Coastal cetaceans in Southeast Asia are poorly studied and are particularly vulnerable to anthropogenic threats, especially in intensive fishing grounds. To investigate the distribution and habitat characteristics of cetaceans in the productive coastal waters of Matang, Perak, Malaysia, boat‐based line transect surveys were conducted between 2013 and 2016. The Irrawaddy dolphin (Orcaella brevirostris) was most frequently encountered at 3.87 sightings per 100 km, followed by the Indo‐Pacific finless porpoise (Neophocaena phocaenoides) at 1.72 sightings per 100 km, and the Indo‐Pacific humpback dolphin (Sousa chinensis) at 0.66 sightings per 100 km. The mean group size was largest for humpback dolphins (8.4 individuals), followed by Irrawaddy dolphins (6.4 individuals), and finless porpoises (2.8 individuals). Humpback dolphins exhibited a clustered distribution concentrated mainly in shallow estuarine waters (<10 m deep and <5 km from river mouths), whereas Irrawaddy dolphins were more widely distributed in farther coastal waters (<15 m deep and <15 km from river mouths), and finless porpoises were mostly found farthest from the shore in coastal waters (10–25 m deep and >15 km from river mouths). The spatial distribution of the three cetaceans overlapped minimally, and this is likely to reflect the distribution of preferred prey resources, species interactions, and their differential responses to anthropogenic activities and species dominance. The results from our study serve as baseline information for future research, conservation, and habitat management of these vulnerable and endangered coastal cetaceans. Conservation actions are recommended for the Matang area.

Differences in the acoustic variables of whistles emitted by Indo-Pacific humpback dolphins (Sousa chinensis) from two coastal locations along western Peninsular Malaysia were investigated. Duration, frequency, and frequency modulation variables were extracted from and used to characterize recordings of free-ranging humpback dolphins that were made using a broadband towed hydrophone. A total of 960 whistles from Matang Mangroves and 823 whistles from Langkawi Island were used in analyses. The whistles of Malaysian humpback dolphins covered frequencies from 1231 to 27 120 Hz with durations from 0.010-1.575 s. Significant multivariate differences were found in whistles emitted between locations. Significant differences were also found between dolphins of the two locations in their whistle duration, frequency modulation, and all frequency variables except for minimum frequency, which is likely under morphological constraints. The differences in whistles may be related to adaptations to the local acoustic habitat or unique whistles may have developed due to social interactions within each location, or broader scale differences resulting from geographic separation between the locations.

1. The paucity of baseline data on coastal cetaceans due to a lack of research in developing countries frequently precludes assessment of their status and informed management actions for conservation.2. This study provides the first abundance estimates of Indo-Pacific humpback dolphins, Irrawaddy dolphins, and Indo-Pacific finless porpoises in the coastal waters of Matang, Peninsular Malaysia. 3. Boat-based surveys covering 1,152 km 2 of coastal waters with 4,108 km of survey effort were conducted between 2013 and 2016 to collect data for line transect analysis of Irrawaddy dolphins and finless porpoises. Photo-identification data of humpback dolphins were concurrently collected for mark-recapture analysis. 4. Estimates of abundance from four sampling strata totalled 763 Irrawaddy dolphins (CV = 13%, 95% confidence interval (CI) [588, 990]) and 600 Indo-Pacific finless porpoises (CV = 27%, 95% CI [354, 1,016]). 5. The annual abundance estimates of humpback dolphins ranged between 171 (95% CI [148, 208]) in 2014-2015 and 81 (95% CI [67, 98]) in 2015-2016, likely due to the presence of offshore individuals that moved in and out of the study area. The estuarine strata were inhabited by 68 (95% CI [63, 73]) inshore humpback dolphins in 2013-2014 to 87 (95% CI [78, 97]) dolphins in 2014-2015. 6. As an International Union for Conservation of Nature important marine mammal area, the productive coastal waters of Matang are shown to support a high density of small coastal cetaceans, and the results serve as an important baseline for future studies to identify population trends for conservation management plans.

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