Status assessment of the Indus River dolphin, Platanista gangetica minor, March–April 2001

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Abstract

A survey was conducted in March and April 2001, to assess the status of the Indus River dolphin, Platanista gangetica minor, throughout its present range. A total of 1535 km of survey effort was conducted, consisting of 1375 km of the Indus River main channel, 136 km of Indus River secondary channels, and 24 km of the Panjnad River, a tributary of the Indus. The effective range of the Indus dolphin has declined by 80% since 1870. The sum of best group size estimates produced an abundance estimate of 965 dolphins. Extrapolation of encounter rates to un-surveyed channels and application of a correction factor to account for missed dolphins indicates that the metapopulation may number approximately 1200 individuals. Dolphins occur in five subpopulations separated by irrigation barrages. A pronounced increase in dolphin abundance and encounter rate was observed in each subsequent downstream subpopulation (except the last). The three largest subpopulations were between Chashma and Taunsa Barrages (84 dolphins; 0.28/km), Taunsa and Guddu Barrages (259 dolphins; 0.74/km) and Guddu and Sukkur Barrages (602 dolphins; 3.60/km). Reasons suggested for the high encounter rate between Guddu and Sukkur Barrages, include high carrying capacity, low levels of anthropogenic threat, effective conservation, and augmentation of the subpopulation by downstream migration of dolphins from upstream.

Introduction

The Indus River dolphin, Platanista gangetica minor, is endemic to Pakistan and occurs only in the Indus River system. The Indus River has five main tributaries; the Jhelum, Sutlej, Chenab, Ravi and Beas Rivers. These rivers merge with one another to form the Panjnad River, which then joins the Indus mainstem (Fig. 1). The Indus leaves the Himalayan foothills and enters the plains at Kalabagh town; 3 km upstream of Jinnah Barrage. From Kalabagh it flows at a gentle gradient (averaging 13 cm/km), primarily SSW, for approximately 1600 km, to its confluence with the Arabian Sea. It runs through semi-desert and irrigated agricultural land, as well as some small remnant areas of native riverine scrub forest located between Guddu and Sukkur Barrages. Human habitation is sparse but increases with proximity to the delta. The only large towns are Dera Ismail Khan in North Western Frontier Province (NWFP), and Sukkur and Hyderabad in Sindh Province. The river is not used for commercial traffic, and the few vessels present are oar-powered ferries and fishing boats.

Historically, the Indus River dolphin occurred in approximately 3400 km of the Indus River and its five tributaries, from the estuary upstream into the foothills of the Himalayas, where distribution was limited by rocky barriers, high velocities, or shallow water (Anderson, 1879). The Indus River dolphin now occupies approximately one fifth of this former range (Reeves et al., 1991) and was listed as ‘Endangered’ in the 2004 IUCN Red List (Braulik et al., 2004).

The Indus River dolphin population was fragmented by construction of the Indus Basin Irrigation System (IBIS). The irrigation system, claimed to be the largest in the world, consists of 19 barrages, 12 inter-river link canals, and two million kilometers of tertiary watercourses (Hassan et al., 1999). The system has immense political and economic importance as its waters irrigate more than 180,000 km2 of arid and semi-arid land, irrigated agriculture accounts for 90% of Pakistan’s agricultural produce and agricultural goods for approximately 55% of Pakistan’s exports (CIA, 2004, FBSP, 2003, Ahmad, 1993). Barrages are low, gated diversion dams comprised of a series of gates used to control the elevation of an upstream ‘head pond’. The head pond is maintained not to store water, but to aid the diversion of water into lateral canals. Barrages also restrict the movement of river dolphins and other aquatic megafauna, thereby separating them into subpopulations. Reeves et al. (1991) questioned the degree to which dolphin subpopulations are isolated, suggesting that individuals may occasionally move downstream through barrages.

Dolphins in the Indus mainstem were split into two subpopulations, and isolated from those in the five Punjab tributaries, in 1932 when Sukkur and Panjnad Barrages were completed. Completion of Taunsa (1959) and Guddu (1962) Barrages, further fragmented the Indus mainstem population into four subpopulations. Dolphins have now been extirpated from the Indus mainstem upstream of Jinnah Barrage, downstream of Kotri Barrage and from the Indus tributaries. Today they occur in five subpopulations bounded by Jinnah, Chashma, Taunsa, Panjnad, Guddu, Sukkur and Kotri Barrages (Fig. 1).

Indus River discharge is highly seasonal. Peak flows of approximately 20,000–22,500 m3/s occur between June and August when the river is fed by Himalayan melt-water and monsoon run-off, while flows as low as 340 m3/s occur in the dry season between December and April. The system is highly modified and managed, and the natural flow regime has been significantly disrupted. Large-scale diversion of river water for irrigation in the dry season causes discharge to diminish as the river flows towards the Arabian Sea. For part of the dry season the river is de-watered downstream of Kotri Barrage and no water flows through the delta into the Arabian Sea. Consequently, dolphins are no longer found downstream of Kotri barrage.

Following partition of British India in 1947, the Indus Waters Treaty of 1960 allocated water in the Ravi, Beas and Sutlej Rivers to India and the Indus, Chenab and Jhelum Rivers to Pakistan. In the dry season, the three rivers allocated to India enter Pakistan virtually dry and almost all flow from the remaining two tributaries is utilised by Pakistan with the result that the lower reaches of all five Indus tributaries are frequently de-watered. While there were occasional reports of dolphin sightings in the Indus tributaries during the 1980s (Pelletier and Pelletier, 1980, Roberts, 1997) reports have ceased, and it is likely that dolphins have been completely extirpated from these rivers due to insufficient and inconsistent water supplies.

Abundance monitoring of the three largest dolphin subpopulations has been conducted since the early 1970s, using direct visual counts from vessels or point counts from the riverbank. Methods were insufficiently recorded to evaluate bias, estimate precision, or reliably detect trends in abundance from the data. In addition, the Sindh and Punjab wildlife departments used different survey methods, precluding direct comparison of counts between Provinces. All published Indus River dolphin abundance estimates for the Guddu–Sukkur, Taunsa–Guddu and Chashma–Taunsa subpopulations are presented in Table 1. This table is an expansion and update to previous compilations of count data published by Reeves and Chaudhry, 1998, Bhaagat, 1999, Gachal and Slater, 2002. Where several counts were conducted in the same year and month, only the highest count is presented.

The survey data implies that each subsequent downstream subpopulation (except Sukkur–Kotri) is larger than the one above. The data also indicate that, since the 1970s, there has been a steady increase in abundance of the Taunsa–Guddu and Guddu–Sukkur subpopulations. This apparent increase may be due to differences in survey methods, a real increase, or a combination of these.

During the last 30 years, there have been occasional reports, but no consistent monitoring, of small dolphin subpopulations, in the Indus mainstem, at the up and downstream ends of the current known distribution. Between Jinnah and Chashma Barrage, from 1 to 4 dolphins were reported in the 1970s and 1980s (Pilleri and Bhatti, 1978, Pilleri and Pilleri, 1979, Niazi and Azam, 1988) and between Sukkur and Kotri Barrage 21 animals were reported in the mid 1980s (Khan and Niazi, 1989) and 30 in the early 1990s (Gachal and Slater, 2002).

There has been considerable international concern regarding the endangered status of the Indus River dolphin and the IUCN Cetacean Specialist Group has urged for cooperation between provincial wildlife departments and for range-wide surveys using accepted river dolphin survey techniques to comprehensively assess the status of the dolphin subspecies (Reeves and Leatherwood, 1994, Reeves et al., 2003). The survey presented in this paper was designed to address this need.

Section snippets

Methods

The survey was conducted between 12 March and 27 April, 2001, and covered 1375 km of the Indus River from Jinnah Barrage (N32° 55.0; E71° 30.9) to Kotri Barrage (N25° 26.7; E68° 18.7) (Fig. 1). This area was selected for survey as it included the location of all confirmed Indus River dolphin sightings since 1980. Early spring is the optimum time to conduct an abundance survey as Indus discharge is at its annual minimum and dolphins are concentrated into a narrower channel and are therefore

Distribution and abundance

A total of 1534.2 km of survey effort was conducted, consisting of 1374.8 km of the Indus River main channel, 135.6 km of secondary Indus River channels, and 23.7 km of the Panjnad River upstream from its confluence with the Indus. Sighting conditions were generally excellent, with 90% of effort conducted in river surface state 0% or 1% and 100% with clear visibility. All potential dolphin habitat was surveyed, including main and secondary channels, except for two reaches between Guddu and Sukkur

Distribution

The linear extent of occurrence of the Indus River dolphin has declined from approximately 3400 km of the Indus mainstem and its tributaries in the 1870s (see Anderson, 1879) to approximately 1000 km of only the mainstem today. This decline is primarily due to fragmentation of the dolphin population by irrigation barrages combined with habitat degradation caused by large-scale water abstraction from the Indus River system. An estimated 99% of the dolphin population occurs in only 690 km of river,

Acknowledgements

This survey was a collaborative effort between WWF-Pakistan, the Wildlife Departments of NWFP, Punjab and Sindh Provinces, the Zoological Survey Department, the Adventure Foundation – Pakistan and Karavan Leaders. I gratefully acknowledge the effort and stamina of the dolphin survey team who spent many weeks working in remote, uncomfortable and often dangerous situations to collect this data: Nasir Ahmed Mirani, Mohammed Akbar, Babar Hussain, Mir Akhtar Hussain, Abdur Razzaq Khan, Khalil Khan,

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