Platanista gangetica ssp. gangetica
|Scientific Name:||Platanista gangetica ssp. gangetica (Roxburgh, 1801)|
Platanista gangetica (Roxburgh, 1801)
|Red List Category & Criteria:||Endangered A2abcde+3bcde+4abcde ver 3.1|
|Assessor(s):||Smith, B.D., Braulik, G.T. & Sinha, R.|
|Reviewer(s):||Reeves, R. & Taylor, B.L.|
Determining the status of this subspecies is especially problematical because of the lack of rigorous quantitative data (current or historical) on numbers, mortality, extent of occurrence, and area of occupancy. However, the diversity and scale of threats – recent, ongoing, and projected – are such that precautionary reasoning is even more appropriate than is usually the case.
Criterion A. Only very limited data are available on the life history of Platanista sp. (reviewed by Brownell 1984). Age at first reproduction is probably between 6–10 years and maximum longevity may be close to 30. Therefore, generation time is probably well over 10 but possibly less than 20 years, which would mean that three generations equals at least 30 years (i.e., from 1974 counting backwards or until 2034 counting forwards) but less than 60 (i.e., from 1944 counting backwards or until 2064 counting forwards).
Subcriterion A1 does not apply because even if the decline has been greater than 70%, the causes are not clearly reversible, understood, or ceased, all of which would have to be true.
Subcriterion A2 can be applied because a population size reduction of more than 50% since some time between 1944–74 is plausible (note that most of the dam and barrage construction has occurred since the late 1950s), and it is certainly true that the reduction and its causes have not ceased (more barrages are planned and under construction — e.g., Kanpur Barrage on the Ganges mainstem; mortality from hunting and net entanglement continue unabated despite protection laws), are not fully understood, and may not be reversible. The basis could rest on any or all of (a) to (e) under A1.
Subcriterion A3 and subcriterion A4 can also be applied because a population size reduction of more than 50% could plausibly be projected over the next 30–60 years, or inferred, projected, or suspected over a period of 30–60 years including both the past and the future, with the causes uncertain, continuing, and possibly irreversible, again in either instance based on any or all of (a) to (e). Evidence for subcriterion A4c is probably the strongest since a precautionary interpretation of life history data indicates a period of 60 years for three generations, which encompasses the dramatic effects of the Farakka Barrage completed in 1974 (see above), as well as at least 19 other barrages and 17 high dams constructed in the GBM system since 1956, and the projected declines in the area of occupancy, extent of occurrence, and/or quality of habitat that will undoubtedly occur if the Ganges-Brahmaputra inter-link canal and dam project is constructed (scheduled to be completed in 2016). The cumulative effects of these projects indicate a probable population size reduction of more than 50% from 1956–2016.
Criterion B. It has not been possible to estimate the extent of occurrence or area of occupancy for this subspecies because its potential total range is vast but severely fragmented, and there is great uncertainty about the continued occurrence of dolphins in many parts of the potential range. In any event, although it would easily qualify under subcriteria B1b and B2b, it would not qualify under either B1ac or B2ac and therefore would not qualify as EN under this criterion even if its extent of occurrence or area of occupancy were within one or both of the stated thresholds.
Criterion C. The substantial investment made to date in surveys has failed to explicitly account for more than about 1,500 animals (Table 1). Although it is possible that the total population size is less than 2,500 mature individuals, current information from field surveys does not allow for this criterion to be credibly applied to the subspecies. If future population assessments ultimately indicate that the population meets the
Criterion D. The population size is definitely greater than 250 mature individuals, so this criterion does not apply.
Criterion E. No quantitative analysis of extinction probability has been attempted for this subspecies.
It is concluded that the subspecies qualifies as EN under criteria A2abcde+3bcde+4abcde, with available evidence strongest for criterion A4c.
The Karnaphuli-Sangu subpopulation should possibly be listed separately as it may qualify for Critically Endangered status.
|Previously published Red List assessments:|
|Range Description:||Ganges-Brahmaputra-Megna (GBM) and Karnaphuli-Sangu (KS) river systems, from the deltas upstream to where rocky barriers, shallow water, fast currents, dams, or barrages (low, gated, diversion dams) prevent upstream movement. The GBM and KS systems are disjunct and therefore so are their respective dolphin populations, although there may be occasional demographic interaction during the high water season if the freshwater plumes of the two river systems meet. There is likely further population separation within the two systems, some of it "natural" but much more of it secondary caused by the presence of physical barriers constructed within the last 100 years. (see Supplementary Material for a distribution map). |
The Ganges River Dolphin was considered by some researchers to be a distinct species for several decades (1970s–1990s) and was listed as such in the 1996 Red List. Its range is disjunct with that of the other subspecies, the Indus River Dolphin, Platanista gangetica minor, and therefore the two have been assessed and listed, and should be managed, separately.
The map shows where the species may occur. The species has not been recorded for all the states within the hypothetical range as shown on the map. States for which confirmed records of the species exist are included in the list of native range states. States within the hypothetical range but for which no confirmed records exist are included in the Presence Uncertain list.
Native:Bangladesh; India; Nepal
|Range Map:||Click here to open the map viewer and explore range.|
|Population:||Although the aggregate range-wide abundance of Ganges River Dolphins was estimated by Jones (1982) as 4,000–5,000 individuals and more recently by Mohan et al. (1997) as fewer than 2,000, these were only guesses. Population assessments have been based on counts conducted within discrete portions of the vast network of riverine and deltaic habitat occupied by these dolphins. Few rigorous estimates of absolute abundance, with estimates of precision, are available. Available counts and estimates are summarized in Table 1 (in Supplementary Material). |
Highest "densities" of Ganges River dolphins (defined as animals per linear river kilometer) have been observed in the Ganges mainstem between Maniharighat and Buxar (1.5/km) (Sinha, unpublished) - and within this segment particularly in the Vikramshila Gangetic Dolphin Sanctuary (2.8/km) (Smith, unpublished) and just downstream between Kahalgaon and Manihari Ghat (near Katihar) (3.4/km) (Sinha, unpublished) - and the lower Sangu River, Bangladesh (1.4/km) (Smith et al. 2001). A few Ganges River dolphins were still present during the mid 1990s as far downstream in the Hoogly River as Kakdwip (Sinha 1997). In the Sundarbans of Bangladesh a minimum of 134 Ganges River dolphins were counted resulting in a "density" estimate of 0.09 individuals/km, with Irrawaddy Dolphins Orcaella brevirostris replacing Platanista in higher salinity waters of the southern and western portions (Smith, unpublished). No information is available on the status of Ganges River Dolphins in the Indian Sundarbans, except for historical reports of occurrence (Anderson 1879, Jones 1982).
Roughly accounting for duplicate counts or estimates in Table 1 (in Supplementary Material), the total of about 1,200–1,800 animals provides a reasonable lower range for the total metapopulation abundance. However, considering that areas with potentially large numbers of animals have not been accounted for at all (e.g., Indian portion of the Sundarbans Delta) and that at least some of the counts and estimates are known to be negatively biased (e.g., see Smith et al. 2001), the true number could be several times as high.
Numerical Declines: Although no credible time series of abundance estimates are available for most of the subspecies' range, the numbers shown in Table 1 (in Supplementary Material) imply downward trends in a number of upstream tributaries (see also Range Declines below).
Range Declines: The range of the subspecies has declined progressively since the nineteenth century when it was mapped by Anderson (1879). No dolphins have been reported in recent years between the Madhya Ganga Barrage at Bijnor and the Bhimgoda Barrage near Haridwar, at the upstream limit of their historical range in the Ganges (Sinha et al. 2000). This suggests a roughly 100 km decline in their range in the Ganges River since the late 1800s. In recent years, dolphins have not been reported in the Yamuna River above the Chambal River confluence during the dry season because upstream channels have become too shallow and polluted to support dolphins, but the segment may still be occupied during the monsoon (Sinha et al. 2000). Historically, they were found year-round in the Yamuna River approximately 400 km upstream to Delhi (Anderson 1879).
Elsewhere in the Ganges mainstem, four extant subpopulations are isolated by barrages, including Farakka Barrage located approximately at the center of the subspecies' overall range. In the northern Ganges tributaries, of the six subpopulations that were isolated above or between barrages, three have been extirpated (in the Gandak River above the Gandak Barrage and in the Sarda River above the upper and lower Sarda barrages) (Sinha et al. 2000) and one reduced to insignificant numbers (in the Kosi River above the Kosi Barrage) (Smith et al. 1994). Ganges River Dolphins have apparently been extirpated from the Son River (at least during the dry season) based on a survey covering ca. 300 km upstream of the Ganges confluence, above and below the Indrapuri Barrage (Sinha and Sharma 2003).
Occasional reports of dolphins in the reservoir behind Kaptai Dam (built in 1961) of the KS system occurred until the mid-1990s (Ahmed 2000), but recent surveys have found no evidence that the subspecies survives there (Smith et al. 2001). Thus, the dam's construction is likely to have caused a substantial reduction in the subspecies’ range in southeastern Bangladesh, but in the absence of any historical information on occurrence in the upper Karnaphuli no quantitative estimate of range reduction is possible.
No surveys have been conducted in the Damodar River system but a single dolphin was rescued after becoming stranded in a deep pool after flow was diverted during the dry season by an upstream barrage. The downstream effects of at least ten dams and barrages constructed in its mainstem and tributaries has probably severely reduced and fragmented dolphin habitat (Smith et al. 2000).
|Current Population Trend:||Decreasing|
|Habitat and Ecology:||Ganges River Dolphins are generally concentrated in counter-current pools below channel convergences and sharp meanders (Kasuya and Haque 1972, Smith 1993, Smith et al. 1998) and above and below mid-channel islands, bridge pilings, and other engineering structures that cause scouring (Smith, unpublished data). Their fidelity to counter-current pools is probably greatest in fast-flowing channels (Smithet al. 1998). Annual monsoon-driven floods cause great variability in the dolphins’ access to large parts of their range. Isolation in seasonal lakes sometimes occurs (especially in the Brahmaputra basin), as does "escapement" from the river channels into artificial water bodies such as canals and reservoirs. Deltaic (brackish) waters are a major component of the total range, but Ganges River Dolphins are not generally known to occur in salinities greater than 10ppt, although they have been recorded in waters as saline as 23ppt (Smith and Braulik, unpublished data).|
|Movement patterns:||Full Migrant|
Water Development Projects
Construction of at least 50 dams and dams within the known or suspected historical range of the subspecies (Smith et al. 2000) has dramatically affected its habitat, abundance, and population structure. The subspecies exists as a metapopulation, with numerous subpopulations isolated to varying degrees by mostly manmade but also natural barriers, as outlined in the preceding section.
In addition to fragmenting dolphin populations, dams and barrages degrade downstream habitat and create reservoirs (known as head ponds (or pondage in India) in the case of barrages) with high sedimentation and altered assemblages of fish and invertebrate species. For example, luxuriant growth of macrophytes and excessive siltation have eliminated suitable habitat immediately above Farakka Barrage (Sinha 2000). Moreover, the insufficiency of water released downstream of this barrage has eliminated dry-season habitat for more than 300 km, or until the Ganges (Padma)-Brahmaputra confluence (Smith et al. 1998). It has also allowed salt water to intrude an additional 160 km into the Sundarbans Delta (Rahman 1986), further decreasing the amount of suitable habitat for this obligate freshwater dolphin (Reeves et al. 1993).
A high dam has been planned for some time just upstream of the dolphins' current (or at least recent) range in the Karnali River, Nepal. If built, this structure would almost certainly eliminate the small amount of dolphin habitat in Nepal’s last river with a potentially viable dolphin population (Smith and Reeves 2000). Disturbance and environmental degradation associated with geotechnical feasibility studies and bridge and road construction for the dam already may have contributed to a decline in the number and range of dolphins above the Nepal-India border (Smith 1993, Smithet al. 1994). Another high dam has been proposed for the Surma River in Cachar, India, which would certainly affect dolphins downstream in the Kalni-Kushiyara distributary (Smith et al. 1994).
Since the 1980s, momentum has been growing within India to proceed with large-scale inter-basin water transfer projects, which will involve additional dam construction and diversion of water from rivers inhabited by dolphins. Although no final decision has been taken to proceed with construction, feasibility studies are to be completed in December 2005 and detailed project reports in 2006. It was anticipated in 2004 that, if built, the entire project would be finished by 2016. During the May 2004 national elections in India all political parties supported the construction of inter-basin water transfer projects and promised to accelerate the construction process. Several key categories of potential threat are: (a) further fragmentation of the dolphin metapopulation, (b) reduction or elimination of habitat simply in terms of dry-season flow, (c) "escapement" of dolphins into canals where they are unlikely to be able to get back into rivers and are therefore doomed, (d) cascading effects from interrupted migrations of prey organisms, degradation of prey spawning habitat etc., (e) contaminant flux leading to significant changes in chronic and/or acute exposure to toxins, (f) loss of complexity (channelization, sediment entrapment upstream of dams, etc.) making the rivers less habitable for dolphins, etc, and (g) downstream effects on the ecology of the delta (e.g., saline encroachment, loss of sediment).
Embankments cause sediments to be deposited in the riverbed instead of on the floodplain, thereby eliminating or reducing the extent of the eddy-counter currents where dolphins are generally found. (Smith et al. 1998). They also restrict access to floodplain habitat critical to the reproduction and growth of riverine fish species (Boyce 1990). Approximately 3,500 km of embankments have been constructed in the Ganges mainstem and Gandak, Buri Gandak, Bagmati, Kamala, Yamuna, and Son tributaries (Mishra 1999). Dolphins were apparently extirpated from at least 35 km of the Punpun tributary of the Ganges after embankments were constructed in 1975 (Sinha et al. 2000). Although plans for constructing an extensive system of embankments in the rivers of Bangladesh under the Flood Action Plan (FAP) coordinated by the World Bank (see World Bank 1990) have been drastically scaled-down, several projects are currently planned or being constructed that will have adverse effects on dolphin habitat. These include the Bank Protection and River Training Project (FAP 21/22), Brahmaputra River Bank Priority Works, and Jamalpur Priority Project (FAP 3.1) (Smith et al. 1998). Environmental assessments of these projects have not considered river dolphins, nor have they acknowledged the cumulative impacts of planned embankments, and others built before the FAP, on the fish and crustacean species eaten by river dolphins.
Other sources of habitat degradation in the GBM system include dredging (Smith et al. 1998) and the removal of stones (Shrestha 1989), sand (Mohanet al. 1998), and woody debris (Smith 1993). These activities compromise the ecological integrity of the riverine environments, especially small tributaries where suitable habitat is limited and disproportionately vulnerable to local disturbance. Dolphin habitat is also threatened by water abstraction from surface pumps and tube wells, especially in the Ganges where the mean dry-season water depth has declined dramatically in recent years (Sinha, unpublished). Although the long-term implications of reduced dry-season flows in the Ganges are catastrophic, both for the survival of river dolphins and a major portion of the world’s human population that inhabits the Ganges basin, the cumulative effects of reduced water supplies have received little attention. Meanwhile, new projects to divert dry-season flow, such as Kanpur Barrage in the upper Ganges, continue to be constructed (Smith et al. 2000).
Organochlorine and butyltin concentrations in samples from the tissues of Ganges dolphins were high enough to cause concern about effects (Kannan et al. 1993, 1994, 1997; Senthilkumar et al. 1999). Pollutant loads can be expected to increase with industrialization and the spread of intensive agricultural practices facilitated by water diversion. River dolphins may be particularly vulnerable to industrial pollution because their habitat in counter-current pools downstream of confluences and sharp meanders often places them in close proximity to point sources in major urban areas (e.g., Allahabad, Varanasi, Patna, Calcutta, and Dhaka). Furthermore, the capacity of rivers to dilute pollutants (e.g., arsenic, DDT) and salts has been drastically reduced in many areas because of upstream water abstraction, diversion, and impoundment. Again, this problem is bound to worsen as more development takes place.
Deliberate killing of river dolphins is believed to have declined in most areas but still occurs at least occasionally in the middle Ganges near Patna, India (Smith and Reeves 2000, Sinha 2002), in the Kalni-Kushiyara River of Bangladesh (Smith et al. 1998), and in the upper reaches of the Brahmaputra River in Assam, India (Mohan et al. 1997). Dolphins are killed by tribal people in the upper Brahmaputra for their meat and by fishermen in the middle reaches of the Ganges for their oil, which is used as a fish attractant.
Mortality in Fishing Gear
Mortality in fishing gear, especially gillnets, is a severe problem for Ganges River dolphins throughout most of their range (Mohan 1995, Smith and Reeves 2000). They are particularly vulnerable because their preferred habitat is often in the same location as the fishing grounds. In the middle Ganges, although harpooning is now "rare", mortality in fishing nets remains "widespread" (Sinha 2002). A specific problem is that, because dolphin oil is highly valued as a fish attractant, fishermen have a strong incentive to kill any animals found alive in their nets and even to set their nets strategically in the hope of capturing dolphins (described by Sinha 2002 as "assisted incidental capture").
Meaningful quantitative data on the magnitude of catches, either deliberate or incidental, are unavailable and unlikely to become available in the absence of a well-organized, adequately funded, and incorruptible fishery/wildlife management system.
|Conservation Actions:||Ganges River dolphins are legally protected from hunting in all range states. The Vikramshila Gangetic Dolphin Sanctuary, Bihar, India, between Sultanganj and Kahalgaon in the mainstem of the Ganges River was designated as a protected area for dolphins in August 1991 but there is little government support to enforce protective measures. The legal protection in India has been described as "completely ineffective" (Sinha 2002), however a small measure of progress was the convening of the Regional Seminar on Environmental Laws in the Vikramshila Gangetic Dolphin Sanctuary, Bhagalpur, India in November 2003 (Anon 2002). Also, recent proceedings of the Patna High Court (C.J.W.C. No. 5628 of 2001) directed the state and federals governments to allocate funds for supporting efforts to conserve and monitor dolphins in the Ganges. In a few smaller tributaries, dolphins receive nominal protection by virtue of the fact that small portions of their habitat are within or adjacent to national parks and sanctuaries (e.g., Kaziranga National Park in Assam, India, National Chambal Sanctuary in Madhya Pradesh, India, and Royal Bardia National Park and Katerniya Ghat Gharial Sanctuary, respectively north and south of the Nepal-India border. Although field trials have shown that shark or fish oils would be efficient substitutes for dolphin oil as a fish attractant and some fishermen in the middle Ganges are now using oil made from fish scraps as an alternative, most apparently continue to use dolphin oil by preference or because suitable alternatives are not widely available in either the Ganges or Brahmaputra systems (Mohan and Kunhi 1996, Smith et al. 1998, Bairagi 1999, Sinha 2002).|
|Citation:||Smith, B.D., Braulik, G.T. & Sinha, R. 2012. Platanista gangetica ssp. gangetica. The IUCN Red List of Threatened Species 2012: e.T41756A17627639.Downloaded on 20 August 2018.|
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