Whistle description of Irrawaddy dolphins (Orcaella brevirostris) in Bay of Brunei, Sarawak, Malaysia

Abstract

Studies of Irrawaddy dolphins' acoustics assist in understanding the behaviour of the species and thereby conservation of this species. Whistle signals emitted by Irrawaddy dolphin within the Bay of Brunei in Malaysian waters were characterized. A total of 199 whistles were analysed from seven sightings between January and April 2016. Six types of whistles contours named constant, upsweep, downsweep, concave, convex, and sine were detected when the dolphins engaged in traveling, foraging, and socializing activities. The whistle durations ranged between 0.06 and 3.86 s. The minimum frequency recorded was 443 Hz [Mean = 6000 Hz, standard deviation (SD) = 2320 Hz] and the maximum frequency recorded was 16 071 Hz (Mean = 7139 Hz, SD = 2522 Hz). The mean frequency range (F.R.) for the whistles was 1148 Hz (Minimum F.R. = 0 Hz, Maximum F.R. = 4446 Hz; SD = 876 Hz). Whistles in the Bay of Brunei were compared with population recorded from the waters of Matang and Kalimantan. The comparisons showed differences in whistle duration, minimum frequency, start frequency, and number of inflection point. Variation in whistle occurrence and frequency may be associated with surface behaviour, ambient noise, and recording limitation. This will be an important element when planning a monitoring program.

Untitled

“…In the Bay of Brunei, whilst Irrawaddy dolphins have been sighted in Brunei waters (Elkin 1992;Gibson-Hill 1950, 1949Jiang et al 2020;Stacey & Letherwood 1997;Weber 1923), much of the literature for the region focuses on the species in Malaysian waters. Past studies on Irrawaddy dolphin in Malaysia side of the bay have been mostly investigated on their distribution, population, abundance, and acoustics (Jaaman et al 2016;Jiang et al 2020Jiang et al , 2019Mahmud 2016;Mahmud et al 2018aMahmud et al , 2018bMuhamad et al 2020Muhamad et al , 2018. Species presence in the bay may be due to favorable environmental conditions.…”