Robust abundance estimate for endangered river dolphin subspecies in South Asia

Abstract

Robust estimates of absolute abundance are vital for management of threatened species, but these have rarely been generated for endangered South Asian river dolphins due to methodological challenges. An estimate of abundance for the Indus River dolphin in 2006 was generated by conducting tandem vessel-based direct counts; conditional likelihood capture-recapture models were then used to correct for missed animals. Group size and sighting conditions were included as covariates, and abundances of the 3 largest subpopulations were estimated as 101 (coefficient of variation, CV = 44.1%) between Chashma and Taunsa barrages, 52 (CV = 14.9%) between Taunsa barrage and Ghazi Ghat, and 1289 (CV = 33.4%) between Guddu and Sukkur barrages. A total of 75.3% of groups were seen by both independent survey teams, and single animals were almost 5 times more likely to be missed than groups of 3 or more. Providing groups can be matched with minimal error, this survey method shows good potential for abundance estimation of dolphins in confined habitat and the shallow rivers of South Asia. Dolphin encounter rates within the Guddu-Sukkur subpopulation (10.35 km −1 ) are the highest reported for any river dolphin. Direct counts conducted over a 35 yr period, suggest that this subpopulation may have been increasing in abundance, probably due to the cessation of hunting and possible immigration from other subpopulations. The future of South Asian river dolphins is intimately tied to water security in the region, and escalating and competing demands for freshwater mean that the long-term future of South Asia's river dolphins is uncertain.

Ganges River Dolphin: An Overview of Biology, Ecology, and Conservation Status in India

“…This method has substantial limitations for the survey of Platanista spp. ; however, because (1) these animals are extremely difficult to photograph, as they surface alone, AMBIO 2014AMBIO , 43:1029AMBIO -1046unpredictably, for about one second or less, and they seldom approach boats or vessels; and (2) they lack a prominent dorsal fin, and the individuals rarely possess any readily identifying marks or features (Braulik et al 2012a). In an earlier survey, not a single individual could be identified from 1,200 photographs of Ganges dolphins taken during that time (Smith and Reeves 2000).…”

“…Generalized additive models showed that the dolphin distribution was dependent on water with low salinity, high turbidity, and moderate depth during both low and high flows, with a preference for wide sinuous channels with at least two small confluences or one large confluence in the tidal zone in the Sundarbans. (3) A study by Braulik et al (2012a) found that dolphins selected locations in the Indus River with significantly greater mean depth, crosssectional area, and hydraulic radius, and significantly narrower river width and a lower degree of braiding. Dolphins with higher frequency at river constrictions and river confluences were also recorded.…”

Habitat use by a freshwater dolphin in the low‐water season

“…Two complementary methods were used to investigate how Indus dolphins use low‐water season habitat. The first examined dolphin distribution with empirically collected data on cross‐sectional geometry in the Indus River main channel (hereafter termed the ‘channel geometry’ analysis) and used data on dolphin sightings collected from a concurrent vessel‐based visual dolphin survey conducted in 2006 (Braulik et al ., ). The second analysis investigated dolphin distribution with respect to remotely‐sensed hydrogeomorphic characteristics of the Indus River (hereafter termed the ‘river geomorphology’ study) and used data on dolphin sightings collected from a vessel‐based visual dolphin survey conducted in spring 2001 (Braulik, ).…”

“…Data on channel geometry and dolphin occurrence were collected in spring 2006 from all five dolphin sub‐populations, with complete spatial coverage of sub‐populations located between Jinnah–Chashma, Chashma–Taunsa, and Guddu–Sukkur barrages and partial coverage of the remaining two river sections (Figure ) (Braulik et al ., ). A hydrological survey vessel followed approximately 2 km behind the dolphin survey, and periodically collected channel depth along cross‐sections.…”

“…Depleted flow and fragmentation of habitat by barrages have led to an 80% reduction in the range of the Indus dolphin since the 1870s (Reeves and Chaudhry, ; Braulik et al ., ) and are likely to be the greatest threats to the long‐term survival of this sub‐species (International Whaling Commission (IWC), ). The three largest dolphin sub‐populations are located between Guddu and Sukkur barrages (6.23 dolphins per linear km in 2006), Taunsa and Guddu barrages (0.74 dolphins per linear km in 2001), and Chashma and Taunsa barrages (0.27 dolphins per linear km in 2006) (Braulik, ; Braulik et al ., ). Indus dolphins are patchily distributed, especially outside of the high‐density Guddu–Sukkur area, but there is no quantitative information on what aspects of river habitat are important to them.…”