Abstract
Behavioural observation and acoustic recordings of Irrawaddy dolphins (Orcaella brevirostris) were performed simultaneously in Brunei Bay, Malaysia. Surface behavioural observations were classified into four categories: travelling, foraging, socialising, and milling. A total of 8.43 hours of acoustic recordings were used to retrieve four different types of sounds: whistles, burst-pulsed sounds, click trains, and biphonal sounds. The recorded whistles were frequency-modulated sounds with durations ranging from 0.06 to 3.86 s and a mean minimum frequency of 6.5±2.5 kHz. The burst-pulsed frequency ranged from >48 kHz in at least four variations. Click trains were made up of broadband clicks with variable inter-click intervals and frequencies >48 kHz. The first biphonic sounds for Irrawaddy dolphins have been revealed in this study. Whistle rates were extremely high in milling. Compared to travelling and foraging, socialising had a significantly higher rate of click trains. Across all surface behaviours, there was no significant difference in burst-pulsed rates. This study laid the groundwork for ongoing monitoring of Irrawaddy dolphins and expanding conservation efforts for this species in Malaysia.
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