Acoustic identification of the sympatric species Indo-Pacific finless porpoise and Indo-Pacific humpback dolphin: An example from Langkawi, Malaysia

The passive acoustic monitoring (PAM) technique has been widely used to detect odontocetes that emit echolocation clicks. However, identifying sympatric dolphins and porpoises using clicks, which is important especially for conservation purposes, remains challenging. In this study, a species identification technique at the family group level using PAM was applied to classify the click trains of two sympatric odontocetes, the Indo-Pacific finless porpoise (Neophocaena phocaenoides, N.p.) and Indo-Pacific humpback dolphin (Sousa chinensis, S.c.). A threshold was identified to discriminate between the two species using the two-band intensity ratios at 130 and 70 kHz. In addition to the significant difference in the two-band intensity ratio, analyses of the click train characteristics revealed that the click train duration and inter-click interval were significantly longer for S.c. than for N.p., indicating that these parameters enable more accurate species identification. Although this study provides a clear means of discriminating between the two species to reveal their distributions and habitat usage, additional studies are recommended to determine whether the same method can be employed to discriminate between other species known to have overlapping distributions in the region, such as Tursiops aduncus or Orcaella brevirostris.