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- Title
Population size, distribution and daylight behaviour of Irrawaddy dolphins (Orcaella brevirostris) in Penang Island, Malaysia.
- Authors
Rodríguez-Vargas, L. H.; Rajamani, L.; Dolar, L.; Porter, L.
- Abstract
The Irrawaddy dolphin (Orcaella brevirostris) is endemic to South and Southeast Asia and occurs in shallow coastal waters, associated with freshwater input, coastal lagoons and in three rivers. The status of all except the riverine populations is poorly understood, however, the proximity of all populations to areas of intense coastal development led to the 2017 IUCN classification of 'Endangered'. This is the first description of a coastal estuarine Irrawaddy dolphin population in Penang Island, Malaysia, and provides baseline information on population size, distribution, group structure, and daylight behaviour. Vessel-based observations were conducted throughout 2013 and mark-recapture analysis of the photo-identification data was used to estimate population size. Thirty individuals were identified from 28 unique left and 26 unique right dorsal fins. Closed and open models were used to estimate a population size of 32 ± 3 (SE) and 43 ± 12 (SE) individuals, respectively. Sightings of Irrawaddy dolphins were recorded throughout the study area in the estuary of Balik Pulau (west Penang), mostly between Sungai [=River] Pinang and Sungai [=River] Burung, usually during high tide and close to shore. The mean group size was 5 ± 0.5 (SE). Juveniles (n=26) were sighted throughout the year. Mother-calf pair (n=16) sightings peaked in May. During the surveys, feeding was the most commonly observed behaviour (60%) followed by milling (16%), travelling (14%), and socialising (2%). Groups of dolphins were observed moving in a circular pattern creating loops while feeding, and returning to the point where the activity started. This appears to be the first documentation of dolphins following prey in large circles over several hundreds of metres. As this population occurs within a restricted habitat, the impact of fishing and other anthropogenic activities in the area is high. As coastal development continues to increase, there is an urgent need to develop a management plan to better protect this critical habitat and the dolphins that live within.
- Subjects
PINANG; MALAYSIA; DOLPHINS; TERRITORIAL waters; DAYLIGHT; COASTAL development; WATER depth; ESTUARINE ecology; FISH food
- Publication
Raffles Bulletin of Zoology, 2019, Vol 67, p671
- ISSN
0217-2445
- Publication type
Article
- DOI
10.26107/RBZ-2019-0048