The Indo-Pacific bottlenose dolphin (Tursiops aduncus) in Tañon Strait, central Philippines
© The Author(s) 2016
Received: 20 June 2016
Accepted: 21 June 2016
Published: 4 October 2016
Previous sightings of bottlenose dolphins in Tañon Strait, central Philippines, in the late 1980’s to early 1990’s were limited to the southern section of the strait, in waters <500 m deep, and the species was formerly identified as Tursiops truncatus. Photo-identification surveys (n = 117) conducted in 2014 and 2015 in Tañon Strait resulted in 50 encounters of Indo-Pacific bottlenose dolphins Tursiops aduncus. No common bottlenose dolphin T. truncatus was seen. The Indo-Pacific bottlenose dolphin has only recently been confirmed to occur in coastal waters in the Philippines and thus very little is known of its local distribution and ecology. In Tañon Strait, the species was seen across a wide range of water depth (4–668 m, x̅ = 309 m) in groups ranging from 1 to ~60 individuals. We provide a photographic verification of this species and first evidence of a seemingly small population in a tropical protected seascape in the Philippines.
The Indo-Pacific bottlenose dolphin (Tursiops aduncus, Ehrenberg 1833) occurs throughout warm temperate and tropical coastal waters, usually less than 100 m deep, in the Indian Ocean and western Pacific (Wang and Yang 2009). In the Philippines, the species was first confirmed only recently from a stranding in Luzon (Aragones et al. 2010). Opportunistic sightings by Wang and Yang (2009) suggested that this species may be distributed throughout the Philippine seas in discrete populations in coastal areas. However, actual sighting records remain sparse.
Bottlenose dolphins have previously been sighted in the southern section of Tañon Strait, but the species identity remained unclear. Leatherwood et al. (1821) identified them as Tursiops truncatus (Montagu 1821) based on the formerly monospecific genus Tursiops. Dolar et al. (2006) identified them only up to the genus level, but subsequently recognised that they were most likely T. aduncus (M.L.L. Dolar pers. comm.). Although both Leatherwood et al. (1992) and Dolar et al. (2006) surveyed the entire Tañon Strait, their sightings of bottlenose dolphins were limited only to the southern section of the strait, near the opening. A short-term follow-up study by Callanta (2009) reported a similar pattern. Our recent work (L. Karczmarski and A.J.C. Tiongson, study in progress) confirms the identity of the species and indicates its considerably wider distribution.
Materials and methods
Boat-based photo-identification surveys (n = 117) were conducted from April through July of both 2014 and 2015 with start off points at Dumaguete City on Negros Island and Moalboal on Cebu Island (Fig. 1). Four small boats of similar size were used (8.5–9.8 m, powered with 40 and 75 HP outboard engines), ranging from one to three boats at a time, during every good-weather day (no precipitation and sea state ≤3 in Beaufort scale) and following a commonly accepted field protocol (similarly as in Karczmarski et al. 2005). The northernmost range of surveys were at Barili (10.2310°N, 123.5487°E) on Cebu Island and Vallehermoso (10.3102°N, 123.3326°E) on Negros Island, while the southernmost range were at Apo Island (9.0607°N, 123.2551°E) off Negros Island and Santander (9.4103°N, 123.3443°E) on Cebu Island (Fig. 1).
Each time a dolphin group was sighted, the animals were slowly approached and their speed and general direction were matched with that of a research vessel to minimise any potential impact caused by the boat presence. Once the dolphins were in a range sufficiently close for species identification, the start time and location of the encounter was recorded using a handheld Garmin 78 s GPS. Group size estimates were made at the beginning and the end of an encounter, and in regular intervals throughout the encounter. Dolphins’ dorsal fins and their upper body were photographed using high-speed digital cameras fitted with 100–400 mm image stabilized lenses. At each encounter, a conscious attempt was made to photograph all individuals in the group and, whenever possible, capture both sides of the dorsal fin. The unique body marks and notches on dorsal fins were used for individual identification (e.g. Karczmarski and Cockcroft 1998) and, subsequently, the software program DISCOVERY (Gailey and Karczmarski 2012) was used for photo-ID database management.
Results and discussion
Water depth associated with sightings of Indo-Pacific bottlenose dolphins in Tañon Strait in 1995 and 2014–2015
Depth range (m)
Mean depth (m)
Dolar et al. 2006
At present, the Indo-Pacific bottlenose dolphin is not listed in the Philippines’ Fisheries Administrative Order 208 (FAO 208) of 2001 which sets out the protection of aquatic species, and it is classified as Data Deficient by the regional Red List assessment (Dolar et al. 2012) and the IUCN Red List of Threatened Species (Hammond et al. 2012). An amendment to the Philippines’ Fisheries Administrative Order 208 that includes T. aduncus is pertinent and urgently needed for the consistency with other protective measures in the Philippines (e.g. Republic Act 8550, 9147, FAO 185/185-1). An appropriate recognition of the species is important as it remains little known not only in the Philippines but across most of its range. Their discontinuous distribution in tropical Philippine waters and apparently small population sizes warrants conservation concern, especially in the face of climate change in the region (Dolar and Sabater 2011). Furthermore, lobomycosis-like disease and other skin disorders have recently been observed in Tañon Strait (A.J.C. Tiongson and L. Karczmarski, unpublished data), likely indicative of the deteriorating habitat quality.
Our preliminary results indicate that the population of Indo-Pacific bottlenose dolphins in TSPS is not large, with considerable site fidelity in the area, although ranging further north and into deeper waters than previously thought. Although the species is known to generally prefer coastal shallow waters, sightings in deeper waters of Tañon Strait are not surprising because the strait’s bathymetry limits the coastal shallow waters to a narrow strip and brings deep waters close to shore. The presence of calves and neonates indicate that the dolphins are reproducing in the strait. The Tañon Strait Protected Seascape (TSPS) is under the National Integrated Protected Area Systems (NIPAS) Act, with a purpose to conserve the biodiversity of the area and implement sustainable development. This provision makes the strait excellently suited for research investigating the bottlenose dolphin population under a rapidly changing environment and such study should be encouraged.
This work was partially funded through a research grant from Ocean Park Conservation Foundation Hong Kong (OPCFHK, grant no. MM05-1415) and internal funding of the Cetacean Ecology Lab at the Swire Institute of Marine Science (SWIMS), The University of Hong Kong. Research clearance (No. 2014-0000-04) was obtained from the TSPS-Protected Area Management Board. The following people are gratefully acknowledged for their assistance at various stages of our field work: Stephen C.Y. Chan, Yuen-WaHo, Mark de la Paz, Edna Sabater and a dedicated team of volunteers. We thank Janet Estacion of Silliman University’s Institute of Environmental and Marine Sciences for her invaluable help; and Louella Dolar of Tropical Marine Research for Conservation (TMRC) for providing access to her earlier data. Raphael Virchaux and Rudy Poitier are gratefully acknowledged for their support, and the staff and crew of ‘Dolphin House’ and ‘Easydiving PH’ are thanked for their assistance during boat surveys.
Both authors contributed equally to this study. AJCT performed this study as part of his ongoing postgraduate research. LK designed the project, secured funding, and supervised the study. Both authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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