170–199, 1959).     

Development and characterization of polymorphic microsatellite loci in the endangered Yangtze finless porpoise (Neophocaena phocaenoides asiaeorientalis)

The finless porpoise (Neophocaena phocaenoides) is one of the smallest cetacean species widely distributed in the shallow coastal waters of the Indo-Pacific Oceans. The population size of the Yangtze subspecies (N. p. asiaeorientalis) has sharply decreased in the last two decades and access to objective data on its population structure and genetic diversity would be of great assistance for their proper management. Here we report on the isolation of nine polymorphic microsatellite using the “Fast Isolation by AFLP of Sequences Containing repeats” (FIASCO) protocol. Polymorphism was assessed using 30 porpoise individuals randomly sampled in the Yangtze River. The number of alleles per locus varies from 2 to 9, with an average value of 5.56, whereas the ranges of observed and expected heterozygosities were 0.300–0.633 (mean 0.496) and 0.473–0.804 (mean 0.659), respectively.     

A new species of freshwater threadfin, <Emphasis Type="Italic">Polynemus aquilonaris</Emphasis>, from Indochina,and redescription of <Emphasis Type="Italic">Polynemus dubius</Emphasis> Bleeker, 1853 (Perciformes: Polynemidae)

 The taxonomic status of two nominal species of Polynemus, viz. P. dubius Bleeker, 1853 and P. longipectoralis Weber and de Beaufort, 1922, is revised. Although regarded as separate taxa up to the present time, examination of the holotype of P. longipectoralis revealed its close similarity to the type series of P. dubius, in the synonymy of which the former is now included. Polynemus dubius is redescribed as a valid species and a lectotype of the species is designated. In addition, a new species, P. aquilonaris, previously identified as P. dubius or P. longipectoralis, is described from Indochina on the basis of 28 specimens. Polynemus aquilonaris differs from P. dubius in having higher counts of pored lateral-line scales [80–86 (mode 81) vs. 69–79 (78) in the latter] and scale rows below the lateral line [14–17 (mode 14, rarely 13 or 17) vs. 13 (rarely 12)], and lower counts of gill rakers [25–29 (mode 27) vs. 29–33 (30), respectively]. The former is known from Indochina (Chao Phraya and Mekong River systems including Lake Tonle Sap), whereas the latter is currently known from the Malay Peninsula, Sumatra, and Kalimantan. Received: March 29, 2002 / Revised: February 2, 2003 / Accepted: February 10, 2003     

<Emphasis Type="Italic">Cabillus atripelvicus</Emphasis>, a new species of gobiid fish from the Ogasawara Islands,with a key to species of the genus

Cabillus atripelvicus is described from three specimens, 26.1–40.6 mm in standard length, collected from depth 17–29 m in the Ogasawara Islands. In addition, two small specimens taken by dredge in 50–55 m from Sagami Bay, Honshu, and one from Tachibana Bay, Kyushu, are identified as this species but not designated as paratypes. Cabillus atripelvicus is distinguished from the four other described species of the genus by its high count of 18–20 pectoral rays, presence of median predorsal scales, longer pectoral fins, and distinctive color pattern, including black pelvic fins. The range of C. lacertops Smith, 1959 is extended east to the Marshall Islands and that of C. tongarevae (Fowler, 1927) east to American Samoa. More species remain to be described in the genus. Supplementary material to this paper is available in electronic format at     

A new sea catfish,Netuma patriciae (Siluriformes: Ariidae), from the Philippines

A new sea catfish (Ariidae), Netuma patriciae, is described based on the holotype and nine paratypes, 127.3–303.4 mm in standard length, collected from Panay and Luzon islands, Philippines. The new species most resembles N. thalassina (Rüppell 1837), in sharing counts of anal-fin rays (17–19 in N. patriciae, 15–17 in N. thalassina) and a distinct dorsomedian head groove (longitudinal length of the groove, 33.7–40.1% of head length, 22.3–31.0%). However, the new species can be easily distinguished from the latter in having fused vomerine tooth patches, and more numerous free vertebrae (43–44 vs. 41–42 in N. thalassina). Although N. bilineata (Valenciennes in Cuvier and Valenciennes 1840a) has also fused vomerine tooth patches, the new species is distinguished by having fewer free vertebrae (43–44 vs. 47–51 in N. bilineata) and longer dorsomedian head groove (33.7–40.1% of head length vs. 0–9.7%). The new species can also be easily distinguished from both N. thalassina and N. bilineata by having a filamentous dorsal-fin ray (longest dorsal-fin ray 26.9–35.9% of standard length vs 20.3–23.3% in N. thalassina, 20.9–25.3% in N. bilineata, standard length > 127 mm) and a U-shaped junction of dorsomedian head ridge (vs. V-shaped in both species).     

Differences in Englemann Spruce Forest Biogeochemistry East and West of the Continental Divide in Colorado, USA

We compared Englemann spruce biogeochemical processes in forest stands east and west of the Continental Divide in the Colorado Front Range. The divide forms a natural barrier for air pollutants such that nitrogen (N) emissions from the agricultural and urban areas of the South Platte River Basin are transported via upslope winds to high elevations on the east side but rarely cross over to the west side. Because there are far fewer emissions sources to the west, atmospheric N deposition is 1–2 kg N ha⁻¹ y⁻¹ on the west side, as compared with 3–5 kg N ha⁻¹ y⁻¹ on the east side. Species composition, elevation, aspect, parent material, site history, and climate were matched as closely as possible across six east and six west side old-growth forest stands. Higher N deposition sites had significantly lower organic horizon C:N and lignin:N ratios, lower foliar C:N ratios, as well as greater %N, higher N:Ca, N:Mg, and N:P ratios, and higher potential net mineralization rates. When C:N ratios dropped below 29, as they did in east-side organic horizon soils, mineralization rates increased linearly. Our results are comparable to those from studies of the northeastern United States and Europe that have found changes in forest biogeochemistry in response to N deposition inputs between 3 and 60 kg ha⁻¹ y⁻¹. Though they are low by comparison with more densely populated and agricultural regions, current levels of N deposition, have caused measurable changes in Englemann spruce forest biogeochemistry east of the Continental Divide in Colorado. Received 22 January 2001; accepted 11 June 2001.     

Thermal adaptation in<Emphasis Type="Italic">Drosophila serrata</Emphasis> under conditions linked to its southern border: Unexpected patterns from laboratory selection suggest limited evolutionary potential

To investigate the ability ofDrosophila serrata to adapt to thermal conditions over winter at the species southern border, replicate lines from three source locations were held as discrete generations over three years at either 19‡C (40 generations) or temperatures fluctuating between 7‡C and 18δC (20 generations). Populations in the fluctuating environment were maintained either with an adult 0‡C cold shock or without a shock. These conditions were expected to result in temperature-specific directional selection for increased viability and productivity under both temperature regimes, and reduced development time under the fluctuating-temperature regime. Selection responses of all lines were tested under both temperature regimes after controlling for carry-over effects by rearing lines in these environments for two generations. When tested in the 19‡C environment, lines evolving at 19‡C showed a faster development time and a lower productivity relative to the other lines, while cold shock reduced development time and productivity of all lines. When tested in the fluctuating environment, productivity of the 7–18‡C lines selected with a cold shock was relatively lower than that of lines selected without a shock, but this pattern was not observed in the other populations. Viability and body size as measured by wing length were not altered by selection or cold shock, although there were consistent effects of source population on wing length. These results provide little evidence for temperature-specific adaptation inD. serrata —although the lines had diverged for some traits, these changes were not consistent with a priori predictions. In particular, there was no evidence for life-history changes reflecting adaptation to winter conditions at the southern border. The potential forD. serrata to adapt to winter conditions may therefore be limited.     

A new, dwarf species of Grammonus (Teleostei: Bythitidae) found off Vietnam

A new species of the viviparous fish genus Grammonus (Ophidiiformes: Bythitidae) is described, based on two ripe males, 32–55 mm SL. They were caught over a muddy bottom in a shrimp trawl at 70–119 m off Central Vietnam. A comparison with the nine hitherto described Grammonus species shows them to be clearly distinct from other species. Except for G. ater, G. minutus differs from all other species by having either more or fewer dorsal (75–76) and anal (54–55) fin rays. It differs from G. ater i.a. by having more pectoral fin rays (22–23 vs. 18–19).     

Mitochondrial DNA hypervariable region-1 sequence variation and phylogeny of the concolor gibbons, Nomascus

The still little known concolor gibbons are represented by 14 taxa (five species, nine subspecies) distributed parapatrically in China, Myanmar, Vietnam, Laos and Cambodia. To set the stage for a phylogeographic study of the genus we examined DNA sequences from the highly variable mitochondrial hypervariable region-1 (HVR-1 or control region) in 51 animals, mostly of unknown geographic provenance. We developed gibbon-specific primers to amplify mtDNA noninvasively and obtained >477 bp sequences from 38 gibbons in North American and European zoos and >159 bp sequences from ten Chinese museum skins. In hindsight, we believe these animals represent eight of the nine nominal subspecies and four of the five nominal species. Bayesian, maximum likelihood and maximum parsimony haplotype network analyses gave concordant results and show Nomascus to be monophyletic. Significant intraspecific variation within N. leucogenys (17 haplotypes) is comparable with that reported earlier in Hylobates lar and less than half the known interspecific pairwise distances in gibbons. Sequence data support the recognition of five species (concolor, leucogenys, nasutus, gabriellae and probably hainanus) and suggest that nasutus is the oldest and leucogenys, the youngest taxon. In contrast, the subspecies N. c. furvogaster, N. c. jingdongensis, and N. leucogenys siki, are not recognizable at this otherwise informative genetic locus. These results show that HVR-1 sequence is variable enough to define evolutionarily significant units in Nomascus and, if coupled with multilocus microsatellite or SNP genotyping, more than adequate to characterize their phylogeographic history. There is an urgent need to obtain DNA from gibbons of known geographic provenance before they are extirpated to facilitate the conservation genetic management of the surviving animals.     

Molecular mechanism for the initial process of visual excitation. IV. Energy surfaces of visual pigments and photoisomerization mechanism

Using the twisted conformations of the chromophores for visual pigments and intermediates which were theoretically determined in the previous paper, energy surfaces of the pigment at −190‡ C were obtained as functions of the torsional anglesθ _9–10 andθ _11–12 or of the torsional anglesθ _9–10 andθ _13–14. In these calculations, the existence of specific reaction paths between rhodopsin (R) and bathorhodopsin (B), between isorhodopsin I (I) and bathorhodopsin, and between isorhodopsin II (I′) and bathorhodopsin were assumed. It was shown that the total energy surfaces of the excited states had minimaC ₁ atθ _9–10 ∼ −10‡ andθ _11–12 ∼ −80‡,C ₂ atθ _9–10 ∼ −85‡ andθ _11–12 ∼ −5‡, andC ₃ atθ _9–10 ∼ 0‡ andθ _13–14 ∼ −90‡. These minima are considered to correspond to the thermally barrierless common states as denoted by Rosenfeld et al. Using the total energy surfaces in the ground and excited states, the molecular mechanism of the photoisomerization reaction was suggested. Quantum yields for the photoconversions among R, I, I′ and B were related to the rates of vibrational relaxations, radiationless transitions and thermal excitations. Some discussion was made of the temperature effect on the quantum yield. Similar calculations of the energy surfaces were also made at other temperatures where lumirhodopsin or metarhodopsin I is stable. Relative energy levels of the pigments and the intermediates were discussed.     

The first results of meiobenthos study in the Cái River (Central Vietnam)

The first data on the composition and quantitative development of meiobenthos in the small tropical Cái River (Central Vietnam) at the end of the “small rainy season” (early June) have been obtained. In the community composition, 101 taxa out of 15 systematic groups have been documented. Specimens which are new for science (Acari), new for the fauna of Vietnam (Nematoda, etc.), and rare representatives of surface waters (Syncarida) are found. The largest part of taxonomic diversity is formed by Nematoda (38 taxa), Annelida (18), and Chironomidae (22). The same groups dominated in abundance. The taxonomic richness, abundance, and biomass of the community varied within 4–45 taxa, 3.8–161.9 thousand ind/m², and 0.05–2.2 g/m². In general, the type of changes of all parameters along the river profile was classical: the values increased from the upper mountain sections to the lower plain parts. The revealed pattern testifies to the important role that hydrodynamics play in the macroscale distribution of meiobenthos in the river during the period under study. The dependence that the community characteristics have on the local features of biotopes (microdistribution) is not as definite and requires an additional study of more extensive material.     

长江流域鱼类物种多样性大尺度格局研究

作者以长江流域鱼类编目数据库为基础数据,研究了长江流域鱼类物种多样性的大尺度格局。长江流域内共记录了鱼类378种(亚种),隶属于14目32科144属。其中淡水鱼338种(亚种),以鲤形目为主,达到269种(亚种),洄游鱼类11种,河口鱼类29种;流域内特有种和受威胁物种分别有162种(亚种)和69种(亚种)。根据鱼类分布特点,按水系将长江流域分为19个区域,除了江源区和金沙江中上游外,物种数和G-F多样性指数上游高于中下游,但各区域内差异不大,然而特有种比例从上游到下游随海拔降低而逐渐降低。利用Jaccard物种相似性系数对19个区域进行聚类分析,将整个流域分成三部分:(1)江源区和金沙江中上游,地理上属于青藏高原东南部波状平原部分和横断山区,(2)上游其他流域,地理上属于川西高原、云贵高原、四川盆地及秦巴山区,(3)中下游流域,地理上属于淮阳山地、江南丘陵和长江中下游平原,基本反映了流域内自然地理环境及我国大陆地势三级台阶变化的特点。     

Blackfly assemblages (Diptera,Simuliidae) of the Carpathian river: habitat characteristics,longitudinal zonation and eutrophication

Investigations were carried out in 2003–04 within a project on trend biomonitoring of macrozoobenthos in the Hron River system. They were focused on blackfly assemblages of the Hron River (Slovakia), using the distribution patterns of blackflies (18 species) from 14 sites, representing a fluent transition from epirithral to epipotamal. Two main “sister” groups were distinguished using TWINSPAN analysis. Prosimulium rufipes is an indicator species within the first group. This group typically occurs in the zone representing a part of the river upstream and its tributaries. Simulium lineatum is an indicator species within the second group that occurs in the zone representing the middle and lower part of the river. The Upstream–Downstream gradient represents, in fact, the ecological position (EP) of the studied localities of the Hron River along the River Continuum Concept. The distribution fitting analysis divides the distribution amplitude data of blackfly species into three sets of nonoverlaping intervals: (1) 0.00–0.19 stenotopic (P. rufipes in the upper course of the river, Simulium erythrocephalum in the lower course of the river), (2) 0.20–0.39 mesotopic (Simulium argyreatum, S. equinum—hyporhithral) and (3) 0.40–0.50 eurytopic (Simulium reptans, S. variegatum). According to CCA the most important variables correlated with the first axis, and were linked to the nitrate and maximal water temperature which gradually increase from the upper stretch of the river. Handling editor: K. Martens     

Insect drift over the northern Arabian sea in early summer

Air borne insects, mostly carried by wind currents, were trapped over the northern Arabian sea (16‡ to 20‡ N; 68‡ to 72‡ E), in the course of cruise No. 111, ORVSAGAR KANYA (March 14 to April 7, 1996). A total of 2,301 insects belonging to 8 different orders, 47 families and 173 species were trapped. Of these, Hymenoptera was represented by the largest number (1082), which was followed by Hemiptera (586), Diptera (552), Coleoptera (51), Neuroptera (10), Trichoptera (03), Lepidoptera (03) and Orthoptera (01). The trapped insects were mostly between 0.6 mm to over 11 mm long. The data was examined for α-diversity as well as for possible correlations between various parameters like the diversity index, size and number of insects trapped on one hand and the distance of the nearest land mass in wind direction, on the other. Deceased.     

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

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 (ATP ase 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.