Habitat use and behaviour of the Irrawaddy dolphin, Orcaella brevirostris and the Indo- Pacific finless porpoise, Neophocaena phocaenoides off the west coast of Penang Island, Malaysia

This study aimed to study the habitat preferences and daylight behaviour of the Irrawaddy dolphin (Orcaella brevirostris) and the Indo-Pacific finless porpoise (Neophocaena phocaenoides) off the west coast of Penang Island, Malaysia. Between February 2019 and April 2021, 87 boat-based surveys were undertaken using predetermined survey routes. Species occurrence, behaviour, and habitat use were recorded. Irrawaddy dolphins (n = 52 sightings) were recorded with a mean group size of 3.6 (SE = 0.4) and finless porpoises (n = 35 sightings) with a mean group size of 2.5 (SE = 0.2). Both species were found between 0.4 km and 7.9 km from the shore. Encounter rate for Irrawaddy dolphins was 0.19 sightings per effort hour and 1.4 sightings per 100 km while for finless porpoises it was 0.13 sightings per effort hour and 0.9 sightings per 100 km. A higher coefficient of area use (AU> 1.5%) for Irrawaddy dolphins occurred at the estuarine areas off Sungai [=River] Pinang and Sungai Burung while higher coefficient of area use for finless porpoises was seen near Teluk [=Bay] Kampi and Sungai Pinang. In terms of activity budget, milling behaviour was the most recorded behaviour for both Irrawaddy dolphin and finless porpoise at 36.4% and 51.6% respectively. Resting behaviour for both the Irrawaddy dolphin and the finless porpoise was recorded for the first time off west Penang. The information obtained for habitat use and behaviour would contribute towards better management of Penang marine areas and conservation strategies for the Irrawaddy dolphin and the finless porpoise.