Recent records suggest that about 850 ‘freshwater' fish species occur in the basin, but many more marine and coastal fishes may at times enter the waters of the Mekong delta
Loss of biodiversity is a major concern for biologists, who warn that we are now well into 'the century of extinctions' during which conservation biology is likely to become an increasingly important discipline (Dubois 2003). While there is increasing understanding of the need to manage habitat to maintain biodiversity, there is much less awareness that rational management of ecosystems faces a major '‘taxonomic impediment', the incompleteness and inaccuracy of inventories of biodiversity, which are the starting point for any conservation program.
Tropical rivers are generally among the most diverse ecosystems and their biodiversity is particularly threatened by habitat changes and introduction of exotic species (Coates et al. 2003, Dudgeon et al. 2006). The Mekong system is thought to be an international 'hotspot' of unusually high biodiversity, so there is an urgent need for an up-to-date inventory of the system's biodiversity, both in an absolute sense and relative to other larger tropical rivers, as well as accurate information on individual species’ distribution and migration patterns.
One element of biodiversity is species richness, the number of species present in a defined area, an easily understood notion and one that is useful in comparisons between systems. The fish species richness of the Mekong system is a topic of particular interest, given the importance of fisheries to Mekong peoples and the claim by many authors that the system has unusually high biodiversity.
It should also be noted that apart from species richness (the subject of this article), other features of biodiversity ensure that the Mekong deserves the title of a biodiversity hotspot and should be given a high priority in conservation. These include the apparently large number of fish families (Kottelat, 2001), a significant proportion of endemic species (about 24%) which include several large or giant fishes, the persistence of the natural flood pulse, and an extraordinary dependence on fisheries by expanding rural populations (Hortle 2009).
What is a species?
The biological species concept (BSC) has been generally accepted by zoologists as the most useful and practical definition of a species as 'actually or potentially reproductively isolated' population(s) of organisms (de Queiroz 2005)1. The BSC can be seen as a special case of the broader 'evolutionary species concept', which covers asexually reproducing organisms and fossils, and which defines each species as a separately evolving lineage (Wiley 1978). Although there is a lively and ongoing debate about the nature and definition of 'species' (de Queiroz 2005; Hey 2006), in practical terms testing for reproductive isolation is usually not feasible, and genetic studies to reveal phylogeny (evolutionary relatedness) are too expensive to be applied to the many wild populations which require classification. To deduce relationships and to diagnose 'species' taxonomists continue to rely on morphology-variation in features, such as number of gill rakers, number of scales, counts of fin rays or even shape and coloration2. Taxonomists generally adjust classifications when evidence is found of intermediates between what were thought to be reproductively isolated populations, or if genetic studies indicate the presence of sympatric but reproductively isolated populations of morphologically similar fishes. The very limited resources available to study in the Earth’s biodiversity in detail, and differences in opinion between taxonomists, ensure that revisions to classification of species that have been described on morphological grounds are likely to continue for the foreseeable future.
The need to adjust for taxonomic effort
The number of species recorded from any particular system tends to increase as species are collected and recorded for the first time (new records) and as new species are found and described, with the rate of addition of species tending to decrease over time to an asymptote. Figure 1 suggests that there are still likely to be many more fish species described from the Mekong catchment, as is also evident from listings of un-described species in recent literature (see below). However, this is not an unusual situation for tropical river systems, indeed it seems likely that the Mekong has by now received comparable or more attention than many other large tropical river systems which are less accessible or subject to security risks. It is also highly likely that in many other tropical river systems flow and habitat alterations and the impact of species introductions may have caused the extinction of many fish species before they could be described. This inference can be reasonably drawn based on the situation in well-studied regions: for example in North America 61 taxa of inland fish are thought to have become extinct and about one third of all inland fish species are considered imperilled (Jelks et al. 2008).
Some other factors that affect species
As well as possible artefacts introduced by differences in taxonomic effort, comparisons between systems should take account of other factors which are well-known to be correlated with species richness; for example, the ecoregion in which the system is situated (Abell 2008), the fact that tropical systems are generally richer in species than temperate systems, and the 'species-area effect': more species are found in larger areas, other factors being equal. Welcomme (1985 Fig 5.1) used data from 47 major rivers to show that fish species richness is well correlated with the size of the river catchment, one indicator of available habitat area. Lévêque et al. (2008) provide updated graphs of the relationship between the size of some major river systems and their species richness, and de Silva et al. (2007 Fig 1.) show that land area alone accounts for over 70% of the variance in published 'finfish' species estimates between countries in east and southeast Asia. It is to be expected therefore that the Mekong will support more species than other smaller tropical river systems in the Oriental region because of the large extent of available aquatic habitat within its catchment of about 800,000 km2.
The limits of the Mekong
Defining a river system's boundaries by its catchment is straightforward, but defining its seaward extent is more problematic. MRC publications generally define the boundary geographically as a line across the mouth, but a more useful ecological boundary would be based on salinity. In the dry season, saline water extends at least 60 km inland, whereas in the wet season a surface layer of fresh water extends several kilometres offshore in the river's plume. Although not well-documented for the Mekong, it can be assumed that the downstream limit of distribution of strictly freshwater species, as well as the penetration of marine species into the estuary changes with season. Estuary fishes tend to prefer water of a particular salinity so they are also likely to move seasonally. Any reference to species counts of the system should therefore refer to whether the figure includes fishes normally found in brackish or marine waters that may only be present in the 'Mekong' seasonally or occasionally.
Published species estimates
Published estimates of the number of species in the Mekong system appear to have increased rapidly in the last few years, as can be seen from the following brief chronology:
Several authors provided similarly high but unsubstantiated figures, perhaps based on Rainboth’s estimate, but misused his figure as if it were a count of species actually recorded or a minimum estimate. In some cases, authors implied that the species count was for the river, when many species are found only in upland tributaries or in the estuary or in specialised habitats such as lakes or caves. Some of these recent species estimates include the following.
The MRC Fisheries Programme published the first compendium of species of the Mekong, the Mekong Fish Database (MFD), in 2003 and widely distributed it as a CD to fisheries scientists in the region. The MFD contained information on 898 indigenous fishes as well as 24 introduced species that were recorded from or thought likely to be present in the lower Mekong basin and in Yunnan. The fishes can be grouped according to their known occurrence in water of different salinities, using information which has been obtained from FishBase (Froese and Pauly 2009).
Fresh- brackish marine:
Brackish and marine:
The marine species and many of the brackish/marine species have only been recorded from the sea off the mouth of the Mekong or from nearby marine waters, and records for several of the freshwater species are questionable, so the MFD supports an estimate of about 750 species likely to be found at times within freshwaters of the system, which can be loosely defined as ‘freshwater’ species3. This figure is quite consistent with Kottelat’s estimate, nevertheless higher unsubstantiated estimates continued to be published, for example:
The citation of species numbers seems to represent a case of 'escalation bias', where a figure is quoted that supports a commonly-held belief (the Mekong has 'high' biodiversity) while ignoring the actual counts of species from the system published by Kottelat in 2001 and in the MFD in 2003. The variability between recent published estimates of species richness certainly creates a credibility issue for those seeking to promote conservation of the system's biodiversity, as well as highlighting the practical problems of classification and nomenclature facing those engaged in field studies or management of habitat for fisheries.
Figure 1: The Sunda shelf
Showing the maximum extent of sea level regression over the
last 250,000 years, the -120 m contour.
Based on (Voris 2000)
Figure 2: Species described from the Mekong River system
Cumulative count does not include species recorded that were first described from elsewhere
Table 1: Species 'split' since material was compiled for publication of the MFD in 2003
The newly described Mekong species is shown in bold after the species which was formerly considered to occupy a large range on both sides of the Sunda shelf, including the Mekong system
|Sister species||Revised distribution summary|
|Amblyrhynchichthys truncatus (Bleeker, 1851)||Malay Peninsula and Borneo|
|Amblyrhynchichthys micracanthus Ng & Kottelat, 2004||Lower and Middle Mekong to Vientiane, Chao Phraya, Mae Khlong, Tapi River|
|Bagrichthys macracanthus (Bleeker, 1854)||Sumatra, Peninsular Malaysia, northern Borneo (Sarawak)|
|Bagrichthys majusculus Ng, 2002||Lower and middle Mekong River system|
|Balantiocheilos melanopterus (Bleeker, 1851)||Kalimantan, Malay Peninsula, Sumatra|
|Balantiocheilos ambusticauda Ng & Kottelat, 2007||Lower and middle Mekong upstream to Nam Ngum, Chao Phraya|
|Hemiarius stormii (Bleeker, 1858)||Eastern Malay Peninsula and west Borneo|
|Hemiarius verrucosus (Ng, 2003)||Lower Mekong to Khone Falls and possibly Bang Pakong R., Southeast Thailand|
|Kryptopterus schilbeides (Bleeker, 1858)||Borneo, Java, Sumatra and Malay Peninsula|
|Kryptopterus paraschilbeides Ng, 2003||Lower and middle Mekong, Mae Khlong, Bang Pakong R., Chao Phraya|
|Kryptopterus schilbeides (Bleeker, 1858)||Eastern Sumatra and western and southern Borneo|
|Kryptopterus paraschilbeides Ng, 2003||Lower and middle Mekong upstream to Vientiane|
|Ompok hypophthalmus (Fang & Chaux, 1949)
||Java and southern Borneo (Barito River drainage)|
|Ompok urbaini Ng, 2003||Mekong, Chao Phraya and Pasak River drainages|
|Ompok rhadinurus Ng, 2003||Sumatra, the Malay Peninsula and Borneo|
|Ompok eugeneiatus (Vaillant, 1893)
||Peninsular Malaysia, central Sumatra and western Borneo|
|Ompok pinnatus Ng, 2003||Lower Mekong River and Chao Phraya drainages|
|Pangasius kunyit Pouyaud Teugels & Legendre, 1999
||Sumatra and Kalimantan (southern Borneo)|
|Pangasius mekongensis Gustiano, Teugels & Pouyaud, 2003 Lower||Mekong River system|
|Pangasius sabahensis Gustiano, Teugels & Pouyaud, 2003||Sabah, northern Borneo|
|Pangasius polyuranodon Bleeker, 1852
||Sumatra and Borneo|
|Pangasius elongatus Pouyaud, Gustiano & Teugels, 2002||Chao Phraya, Mekong and Bang Pakong basins|
|Pangasius mahakamensis Pouyaud, Gustiano & Teugels, 2002||Mahakam River, East Kalimantan, Borneo|
|Polynemus dubius Bleeker, 1853
||Malay Peninsula, Sumatra, Kalimantan|
|Polynemus aquilonaris* Motomura, 2003||Chao Phraya and Mekong River basins|
|Wallago leerii Bleeker, 1851
||Southern Thailand, Malay Peninsula and western Indonesia|
|Wallago micropogon Ng, 2004||Lower and middle Mekong to Luang Phabang, and middle Chao Phraya|
* also includes material formerly identified as Polynemus longipectoralis
An updated species count
Since the MFD was published there has been considerable taxonomic work which has resulted in many changes to classification of Mekong system fishes, which include the following.
Table 2: New species described from the Mekong River system Since material was compiled for publication of the Mekong Fish Database in 2003
|Scizothorax nudiventris Yang Chen & Yang, 2009||Upper Mekong, Yunnan|
|Bangana brevirostris Liu & Zhou, 2009||Upper Mekong, Yunnan|
|Mekongina lancangensis Yang, Chen & Yang, 2008||Upper Mekong, Yunnan|
|Minyclupeoides dentibranchialus Roberts, 2008||Lower Mekong, Cambodia|
|Oreoglanis jingdongensis Kong, Chen & Yang, 2007||Upper Mekong, Yunnan|
|Pareuchiloglanis abbreviatus Li, Zhou, Thomson, Zhang & Yang, 2007||Upper Mekong, Yunnan|
|Pareuchiloglanis prolixdorsalis Li, Zhou, Thomson, Zhang & Yang, 2007||Upper Mekong, Yunnan|
|Tonlesapia tsukawakii Motomura & Mukai, 2006||Great Lake, Tonlé Sap drainage in Cambodia.|
|Polynemus bidentatus Motomura & Tsuwaki, 2006||Mekong delta, Viet Nam|
|Pseudobagarius filifer (Ng & Rainboth, 2005)||Tonlé Sap drainage near Phnom Penh, Cambodia|
|Pseudobagarius nitidus (Ng & Rainboth, 2005)||Mekong River downstream of Khone Falls near Cambodia-Lao border|
|Akysis fuliginatus Ng & Rainboth, 2005||Lower Mekong River in northern Cambodia|
|Betta stiktos Tan & Ng, 2005||Mekong Drainage, Cambodia|
|Schistura bannaensis Chen, Yang & Qi, 2005||Upper Mekong, Yunnan|
|Oreoglanis macronemus Ng, 2004||Xieng Khouang, Lao PDR|
|Hemimyzon ecdyonuroides Freyhof & Herder, 2002||Headwaters of Se San River, Viet Nam|
The count of all fish species from 'the river system' is likely to be heavily influenced by the coastal species which enter fresh water for short periods, or marine species which may cross the geographical boundary (a line across the mouth) during dry periods. Systematic collection and accurate identification of fishes from the delta region would do much to remove the uncertainty in species counts and resolve the discrepancies in published estimates. If marine and coastal 'visitors' are included, there are about 1100 species recorded from or thought likely to occur in the system at times based on their occurrence in the Mekong's plume. More species will continue to be found, but at present there are no grounds for quoting higher estimates.
The available data indicate that there are about 850 freshwater fish species recorded from the Mekong (a total that includes some undescribed species of uncertain status), with a total estimate of about 1100 if the possible coastal or marine visitors are included. As mentioned above, comparison with other river systems should take into account the biogeographic province or ecoregion, the area of each system, the relative taxonomic effort that has been applied, and the extent to which biodiversity may have been lost before it could be documented. If it is evident that the Mekong does indeed have more species than expected for a river of its size and location, it would be of interest to explore the possible causes, which might include its recent geological history and the geography of the system.
Unless great care is taken, comparisons with other river systems may lead to erroneous conclusions. For example, according to FishBase there are 405 species recorded from the Zaire (Congo) and 1212 species recorded from the Amazon, but these are clearly under-estimates, both because existing records have not been systematically compiled and because many species remain to be described. Roberts (1975) recorded 650 fishes from the Zaire basin and Tedesco et al. (2005) quoted a figure of 686 species, but Lévêque et al. (2008) cautioned that existing information for this basin needs to be synthesised and that many new discoveries are anticipated. For the Amazon, Tedesco et al. (2005) used an estimate of 1761 species and Lévêque et al. (2008) recorded 2416 species, but Kottelat (2001) cited an opinion that the number of species in South America could more than double with new collections and taxonomic effort. The Amazon's catchment is about seven times and the Zaire five times the size of the Mekong's, so it is likely that they support more fishes, but at present it is impossible to know whether any of these large river system's species richness is above or below the average that would be predicted for large tropical rivers of their size.
The Water Studies Centre of Monash University kindly provided office space and library facilities to assist in the preparation of this paper. I thank Eamonn Hortle for assisting with compilation of species information from FishBase and Chris Burridge and Chris Barlow for helpful comments on a draft manuscript.
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2 The basis for the current system of scientific classification (taxonomy) of animals was set out over 200 years ago by Linnaeus in his Systema Naturae, with rules now formalised in the Code set by the International Commission on Zoological Nomenclature (www.iczn.org).
3 It should be noted that 'freshwater' species have been defined by some authors as only those within primary freshwater families, i.e. families which have a long history of evolution in freshwaters, and if this approach were used the total for the Mekong system would fall to about 530 species because many Mekong fishes are within peripheral division families, those with recent marine ancestors. See: Berra TM (2001) Freshwater Fish Distribution. Academic Press: San Diego, USA.