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Habitat use and behaviour of Irrawaddy dolphins (Orcaella brevirostris) in the Mekong River of Laos

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  • This field study addressed a number of natural history parameters of Irrawaddy dolphins (Orcaella brevirostris) in the Mekong River of Laos that were not well-documented, including diving behaviour, foraging ecology, habitat use, social behaviour, group size, and diurnal movements. Irrawaddy dolphin sightings were most common in the morning and decreased throughout the day. This could indicate diurnal feeding, given that foraging is suggested by repeated direction changes, lack of through travel, and observed fish consumption. Habitat use was highest off the tributary mouth and adjacent Sandy Island. Mean water depth at the study site was 18.4 m, current speed of the main channel was 0.15 m/s, and water temperature was 31degreeC. Mean dive duration of dolphins was 115.3 s and similar for all group sizes. When no boats were within 100 m of dolphins, mean dive duration was significantly shorter than when boats were present. The longest dive times occurred in the shallowest and the deepest water. There was no significant relationship between surface direction or group cohesiveness and dive duration. The mean number of respirations per dolphin on each surface run was 1.96, and did not change with boat presence. The average dolphin group size was 3. There was no significant relationship between group cohesiveness and boat type or speed. Discrete surface activities occurred on 14% of surface runs and neither boat speed or the distance dolphins surfaced from boats were associated with the presence of these surface activities. These activities occurred most frequently in shallower water, especially in one area away from the tributary mouth. Dolphins tended to surface closer to paddle boats than to large boats. Of over six hundred photographs taken, only 11 were of sufficient quality to identify individuals. Two animals, and tentatively up to six, could be identified. The conclusions of this study must be placed into the context of the limited time and space over which the research was conducted. Irrawaddy dolphin behaviours and environmental interactions observed are likely site- and time-specific, especially in this dynamic habitat. This study provided a portrait of Irrawaddy dolphins that is far from complete, but is an example of baseline research that is needed before being able to progress to question-oriented studies. Long-term, in-depth observations over a larger area are needed.

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  • Type of Item
    Article (Published)
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    Attribution-NonCommercial 4.0 International
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  • Citation for previous publication
    • Stacey, P.J. and G.T. Hvenegaard. 2002. Habitat use and behaviour of Irrawaddy Dolphins (Orcaella brevirostris) in the Mekong River of Laos. Aquatic Mammals 28(1): 1-13.