Review of Globally Important Ecosystems and Their Biodiversity of the Lower Mekong Basin
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AmphibiansThere have been few systematic studies of the amphibians of the Lower Mekong Basin. Over 100 species of frogs are reported from Laos.
A recent IUCN study lists the Mekong River as one of the nine richest watersheds in the world for fish. Of some 1,300 fish species proposed to occur in the Mekong system (including estuarine and deltaic species), over 850 have been suggested to occur in the Lower Mekong Basin. This figure is a little controversial, since not all species have been taxonomically verified. In comparison, Kotellat’s (Kotellat 2000) taxonomic studies have recorded 456 species of freshwater fish species in the Lower Mekong Basin, of these 179 (40%) are known to be endemic and 53 species (11%) are known from only a single sub-basin, illustrating the high level of local endemism. The total number of fish species in the Lower Mekong Basin probably lies between these two figures. In recent geological times, the Lower Mekong Basin has been through a number of dramatic changes. During the last ice age, the Mekong River may have been associated with the now submerged North Sunda River, which linked with the Chao Prya River and rivers of the eastern Malaya Peninsula. Thus, the fish species of the main channels and floodplains, although exhibiting a great diversity, have much in common with the rivers of the former system and do not exhibit high amounts of endemism. The fish of Lake Tonle Sap and the main channels of the Mekong River are well known, reflecting the high fishing pressure and the economic importance of these species. Relatively few endemic species occur in these areas. The greatest numbers of endemic and undescribed species are found in the tributaries, head waters, rapids, caves and peat swamps. Each new taxonomic survey of a tributary identifies undescribed, often endemic, fish species. This is demonstrated by a recent survey of the Nam Thuen in areas above Geng Wang Fang. Eighty-seven species of fish were identified, of which 33 (38%) were tentatively classified as new species, most probably endemic to the Lower Mekong Basin. Additional surveys of limestone caves, rapids, peat swamps and waterfalls will add to the known fish species of the Lower Mekong Basin. Cyprinids are the dominant species in the Lower Mekong Basin, reported to comprise 50% of all fish species in the lower areas, increasing to 60% of all species in the middle reaches of the Lower Mekong Basin. The diversity of the Lower Mekong Basin fish is illustrated by some of its more interesting fish species:
Table 5 provides a list of fish species from the Lower Mekong Basin listed in the IUCN Red List of Threatened and Endangered species. Two are regarded as Globally Critically Endangered in the Lower Mekong Basin – Leaping Barb Chela caeruleostigmata and Dwarf Botia Botia sidthimunki. Of the remainder, the Giant Catfish Pangasianodon gigas is probably the best known. There are known to be two discrete populations of Giant Catfish. One is believed to migrate many hundreds of kilometres each year from the mainstream Mekong north of the Khone Falls, to stretches of the Mekong upstream of Vientiane where it is believed to spawn. The second population is below Khone Falls in Cambodia and Viet Nam. Although this population is also known to undergo seasonal migrations, the location of the breeding sites is not known. Table 5: Globally threatened fish species from the Lower Mekong Basin
It is certain that a number of additional species should be added to this list. However, due to the lack of a broad-based understanding of fish diversity, fish life histories, and an evaluation of threats to species, only a superficial analysis can be made. There is currently insufficient information to make useful and comprehensive decisions related to fish biodiversity conservation in the Lower Mekong Basin. In the last 10 years there has been a dramatic increase in fishing effort, combined with an increase in the use of modern gears and destructive fishing practices. This has placed severe pressure on some fish species. Of grave concern, is the use of illegal fishing methods, i.e. electro-fishing and fish bombing, in sensitive habitats such as small ponds and deeper pools, which act as dry season refuges for many species. Although fish bombing is decreasing, electro-fishing is still occurring in a number of wetlands. Fish species populations reported to be decreasing sharply in numbers, or in some cases reported as locally extinct, include (Kotellat 2000):
A number of different types of distinct fish migrations occur in the Lower Mekong Basin. These involved extensive movements ranging from thousands of kilometres down to very localized movements. One of the commonest methods used to refer to fish migrations in the Mekong floodplain is to classify fish as black fish and white fish. These groups can be broadly categorized as:
The seasonal migration to and from Lake Tonle Sap is the largest and most significant in the entire Mekong Basin. During the seasonal inundation many species migrate to feed on the inundated plain and inundated forest of Lake Tonle Sap. As the water recedes, many fish species migrate upstream to dry season habitats, spawning areas and refuges. The Tonle Sap, Mekong mainstream channels, and larger tributaries are vitally important for the seasonal longitudinal migration of white fish species. Over 100 species are probably involved in these migrations, important one include large cyprinid species Bangana behri, Cyclocheilichthys enoplos, Hypsibarbus sp., Labeo chrysophekadion, Labeo pierrei, Mekongina erythrospilus and Scaphognathops bandonensis. Other smaller cyprinid species include Henicoryhnchus lineatus, H. lotabus (locally called ‘trey reil’), H. microlepis, and H. siamensis. Labiobarbius leptochelius, Lobocheilos melanotaenia and Paralaubuca typus. Other highly migratory groups include all the catfish species, including the endangered Giant Catfish Pangasianodon gigas, Pangasius sanitwongsei, P. sutchi, P. larnaudi, and Jullien’s Barb Probarbus jullieni. The Khone Falls on the Mekong mainstream in southern Laos, presents a physical barrier to some species, especially those of secondary freshwater families (evolved from marine taxa), i.e. Ariidae, Polynemidae, Soleidae and Cynoglossidae, and larger fish species, including the Giant Catfish Pangasianodon gigas. Other species are able to negotiate these falls and have been tracked over 1,000 kilometres to their dry season habitats, e.g. Henicorhynchus lobatu.
A number of migratory diadromous fish species, i.e. fish that migrate between marine/estuarine and freshwater or visa-versa) are found in the Lower Mekong Basin. Those species include:
Migration patterns of species in the upper tributaries are not well understood and more intensive research in the future.
The wetlands of the Lower Mekong Basin also contain a wealth of diversity of invertebrate groups. However, there have been only very limited surveys of these taxa: Insects : There have been few detailed studies of the insects of the Lower Mekong Basin. Crustacea : The giant fresh water prawn Macrobrachium rosenbergi undertakes an impressive migration. Adults live in the freshwater areas of the Mekong including tributaries, natural lakes, rice fields and ditches. On maturing the prawns migrate towards the estuary to spawn in saline water. After 2-6 months in the estuary, juveniles migrate back into freshwater areas. Adults can be found up to 200 km from the estuary. Molluscs : An example of invertebrate diversity can be seen from the limited surveys for molluscs. A survey of only 500 km of the Mekong River in Thailand and Laos identified 160 mollusc species. Of these, 116 (72%) species were endemic. Full surveys are certain to identify additional species. Coelenterates : It is urgent that further studies of all water bodies of the Lower Mekong Basin be conducted to determine the status of invertebrates. An as yet unidentified freshwater jellyfish Moerisia sp. has been found just south of Khone Falls, and a diverse assemblage of invertebrate fauna, including sponges, are expected to be found. 1 Rapids: Assume each rapid has an average area = 30 m. (river width) x 10 m. (length of rapid) = 300 m2 (0.0003 km2), thus a species occurring in 100 rapids would have a total habitat confined to 0.03km2. Thus meeting the criteria as Critically Endangered (CR B1) – confined to a limited habitat. ____________________________________________________________________________________________________ of |