Biology Reference
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percentage of sightings by the primary observer was 348 ± SD 65.26 whereas by the
secondary observer was 95 ± SD 10.96. Thus the mean percentage of sightings missed by the
primary observers but detected by the secondary observers was 27.3%. The percentage of
observed neonates was relatively high during the surveys conducted in May-June 2006 (15.74
and 15.49% for upstream and downstream surveys, respectively) and low for those conducted
in March 2007 (7.08 and 7.68% for upstream and downstream surveys, respectively).The
difference in the proportion of neonates observed during May-June 2006 versus March 2007
may be due to the fact that maximum breeding takes place during April-May in the Ganges.
Choudhary et al. (2006) recorded 21% and 18.4% calves in May and December 2001,
respectively in the Ganges and opined that the difference in the proportion of neonates may
be accounted for by the apparent preference of cow-calf pairs to congregate in large counter-
current pools and the fact that the availability of these features varies in different river
segments from year to year.
The Ganges River dolphins were sighted all along the stretch of the Ganges under study,
however, the dolphin population showed spatial and temporal variation. The climatic and
weather conditions, viz. choppy water surface due to high wind, glaring due to sunlight, foggy
weather, rain etc also had impacts on sighting records of the dolphins. Usually more dolphins
were sighted near confluences, meanderings, behind sand bars, and in the river stretches
having deep pools, shallow waters and sand bars.
Our dolphin encounter rates in the Ganges compare favorably to other rivers and in the
Ganges where the species has been surveyed using similar techniques. For example, the
encounter rate for upstream and downstream surveys in 65 km of the Vikramshila Gangetic
Dolphin sanctuary between Sultanganj and Kahalgaon in Bihar within the Ganges was 1.8
dolphin km-1 and 1.2 dolphins km-1, respectively during 2001-2003 (Choudhary et al.,
2006). This stretch falls within our area of survey. Similarly, the encounter rate for a
downstream survey in the middle Brahamaputra River between Guwahati and Goalpara was
0.03 dolphin km
-1
during April 1999, and 0.76 dolphins km
-1
for downstream surveys in the
Karnaphuli-Sangu river of Bangladesh from January to April 1999 (Smith et al., 2001). 197
dolphins were recorded in the entire 856 km (i.e. 0.23 dolphin km
-1
) of the River Brahmaputra
stretch from Assam-Arunachal Pradesh border to the India-Bangladesh border (Wakid, 2009).
Overall the pattern of dolphin occurrence was consistent with a preference for reaches
characterized by complex morphological features that induce hydraulic heterogeneity and
bottom scouring (Smith, 1993; Smith et al., 1998, 2001).
In areas with large seasonal fluctuations in water level, dolphins are most abundant in the
long stretches of deep water that remain during the dry season (Singh & Sharma, 1985).
Occasionally susu enter shallow water (2 m) while chasing prey, but they remain primarily in,
or near (with ready access to) deeper river channels (Reeves et al., 1993). Habitat preferences
of the susu have been studied in the River Karnali in Nepal by Smith (1993). Two categories
of habitats were identified: primary habitats characterized by an eddy-counter system in the
main river flow caused by a point bar formed from sediment deposits of a convergent stream
branch or a tributary, and marginal habitats having a smaller eddy counter-current system
caused by an upstream meander. The stretch of the Ganges under study has very low gradient
(1:13000) and receives the three major tributaries, the Ghaghara (Mahakali+ Karnali rivers in
Nepal), Gandak (Narayani in Nepal), and Kosi (saptakosi in Nepal) originating in the
Himalayas and smaller tributaries like the Sone and the Punpun from the south, originating in
central India. The primary habitats were identified at the confluence of the tributaries. Bottom
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