J. Mar. Biol. Ass. India, 54 (2), July-December 2012 Behaviour of Indo-Pacific humpback dolphin, Sousa chinensis (Osbeck) in the Ashtamudi estuary, southwest coast of India Original Article A. Bijukumar* and R. Smrithy Department of Aquatic Biology and Fisheries, University of Kerala, Thiruvananthapuram, Kerala 695581, India. *Correspondence e-mail: abiju@rediffmail.com Received 7 Sep 2011, Accepted: 25 Jul 2012, Published: 8 Oct 2012 Abstract This paper documents the behaviour of Indo-Pacific humpback dolphin, Sousa chinensis (Osbeck) found in the Ashtamudi estuary situated along the southwest coast of India. The major foraging strategy of the dolphins observed was beach hunting. The feeding-related behaviours noted were sudden bout towards the prey with shuttling or speedy zigzagging backwards and forwards and fish-stunning by tossing them into the air. The surface feeding was primarily on mullets. The dolphins displayed a stereotyped surfacing–breathing pattern, with the rostrum rising steeply above the water before the forehead breaks the surface. While travelling, they move in the same direction and the group members dive and surface synchronously for a longer time. The resting activity was primarily floatation in water. As part of socializing behaviour, the humpback dolphins exhibited vertical leaps, side leaps, quasi leaps and somersaults. The principal threat to dolphins in the Ashtamudi estuary is the heavy traffic of trawlers and other motorised vessels. Keywords: Humpback dolphin, group size, ethology, feeding, social life, resting behaviour. Introduction The Indo-Pacific humpback dolphin, Sousa chinensis (Osbeck), is widely distributed in the shallow coastal and inshore waters of the Indian and western Pacific oceans (Jefferson and Available online at: www.mbai.org.in doi: 10.6024/jmbai.2012.54.2.01699-0x Karczmarskil, 2001). In the coastal waters of India, it is one of the commonly recorded species (Sathasivam, 2004; Sutaria and Jefferson, 2004; Afsal et al. 2008; Jayasankar and Anoop, 2010). Just as other cetaceans, they occupy the summit of the marine trophic webs with very low reproductive rate, and are thus particularly vulnerable to threats from human activities (Chen et al., 2011). This species, typically with a near-shore distribution throughout its range, often enter estuaries and sometimes rivers (Ross, 2002), is listed as ‘Near threatened’ by the IUCN in the Red Data Book, besides being listed in Appendix I of the CITES and in the Appendix I of the CMS (Reeves et al., 2008). The behaviour and ecology of S. chinensis has not been adequately studied throughout its range and therfore little is known about the ethology of this species inhabiting the coastal waters of India. Reports on the behaviour of S. chinensis are available from the coasts of Australia (Corkeron, 1990; Ross, 2006), South Africa (Karczmarski et al., 1997, 2000; Karczmarski and Cockcroft, 1999), Hong Kong (Parsons, 1998a; Jefferson, 2000) and China (Chen et al., 2011). Lal Mohan (1983) documented the death of S. chinensis kept in captivity in India. Parson (1998b) observed the presence of S. chinensis in Goa, recorded a few behavioural patterns of the species and stressed the need for ethological studies in Indian waters. Knowledge on the