The IUCN/SSC Shark Specialist Group
Shark News 5: October 1995
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Status of the Giant Freshwater Stingray (whipray)
Himantura chaophraya (Monkolprasit and Roberts 1990)
Compiled by Leonard J.V. Compagno and Sid F. Cook
Taxonomy
The giant freshwater stingray (whipray) is one of eight apparently
obligate freshwater species of dasyatids (four Dasyatis spp. and four
Himantura spp.) of the much larger Family Dasyatidae (whiptail stingrays).
Other members of the family believed to be found at least occasionally
in freshwater include the brackish-marginal Himantura schmardae and
several euryhaline species: two Dasyatis spp., two Himantura spp. and
Pastinachus [= Hypolophus] sephen.
H. chaophraya was only formally described in 1990, though its
existence had been known for some years. The type locale is listed as
the Central Chao Phraya River of Thailand. It has been previously mis-identified
in Australia as Dasyatis fluviorum Ogilby, 1908 [estuary
stingray] and may have been listed under the old name of Himantura
polylepis (Bleeker) in Indonesia.
Distribution
The giant freshwater stingray is known from highly disjunct locales
including fresh waters in Thailand in the Chao Phraya, Nan, Mekong,
Bongpakong, Tachin and Tapi Rivers. It is also recorded from Mahakam
Basin [Borneo], the Fly River Basin [New Guinea], and from Australia
in the Gilbert River [Queensland], the Daly and South Alligator Rivers
[Northern Territory], Pentecost and Ord Rivers [Western Australia]. It
may occur in most of the large rivers of tropical Australia. However, it
has not been recorded from marine waters in any of its known range.
Sid Cook with air-dried specimens of adult and newborn Himantura chaophraya
at Chainat, Thailand. Photo: Sarah Fowler.
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Description
This species, one of the largest living dasyatids, has a characteristic
rounded disk, a prominent snout tip and a long whip-like tail without
cutaneous folds. It reaches a size of up to 200 cm disk width, and 600
kg in weight (Thailand and most other locales in range). However,
Australian specimens are reported as only reaching slightly more than
100 cm (disk width). Males mature by 110 cm disk width. Young are
born at about 30 cm disk width. Maximum lifespan in the wild is
unknown.
Conservation status
The giant freshwater stingray has been taken by fishermen on the
rivers of Central Thailand, in fisheries for bony fishes, notably giant
gouramy ( Osphrenemeus gouramy) and giant river catfishes (Pangasius
spp.). In 1992 Thai fishermen reported 25 individuals of H. chaophraya
in their catch, but by 1993 reported landings had dropped to three.
Due to a complex series of factors causing degradation or habitat
alteration in riverine habitats in the region, only about 30-31 of the
190 species of indigenous Thai freshwater fishes are estimated to
reproduce in the wild, although it is likely that a somewhat higher
biodiversity exists in backwater habitats where small, isolated pockets
of endemism undoubtedly occur.
Habitat-degrading factors having a negative impact on Thai
riverine environments include over-harvesting of forest canopy, leading
to drought upstream and flooding downstream during monsoon
conditions which further leads to excess siltation; dam building to
control flooding, which leads to silt build-up and retention of
agrochemicals behind impoundments; and development of lands
adjoining river habitats, which facilitates degradation and
destruction of ray habitats with deposition of broad-spectrum
wastes. The dams effectively isolate portions of the reproductive
populations of all riverine stingrays ( H. chaophraya, H. oxyryncha
[= krempfi], H. signifer and Dasyatis laosensis) from intermixing during
mating, dramatically cutting the diversity of the gene pool for any given
species. In the case of some very low density riverine elasmobranch
species, like the sawfishes, a combination of fisheries and habitat
changes have effectively eliminated them from the Chao Phraya and
adjoining freshwater habitats, where they have not been reported for
some 40 years.
The precipitous decline of riverine stingrays in Thai fresh waters
has led the Thai government to implement an experimental program
for captive propagation to try to stabilise levels of biodiversity while
they attempt to solve problems with degradation of river habitats. The
authors observed the operations at Chainat, Suppraya Province,
Central Thailand, in December 1993, where healthy individuals of
H. chaophraya ranging in size from 0.45 m to 1.6 m in disk width and
ranging from an estimated 50 kg to 500 kg were observed, along with
healthy individuals of Himantura signifer (white-edged freshwater
whipray) and Dasyatis laosensis (Mekong freshwater stingray). One
Himantura oxyryncha [= krempfi] (marbled freshwater stingray) in
poor condition died while the authors were at the facility.
In the South Alligator (and possibly East Alligator) River which runs
through Kakadu National Park, concern has arisen for both the giant
freshwater stingray and riverine occurrences of the bull shark
( Carcharhinus leucas), related to possible adverse effects of silt carrying
heavy metals and radio-isotopes from experimental uranium mines
around Coronation Hill and along the Alligator Rivers in the Park.
Further research is urgently needed to ascertain the status and
possible threats to this species in other parts of its range (Borneo, New
Guinea and Indonesia).
IUCN threatened species assessment
This species should be considered Critically Endangered throughout its
known range. It has been and will continue to be affected by the
complex and synergistic effects of the restrictions of its obligate freshwater
habitat, fishing pressures and habitat alteration/destruction. The
possibility of biological extinction in the wild is considered extremely
high.
Editor's note: The above is a greatly abbreviated version of the draft
account supplied by the authors for the Shark Action Plan. The original
includes many references and is available from the Editor. The
threatened species assessment is provisional until agreed by the
Shark Group, and based on criteria given in: IUCN (1994). IUCN Red
List Categories. Gland, Switzerland.
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