Behavioral observations of coastal Irrawaddy dolphins (Orcaella brevirostris) in Trat Province, Eastern Gulf of Thailand

ABSTRACT

The Irrawaddy dolphin (Orcaella brevirostris) is found in Southeast Asia in nearshore coastal, estuarine habitats, in three freshwater riverine habitats of the Mahakam, Mekong, and Ayeyarwady Rivers, and in brackish water lakes in Chilika, India, and Songkhla, Thailand (Reeves et al., 2008). Throughout its range, the Irrawaddy dolphin has been studied primarily in riverine and brackish habitats such as the Mekong Smith et al., 2004). Coastal populations have been studied to a lesser extent, mainly in areas such as Australia (Parra et al.,. Most research efforts have been focused on estimating population sizes for conservation efforts as this species is listed as Vulnerable on the International Union for Conservation of Nature’s (IUCN) Red List of Threatened Species and thought to be undergoing decline (Reeves et al., 2008); however, certain subpopulations such as the ones in the Songkhla Lake (Smith & Beasley, 2004) and the Mekong River are listed as Critically Endangered (Smith & Beasley, 2004; Beasley et al., 2013). Very few detailed behavioral studies on this species are available in the literature on the whole (Stacey & Hvenegaard, 2002; Kreb, 2004b; Kreb & Rahadi, 2004; Beasley, 2007; Parra et al., 2011) or are unpublished, leaving much to be explored. In this short note, we describe Irrawaddy dolphin behaviors that were observed during boat and aerial surveys in the eastern Gulf of Thailand. Documented behaviors included probable foraging for cephalopods (as suggested by squid ink and loose tentacles floating at the surface), and herding and tactile social actions with animals layered vertically in the water column, suggesting mating in large aggregations (Table 1). Trat Province in the eastern Gulf of Thailand is the easternmost province of Thailand, approximately 315 km east of Bangkok, with an open stretch of coastline extending to the Cambodian border. The area consists of a very shallow bay (< 5 m depth) to the northwest and several estuaries along the southeast-oriented coast (Figure 1). The coast has a gently sloping shallow bathymetry with an average depth of 7 m within 3 km of shore. It was chosen as the site for our research on Irrawaddy dolphins after aerial surveys in 2003 to 2005 for dugongs (Dugong dugon) conducted by Hines et al. (2003, 2004, 2005) yielded numerous incidental sightings of these dolphins, including herding behavior during a dugong survey in 2004. At that time, coastal Irrawaddy dolphins had not previously been studied in this area. We made behavioral observations of Irrawaddy dolphins off the coast of Trat Province from aerial surveys in 2004, 2008, and 2009, and from boat-based line transect surveys conducted.