|Scientific Name:||Himantura dalyensis|
|Species Authority:||Last & Manjaji-Matsumoto, 2008|
|Taxonomic Notes:||The Freshwater Whipray (Himantura dalyensis) is a recently described species which was previously referred to as Himantura chaophraya Monkolprasit & Roberts, 1991 (for example, in Last and Stevens 1994). Himantura chaophraya was considered to occur in Southeast Asia and in northern Australia. However, Last and Manjaji-Matsumoto (2008) consider the Australian form to be distinct from the Southeast Asian form, and they described the former as H. dalyensis. They also clarified the correct name for the Southeast Asian species as H. polylepis (Bleeker, 1852). Thus, the name H. chaophraya is invalid.|
|Red List Category & Criteria:||Data Deficient ver 3.1|
|Reviewer/s:||Carlson, J. & Dulvy, N.|
The Freshwater Whipray (Himantura dalyensis) is a recently (2008) described species which is possibly endemic to northern Australia and was previously referred to as Himantura chaophraya. The new valid name for H. chaophraya is H. polylepis and that species is found only in Southeast Asia. This assessment partially replaces the previous H. chaophraya assessment (that is, it replaces the Australian component).
The Freshwater Whipray is a poorly known species, which is possibly endemic to fresh and estuarine waters of tropical northern Australia, although its distribution is not fully mapped and its occurrence in New Guinea requires investigation. There may be minimal exchange between rivers where it occurs, and so separate subpopulations may exist across northern Australia. The ecology of this uncommon, large (reaching 124 cm disc width, but more commonly <100 cm disc width) stingray is poorly known; there is a lack of information on life history parameters, key habitat requirements and residency and/or migration patterns. Similarly, there is a lack of concrete information available on threats facing this species. Freshwater elasmobranchs are generally of conservation concern given the coupling of the usual biological limitations of the group with the physical constraints of their environment which limits their ability to evade both habitat alteration/degradation and exploitation. The restricted geographical distribution of the Freshwater Whipray, the fact that it is not particularly common and the presumed limited interchange between rivers increases the species’ inherent vulnerability. Details are required on how several potentially threatening processes may be impacting upon the species, namely: commercial fishing (the species’ distribution overlaps with net fisheries, however the Freshwater Whipray is not very susceptible to capture in gillnets), recreational fishing (parts of the species’ range are extremely popular for recreational fishing, an activity which continues to grow in northern Australia), impoundments and artificial barriers (which have the capacity to impact movements and alter habitat availability) and broader-scale catchment practices. More remote parts of the species’ range may provide some refuge from fishing, but areas closed to both commercial and recreational fishing should be considered as a management tool for limiting pressure on the species. At this time, although the inherent vulnerability of the species and the existence of potential threatening processes can be recognized, there is no support for the species meeting a threatened category. Indeed, a lack of information precludes its assessment beyond Data Deficient, but data are rapidly needed to accurately assess the species’ conservation status.
|Range Description:||The Freshwater Whipray is possibly endemic to estuarine and freshwater environments of northern Australia, although it may also occur in the Fly River Basin of Papua New Guinea (Last and Manjaji-Matsumoto 2008, Last and Stevens 2009). In northern Australia it has been recorded from the Ord, Fitzroy and Pentecost Rivers in Western Australia, the Daly, Roper and South Alligator Rivers in the Northern Territory and the Mitchell, Gilbert, Normanby and Wenlock Rivers in Queensland (Thorburn et al. 2003, Last and Manjaji-Matsumoto 2008, Last and Stevens 2009). However, its distribution is not fully mapped, and it has been suggested that it may occur in most large tropical rivers of northern Australia (Last and Stevens 2009). The Daly River (Northern Territory) and Normanby River (Queensland) systems have been identified as sites of significance for the species, given abundances relative to other systems (Thorburn et al. 2003).|
Native:Australia (Northern Territory, Queensland, Western Australia)
|FAO Marine Fishing Areas:||
Indian Ocean – eastern; Pacific – western central
|Range Map:||Click here to open the map viewer and explore range.|
|Population:||There is no information available on the population size or trends for the Freshwater Whipray. It is possible that separate subpopulations exist in each of the different river systems in which the species occurs, particularly given that some of these are widely separated, and so exchange between them would be minimal. Although its exact status is unknown, it was considered to be uncommon by Larson et al. (2004).|
|Habitat and Ecology:||
The Freshwater Whipray occurs in freshwater and estuarine habitats. It has been recorded in both tidal and non-tidal reaches of rivers in waters of <30 ppt salinity (usually <10 ppt) and frequents both turbid and clear waters (Thorburn et al. 2003). It has not been recorded from euhaline marine waters (Pogonoski et al. 2002, Last and Manjaji-Matsumoto 2008). Thorburn et al. (2003) documented the species on sparsely covered finer substrates (sand and silt) in the main channel of rivers, to a maximum depth of 3.5 m, although it generally occurred in shallow waters of 0.3–1.0 m. Maximum depth of occurrence is certainly deeper than 3.5 m given the depths that some northern Australian rivers can reach during the wet season (15 m +). Maximum recorded size is 124 cm disc width (DW), but most specimens have been <100 cm DW; an 88 cm DW male was adolescent, but exact sizes at maturity are not known for either sex (Thorburn et al. 2003, Last and Manjaji-Matsumoto 2008, Last and Stevens 2009). Thorburn et al. (2003) noted that several individuals are often encountered together, suggesting some sort of aggregation behaviour or concentration in suitable habitat. Most aspects of the species’ ecology and biology are unknown, including reproductive biology (although it is known to be viviparous). There is a lack of knowledge regarding its full habitat requirements and its residency and/or movement patterns.
There is little concrete information available on threats facing the Freshwater Whipray. Freshwater elasmobranchs are generally of conservation concern given the coupling of the usual biological limitations of the group with the physical constraints of their environment which limits their ability to evade both habitat alteration/degradation and exploitation (Compagno and Cook 1995). Broader-scale catchment practices may also impact upon the species’ habitat through such activities as overgrazing, land clearing and mining. The restricted geographical distribution of the Freshwater Whipray, the fact that it is not particularly common and the presumed limited interchange between rivers increases the species’ inherent vulnerability.
Impoundments and artificial barriers on rivers may have caused reductions in habitat available for the Freshwater Whipray, thus possibly altering the species’ distribution. Although poorly known, migrations and movement patterns have certainly been impacted by these barriers which can limit or completely restrict movements. Thorburn et al. (2004) noted a concentration of Freshwater Sawfish (Pristis microdon) and Bull Shark (Carcharhinus leucas) below Camballin Weir on the Fitzroy River, which acted as an obstacle to upstream movement during the lower water levels of the dry season. Similarly, Freshwater Whiprays are easily observed immediately downstream of Beeboom Crossing on the Daly River, which is impassable to upstream moving elasmobranchs for much of the year (pers. obs.).
During the dry season, when water levels are at their seasonal low, some freshwater elasmobranchs become isolated in disconnected pools, and this may make them more susceptible to capture as they are concentrated in or restricted to smaller areas. Any human-induced reduction in dry season water levels (i.e., through water extraction for agriculture or other purposes) may increase this susceptibility to capture as well as reduce available habitat.
It is not known how fishing activities, both recreational and commercial, have impacted upon this species both historically and presently. Parts of the species’ range are extremely popular recreational fishing locations, such as the Daly and Fitzroy Rivers, and elasmobranchs are a bycatch of recreational fishing for Barramundi (Lates calcarifer) (Thorburn et al. 2003). In some areas of these rivers (for example the Douglas-Daly Region of the Daly River which is close to Darwin and is easily accessible) recreational fishing activity is considerable. Given the general difficulty and danger of handling stingrays, some fishers who do unintentionally hook this species may cut their line to release the animal. Post-release stress and injuries caused by the remaining line and hook may result in subsequent mortality. Thorburn et al. (2003) observed discarded Freshwater Whipray at a site on the Fitzroy River, a result of unwanted catch from recreational fishing (Freshwater Sawfish and Bull Sharks were also found at the site). Discarded elasmobranchs have been observed at other popular fishing spots on other rivers (pers. obs.), and this highlights that mortality (without utilization) is occurring from recreational fishing.
Given the morphology of the species, it is not as susceptible to capture in gillnets (which are used across northern Australia to target commercial species such as Barramundi) as other elasmobranchs such as sawfishes or sharks; it is however susceptible to capture by longline and handline (Thorburn et al. 2003, 2004). The extent of interactions with commercial fisheries targeting Barramundi is largely unknown. Given that the Freshwater Whipray has not been recorded in euhaline marine waters, there are likely no interactions where these fisheries operate in coastal marine waters. These fisheries do operate in some river and rivers mouths, for example the NT Barramundi Fishery can operate within a restricted number of rivers. However, as these fisheries use monofilament gillnets, interactions with this species are likely to be minimal. Field et al. (2008) did not record the species in the NT Barramundi Fishery, but gear considerations aside, they largely sampled outside known rivers of Freshwater Whipray occurrence.
Chin et al. (2010) classified the Freshwater Whipray as one of the most vulnerable chondrichthyans to the impacts of climate change in the Great Barrier Reef and adjacent regions of the Queensland east coast (where the Freshwater Whipray occurs in the Normanby River). This assessment was a result of a high exposure to climate change factors (rising sea levels, rising temperature, changing freshwater input etc.) within its riverine/estuarine habitat; and the species’ high sensitivity and low adaptive capacity given its relative rarity, habitat specificity and restricted range (Chin et al. 2010).
The Freshwater Whipray is poorly known, and a priority is to conduct further field research to ascertain the full extent of its distribution across northern Australia (and verify if it occurs in New Guinea), its habitat use and requirements, its biological parameters, its migration and movement patterns or residency, and the extent to which it is impacted upon by various potentially threatening processes (particularly artificial barriers, fishing activities, land use, and the future potential alteration of flow regimes).
Within the Northern Territory, commercial fishing is prohibited in the Roper (since 1988) and Daly (since 1989) Rivers (Handley 2010). However, this is not a conservation action to aid in the protection of the river’s fauna or habitats, but rather it is to enhance recreational fishing and tourism opportunities. Recreational fishing pressure is high in some parts of the Daly River, particularly during the peak of the Barramundi season. Some other parts of the species’ range are more remote, and this may benefit the species by limiting access for fishing. However, a growing population, improvements in technology, larger recreational boats, greater access to the coast and an increase in fishing tour operators (Handley 2010) means that remote areas are now becoming more accessible.
Areas closed to both commercial and recreational fishing should be considered as a management tool for limiting pressure on the species (Thorburn et al. 2003). Even though the species occurs in some protected areas, such as Kakadu National Park (where it has been documented in the South Alligator River), recreational fishing is still permitted there. Education on the safe release of all freshwater elasmobranchs caught by recreational fishers, like that which has been conducted for sawfish and Glyphis species in parts of northern Australia, should be conducted through community engagement programs.
The Freshwater Whipray shares its habitat (large northern Australian tropical rivers) with other freshwater elasmobranchs (namely Freshwater Sawfish and Speartooth Shark Glyphis glyphis) which are considered to be threatened. These two species are protected where they occur in Australia. Many of the pressures identified for these species are likely to also impact upon the Freshwater Whipray and therefore, consideration should be given to offering this species the same level of protection (i.e. make it a protected or ‘no-take’ species in all jurisdictions where it occurs).
The middle reaches of the Daly River, in which Freshwater Whipray occurs, is considered to be a site of national conservation significance by the Northern Territory Government, recognizing the special biodiversity values of the area (Harrison et al. 2009). This does not translate to actual regulatory or legislative protection of the river, but should theoretically guide management towards the conservation of biodiversity.
Compagno and Cook (2005) considered H. chaophraya to be globally Vulnerable A1bcde+2ce, but commented that its status in Australia is ‘probably favourable’. Pogonoski et al. (2002) stated ‘[Freshwater Whipray (H. dalyensis)] should be assigned the conservation status of Vulnerable in Australian waters. It has been and will continue to be affected by the complex and synergistic effects of the restrictions of its obligate freshwater habitat, fishing pressure and habitat alternation/destruction.’ While considerable declines in H. polylepis (the valid name for what was previously referred to as Southeast Asian H. chaophraya) have occurred throughout its Asian range (Compagno and Cook 2005), there is no data to support a Vulnerable listing in Australia for the Freshwater Whipray. While the inherent vulnerability of the species due to it probable life history characteristics, and the restrictions of its habitat, imply that it is of conservation concern, data are rapidly needed to accurately assess the species’ conservation status. Data may show that it meets criteria for a threatened category, or meets Near Threatened, but it would be remiss to carry the global assessment of Vulnerable for ‘H. chaophraya’ from Compagno and Cook (2005) across to the Australian ‘H. dalyensis’. Information on threats and how these are impacting the species need to be urgently gathered.
Under the Territory Parks and Wildlife Conservation Act 2000, Freshwater Whipray (H. dalyensis) is listed as Data Deficient in the Northern Territory.
|Citation:||Kyne, P.M. 2011. Himantura dalyensis. In: IUCN 2013. IUCN Red List of Threatened Species. Version 2013.2. <www.iucnredlist.org>. Downloaded on 07 December 2013.|
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