Sightings and behavioral observations of Indo-Pacific Humpback Dolphins Sousa
chinensis (Osbeck, 1765) along Chennai coast, Bay
of Bengal
Rahul Muralidharan
E4/253, 5thStreet, D.S. Nagar, Vandalur, Chennai, Tamil Nadu
600048, India
rahulmuralidharan29@gmail.com
Abstract: Boat-based surveys were used to
investigate the presence of Indo-Pacific Humpback Dolphins Sousa chinensis along the coast of Chennai, Tamil Nadu,
India. Notes were collected on behavior, group size, coloration patterns and group
composition on sighting cetaceans during the surveys. Four groups of Indo-Pacific Humpback
Dolphins were sighted near-shore in the month of February 2011, between
10–25 m depth with an average group size of 20
individuals of which 10 individuals were photo-identifiable. Dominant group behaviorwas aerial display, feeding and traveling. This study gives a basic idea of presence, threats and habitat use of
Humpback Dolphin areas along Chennai coast.
Keywords: Bay of Bengal, behavior,
Chennai coast, habitat use, photo identification, sightings, Sousa chinensis, Tursiops sp.
doi: http://dx.doi.org/10.11609/JoTT.o3454.5002-6 | ZooBank:urn:lsid:zoobank.org:pub:24BC1AD1-48DC-4469-A97D-BB6527D6DA52
Editor: Kumaran Sathasivam, Marine Mammal Conservation Network of India. Date of publication:26 November 2013 (online & print)
Manuscript details: Ms #
o3454 | Received 23 December 2012 | Final received 20 October 2013 | Finally
accepted 24 October 2013
Citation: Muralidharan,
R. (2013). Sightings and behavioralobservations of Indo-Pacific Humpback Dolphins Sousa chinensis(Osbeck, 1765) along Chennai coast, Bay of Bengal. Journal
of Threatened Taxa 5(15): 5002–5006; http://dx.doi.org/10.11609/JoTT.o3454.5002-6
Copyright: © Muralidharan 2013. Creative Commons
Attribution 3.0 Unported License. JoTT allows unrestricted use of this article in any medium,
reproduction and distribution by providing adequate credit to the authors and
the source of publication.
Funding: TREE Foundation, Chennai, India.
Competing Interest: None.
Acknowledgements: I would like to
thank TREE Foundation for funding this study and my supervisor Dr. Thirunavukkarasu for
supporting me through the study period. I would also like to thank Mr. R.P. Kumarran and Mr. Nithyanandan Manickam for their encouragement and initial assistance.
The surveys would not have been possible without the assistance of my friend Pugalarasu, who took me into the sea and brought me back
safe to the shore. Thanks to Abdul Saeef for his
excellent photos of dolphins during the survey, Sudharshan,Siddharth and Kaushik for
assisting me with the software. I thank Dipani Sutaria for helping me write this note and the anonymous
reviewers for their critical comments.
For figures, images, tables -- click here
Indo-Pacific Humpback Dolphins Sousa chinensis have been recorded all along the Indian
coast, from vessel-based and shore-based sightings, stranding records,
incidental catch data, and museum specimens (Parsons 1998; Kumarran2002; Parsons 2004; Sathasivam 2004; Sutaria & Jefferson 2004; Afsalet al. 2008; Anoop et al. 2008; Reeves et al. 2008; Yousuf et al. 2008; Muralidharan2011; Kumaran 2012; Vivekanandan& Jeyabaskaran 2012). S. chinensis is placed under ‘Near Threatened’ (NT)
category on the IUCN Red List (Reeves et al. 2008), but very few ecological
studies on cetaceans have been carried out in India. Sutaria &
Jefferson (2004) studied the density and distribution of humpback dolphins
along the Gujarat and Goa coasts of western India and provided records of strandings, sightings and incidental catches from
India. The species has also been
recorded from the coastal waters of Maharashtra (K. Jog & M. Sule pers. comm. February 2012), from Kerala (Bijukumar et al. 2012; D. Panickerpers. comm. February 2012), from Karnataka (Sutariapers. comm. February 2012) on the western coast, and Orissa (Sutaria 2009), Andhra Pradesh (Tripathy& Choudhury 2004) and Tamil Nadu (S. Dharini pers. comm. February 2011) on the eastern
coast. Being one of the four
predominant delphinids and most recorded species in
incidental catches in fishing gears in India (Afsalet al. 2008), research on humpback dolphins is
urgently needed (Sutaria & Jefferson 2004). In this study, we surveyed the Chennai
coast for presence and behavior of S. chinensis, in relation to depth, distance from the
shore and time of the day.
Materials and Methods
Study
area: Chennai
city is located in the state of Tamil Nadu, which lies on the southeastern Coromandel Coast adjoining the Bay of Bengal
(Image 1) of India. Approximately
300km2 area between Adyar Estuary and Muttukadu Estuary was surveyed during this study (Image
1). The mouth to the estuaries
where brackish water empties into the sea is considered the preferred habitat
for S. chinensis (Jefferson & Karczmarski 2001). The coast is surf beaten and interspersed with rocky reefs. The mean
spring and neap tides are 1.21m and 0.61m, respectively.
Field
Methods: Surveys
were carried out in the morning between 07:00 and 11:00 hr(Table 1). The survey tracks were
perpendicular to the shore and were placed between AdyarEstuary in the north and Muttukadu backwaters to the
south (Images 1 & 2). All the
surveys originated from and ended at Neelangarai. Surveys were carried out using 10m long
FRP (Fiber-Reinforced Plastic) boat with 12hp
outboard engine. The average speed
of the boat was maintained at 10km/hr. Three observers, one at the bow and two on either
sides of the boat-searched for dolphins with naked eyes. When dolphins were sighted, the survey
boat approached the group at a slower speed. Parameters such as depth, time, beaufort scale (BS), GPS
coordinates, distance from shore, group size and associated species were
noted. Bathymetry profile data for 25m depth mark along Chennai coast were ascertained with GPS
coordinates (Institute of Ocean Management, Anna University). We used Nikon D70 digital single-lens
reflex camera 6.1 megapixel with 55–200 mm Nikkorlens and Canon Power Shot SX20 IS Digital Camera, with 12.1 Megapixels 20x
Wide-Angle Zoom Lens (28–560 mm) to photograph dorsal fins of individuals
whenever possible.
Results and Discussion
The survey consisted of a total of 12
hours and 120km of survey effort. The sea state was calm during early morning hours of the survey and
changed gradually during the end of the survey period.
Four groups of humpback dolphins and two
groups of Bottlenose Dolphins (Tursiops sp.)
were sighted. All groups were
sighted between 07:00am and 11:30am. Indo-pacific humpback dolphins were identified based on the coloration
pattern and hump under the dorsal fin (Jefferson et al. 1993). Bottlenose dolphins were identified
using body shape, triangular fin, shape and size of their snout, shape of
rostrum, and relative length of the dolphin to the boat (Jefferson et al.
1993).
The observed bottlenose dolphins may be
the Indo-Pacific Bottlenose Dolphin Tursiops aduncus, a distinct morphotypeof Tursiops truncatus,which is an off-shore form. T. aduncus have been recorded before, during a
near-shore research survey in the Bay of Bengal (Afsalet al. 2008) but further studies are required to ascertain the Tursiops species sighted in our surveys.
The estimated size of the S.chinensis groups ranged between 10–30
individuals with an average of 20 individuals in a group. A “group” here is defined as an
aggregation of more than one dolphin, including all age classes or dolphin
individuals within 20m of each other, visible from the survey boat (Karczmarski 1999; Sutaria &
Jefferson 2004). S. chinensis are usually found in
group sizes of 25 and less, but groups of less than 10 are most common (Jefferson & Karczmarski2001). All S.chinensissightings were at depths less than 25m with maximum and minimum depth recorded
at 25m and 10m, respectively (Table 1). S. chinensisgroups were sighted around 100–200 m from the shore, however, mixed
groups of S. chinensis and Tursiops sp. were sighted nearly 5km from the shore. S. chinensis have been frequently sighted in the study area in the years 2012 and 2013,
after this study was completed.
Photographs with dorsal fins were analyzed based on picture quality (Excellent, Good,
Medium). Of the totally usable
photographs only 14% (good and excellent pictures) were used to identify
individual fins. We
photo-identified 10 individuals from 70 photographs (Images 3 & 4). We used DARWIN software, Eckerd College, to produce a database for photo-identification
based studies. We hope to build
this database in the future to estimate population size and study movement
patterns.
Behavior observations
On two occasions, mixed groups of S. chinensis and Tursiopssp. were encountered. On 17
February 2011, 09:35–09:50 hr, S. chinensis and Tursiops sp. were observed milling, i.e., swimming in a directionless fashion, but
in the same location, off shore of Injambakkam.
Again, the same mixed group was observed milling, diving randomly and circling
in the same location, from 10:00–10:10 hr. Such milling behavioris seen during a foraging state in most cetaceans (Perrin et al. 2009). Short-term associations between humpback
dolphins and bottlenose dolphins are known to occur elsewhere, in feeding areas
that overlap or during foraging events (Karczmarskiet al. 1997; Sayaman & Tayler1979). This kind of behavioral association between the two species has also been
well documented in South Africa and Australia (Jefferson & Karczmarski 2001).
On 21 February 2011, at 11:10 hr, S. chinensis and Tursiops sp. were observed traveling
perpendicular to the coast into deeper waters, near Injambakkam. However, the survey boat could not
follow the group due to unfavorable sea
conditions. We hypothesize that S.chinensis might swim farther offshore till the
20m bathymetry line, or alternatively upriver due to tidal changes as these are
predicted to have influence on distribution of S. chinensis(Parsons 1998; Sutaria & Jefferson 2004).
A group of humpback dolphins, ranging from
10–35 individuals, predominantly traveling with occasional aerial
displays (Image 4), were followed from Neelangarai to
Marina Beach covering a parallel shore distance of 7km and a total of 02:20 hr, on 22 February 2011. Interestingly, two calves of 1m length were noticed travelling with the adult dolphins. Calves are defined as animals two-thirds
or less the length of an adult, regularly accompanying a larger animal,
presumed to be the mother (Karczmarski 1999). Calves are usually more active in aerial
displays and we hypothesize this large group, to be two ‘mother-calf’ groups
meeting and socializing. Saayman & Tayler (1979)
interpreted aerial behavior as greeting displays when
different groups of humpback dolphins meet.
S. chinensis and near-shore Tursiops sp. are
susceptible to coastal gillnet fisheries (Jayasankaranet al. 2009). In our study area,
both species have been reported as incidental catch by fishermen (S. Dharini pers. comm. February 2011). During this study, we observed the
interactions between the dolphin groups and fisheries, in cases where both were
present at a sighting. Most of S.chinensis sightings were observed in concurrence
with shore seine fishing activity along the Chennai coast but direct feeding
was not observed. It is probable
that S. chinensis follow schools of fish to
the shore, which the shore seiners also target. We also noticed that S. chinensis groups carefully avoided shore seine nets by swimming
around them. Fishermen in the study
area perceive that the presence of S. chinensisin their fishing grounds has reduced over the years. However, the exact reasons are unknown and
studies are required to ascertain the conservation status of coastal cetaceans
along the Chennai coast.
Conclusion
Due to the limited survey effort in our
study area, we cannot ascertain if humpback dolphins are resident along the
Chennai coast or are local migrants from close-by estuaries, but we hypothesize
the latter based on scattered reports from fishers and from our
observations. The Injambakkam area seems to be an important foraging habitat
that overlaps for both S. chinensis and Tursiops sp. and is important for such mixed
species groups. An increase in the
number of records of S. chinensis off Chennai,
in the months of Jan–Mar and May–Aug, suggests an overlap with an
increase in near-shore prey. But,
fishery interaction is recognized to be a primary threat to humpback dolphin
population and needs to be assessed immediately to conserve the species (Mohan
1994; Kumarran 2002; Sutaria& Jefferson 2004; Yousuf et al. 2008; Kumarran 2012). Long term seasonal observation and further investigation will yield
critical information to understand the species and their range in detail, to
mitigate threats.
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