4Catch and Culture Volume 15, No. 2 August 2009
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,
an easily understood notion and one that is useful
richness of the Mekong system is a topic of particular
Fishes of the Mekong–howmany
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
rural populations (Hortle 2009).
The biological species concept (BSC) has been
generally accepted by zoologists as the most useful
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
species as a separately evolving lineage (Wiley 1978).
Although there is a lively and ongoing debate about
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
and to diagnose ‘species’ taxonomists continue to
number of gill rakers, number of scales, counts of
1An alternative ‘phylogenetic’ system based only upon degree of genetic relationship has been developed recently and is being increasingly used in
cladistics (Rieppel 2006).
Naturae, with rules now formalised in the Code set by the International Commission on Zoological Nomenclature (www.iczn.org).
By Kent G. Hortle *
August 2009 Catch and Culture Volume 15, No. 2
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
to study in the Earth’s biodiversity in detail, and
differences in opinion between taxonomists, ensure
been described on morphological grounds are likely to
continue for the foreseeable future.
The number of species recorded from any particular
system tends to increase as species are collected
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
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
species introductions may have caused the extinction
This inference can be reasonably drawn based on the
situation in well-studied regions: for example in North
species are considered imperilled (Jelks et al. 2008).
As well as possible artefacts introduced by differences
in taxonomic effort, comparisons between systems
should take account of other factors which are
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
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
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 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.
salinity so they are also likely to move seasonally.
Any reference to species counts of the system should
normally found in brackish or marine waters that
may only be present in the ‘Mekong’ seasonally or
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
1970s and estimated that there were about 300
species known from the lower Mekong basin.
were about 600 species in the Mekong, and
ranked the system third in terms of its species
richness after the Amazon and the Zaire, both of
which are larger rivers.
6Catch and Culture Volume 15, No. 2 August 2009
Cambodia and estimated that the Mekong “has
this estimate in an interview (Jensen 1996). This
estimate however was clearly his expectation
of the total that might be eventually found after
more collection and taxonomic work.
documented records of about 700 species from
the Mekong basin, and because there were
suggested that they should not be quoted.
Several authors provided similarly high but
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.
between 1200 and 1600 described species of
Basin’ (Coates et al. 2003).
(sic) (MRC 2003).
compendium of species of the Mekong, the Mekong
Fish Database (MFD), in 2003 and widely distributed it
as 24 introduced species that were recorded from or
thought likely to be present in the lower Mekong basin
to their known occurrence in water of different
salinities, using information which has been obtained
from FishBase (Froese and Pauly 2009).
Freshwater only : 539 species
Freshwater-brackish: 79 species
Fresh- brackish marine: 113 species
Brackish only: 4 species
Brackish and marine: 115 species
Marine: 48 species
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
consistent with Kottelat’s estimate, nevertheless higher
unsubstantiated estimates continued to be published,
found in the (Mekong basin) exceeds 2000’ (van
Zalinge et al. 2004).
The citation of species numbers seems to represent
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
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
Distribution. Academic Press: San Diego, USA.
August 2009 Catch and Culture Volume 15, No. 2
Figure 1: TheSundashelf
Showing the maximum extent of sea level regression over the last 250,000 years, the -120 m
contour. Based on (Voris 2000)
Figure 2: SpeciesdescribedfromtheMekongRiversystem
8Catch and Culture Volume 15, No. 2 August 2009
Amblyrhynchichthys truncatus (Bleeker, 1851) Malay Peninsula and Borneo
Amblyrhynchichthys micracanthus Ng&Kottelat,2004 LowerandMiddleMekongtoVientiane,ChaoPhraya,MaeKhlong,Tapi
Bagrichthys macracanthus (Bleeker, 1854) Sumatra, Peninsular Malaysia, northern Borneo (Sarawak)
Bagrichthys majusculus Ng,2002 LowerandmiddleMekongRiversystem
Balantiocheilos melanopterus (Bleeker, 1851) Kalimantan, Malay Peninsula, Sumatra
Balantiocheilos ambusticauda Ng&Kottelat,2007 LowerandmiddleMekongupstreamtoNamNgum,ChaoPhraya
Hemiarius stormii (Bleeker, 1858) Eastern Malay Peninsula and west Borneo
Hemiarius verrucosus (Ng,2003) LowerMekongtoKhoneFallsandpossiblyBangPakongR.,Southeast
Kryptopterus cryptopterus (Bleeker, 1851) Borneo, Java, Sumatra and Malay Peninsula
Kryptopterus geminusNg, 2003 LowerandmiddleMekong,MaeKhlong,BangPakongR.,ChaoPhraya
Kryptopterus schilbeides (Bleeker, 1858) Eastern Sumatra and western and southern Borneo
Kryptopterus paraschilbeidesNg,2003 LowerandmiddleMekongupstreamtoVientiane
Ompok hypophthalmus (Fang & Chaux, 1949) Java and southern Borneo (Barito River drainage)
Ompok urbaini Ng,2003 Mekong,ChaoPhrayaandPasakRiverdrainages
Ompok rhadinurus Ng, 2003 Sumatra, the Malay Peninsula and Borneo
Ompok eugeneiatus (Vaillant, 1893) Peninsular Malaysia, central Sumatra and western Borneo
Ompok pinnatusNg,2003 LowerMekongRiverandChaoPhrayadrainages
Pangasius kunyit Pouyaud Teugels & Legendre, 1999 Sumatra and Kalimantan (southern Borneo)
Pangasius mekongensis Gustiano, Teugels&Pouyaud,2003 LowerMekongRiversystem
Pangasius sabahensis Gustiano, Teugels & Pouyaud, 2003 Sabah, northern Borneo
Pangasius polyuranodon Bleeker, 1852 Sumatra and Borneo
Pangasius elongatus Pouyaud,Gustiano&Teugels,2002 ChaoPhraya,MekongandBangPakongbasins
Pangasius mahakamensis Pouyaud, Gustiano & Teugels, 2002 Mahakam River, East Kalimantan, Borneo
Polynemus dubius Bleeker, 1853 Malay Peninsula, Sumatra, Kalimantan
Polynemus aquilonaris* Motomura,2003 ChaoPhrayaandMekongRiverbasins
Wallago leerii Bleeker, 1851 Southern Thailand, Malay Peninsula and western Indonesia
Wallago micropogon Ng,2004 LowerandmiddleMekongtoLuangPhabang,andmiddleChaoPhraya
* alsoincludesmaterialformerlyidentiedas Polynemus longipectoralis
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
August 2009 Catch and Culture Volume 15, No. 2
Since the MFD was published there has been
considerable taxonomic work which has resulted in
species that was considered widespread in
rivers that formerly ran across the Sunda shelf
has been ‘split’ into ‘sister’ species, a mainland
Indochinese (including Mekong) species and
one or more ‘Sundaic’ species, found in river
systems to the south (Figure 2, Table 1). Such
splitting does not change the species count
for the Mekong, only the name applied to the
Mekong species. But it can cause confusion
and double-counting when distribution records
for the Sundaic species are not updated,
which is a problem for example in the current
version of FishBase. Examples of split species
are shown in Table 1 with updated distribution
still being described, either from new collections
or when taxonomists re-examine existing
collections which may have been stored in
museums for many years. At least 16 new inland
species have been described from the Mekong
River system since the MFD was published in
2003 (Table2), adding to the species count for
to be distinct are synonymised when additional
examination shows that there is overlap in
the range of variation of diagnostic characters
between populations that have been named
as separate species, or when it is found that
a species has been named more than once.
Since 2003 ‘lumping’ of species has reduced
the count for the Mekong system by about 20
species, more than compensating for the effect
of description of new species.
or families) have been revised since the MFD
was published, with many new generic names
species. For example, the widespread genus
Botia, which includes several common Mekong
species, was split into seven genera by Kottelat
(2004), with Mekong species now falling within
three genera. Thus the red-tailed botia, formerly
Botia modesta is now known as Yasuhikotakia
Scizothorax nudiventris YangChen&Yang,2009 UpperMekong,Yunnan
Bangana brevirostris Liu & Zhou, 2009 UpperMekong,Yunnan
Mekongina lancangensis Yang,Chen&Yang,2008 UpperMekong,Yunnan
Minyclupeoides dentibranchialus Roberts, 2008 Lower Mekong, Cambodia
Oreoglanis jingdongensis Kong,Chen&Yang,2007 UpperMekong,Yunnan
Pareuchiloglanis abbreviatus Li,Zhou,Thomson,Zhang&Yang,2007 UpperMekong,Yunnan
Pareuchiloglanis prolixdorsalis Li,Zhou,Thomson,Zhang&Yang,2007 UpperMekong,Yunnan
Tonlesapia tsukawakii Motomura & Mukai, 2006 Great Lake, Tonlé Sap drainage in Cambodia.
Polynemus bidentatus Motomura & Tsuwaki, 2006 Mekong delta, Viet Nam
Pseudobagarius lifer (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 UpperMekong,Yunnan
Oreoglanis macronemus Ng, 2004 Xieng Khouang, Lao PDR
Hemimyzon ecdyonuroides Freyhof & Herder, 2002 Headwaters of Se San River, Viet Nam
10 Catch and Culture Volume 15, No. 2 August 2009
has also led to many new names for genera
(Marceniuk and Menezes 2007). Such name
changes are relatively easy to follow by updating
species lists using FishBase and do not affect
the species count for the system.
are already described from elsewhere are
the Mekong system. Such information has not
been updated in any systematic way since
the MFD was published. Summary distribution
records are often woefully inadequate for
assessing whether it is likely that a species
is present in the area of interest, in some
cases simply reading “Asia”, while original
locality data on museum specimens are often
published in ecological surveys typically include
as ‘sp1’, ‘sp2’ etc.) or may contain errors (for
example non-Mekong species), hardly surprising
correctly. Ideally, distribution data for inland
species in FishBase would accurately identify
the major river basins in which each species
task to check and carefully summarize and
update such information. A review and cross-
checking of Mekong system species listed in
Baird et al. (1999), Kang et al. (2009), Kottelat
(2001), Rainboth (1996) and Vidthayanon
(2008) suggests that there are about 100
additional species that have been recorded
from freshwaters of the Mekong basin, although
most of these are not formally described or are
of uncertain taxonomic status. Therefore it is
reasonable to estimate that there are about 850
‘freshwater’ species known from the system.
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
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
(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
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
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
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.
August 2009 Catch and Culture Volume 15, No. 2
The Water Studies Centre of Monash University kindly
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.
*Mr Hortle was a technical ofcer at the MRC Fisheries Programme
from 2001 to 2005 and currently works as a consultant on sheries
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