Thread: Fresh Water Stingray

Fresh Water Stingray

Some articles on Thailands giant stingrays can be found here (Thailand : Giant Stingrays).

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.

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.
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Thailands Freshwater Fish