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Distribution and abundance of tropical inshore dolphins in Cygnet Bay, Western Australia

Brown, A., Pollock, K., Bejder, L. and Allen, S. (2013) Distribution and abundance of tropical inshore dolphins in Cygnet Bay, Western Australia. In: 20th Biennial Conference on the Biology of Marine Mammals, 9 - 13 December, Dunedin, New Zealand.

Abstract

Little is known of inshore dolphin populations, particularly snubfin (Orcaella heinsohni) and humpback (Sousa chinensis) dolphins, in tropical north-western Australia. Studies elsewhere suggest that they are sensitive to habitat modification and vulnerable to localised extinctions. Both species are listed ‘near-threatened’ by the IUCN. In north-western Australia, large-scale industrial development is ongoing, but a complete absence of quantitative data on dolphin abundance and distribution precludes assessment of their conservation status. Here, we report initial results from systematic boat-based photo-identification surveys of ca. 130 km2 of the near-shore waters (<2 nm offshore) of Cygnet Bay, in the western Kimberley region, from 2012-2013. Data are presented for snubfin, humpback and bottlenose (Tursiops aduncus) dolphins. The total numbers of marked individuals (excluding calves) identified during 2012 were 52 snubfin, 22 humpback and 53 bottlenose dolphins. Preliminary mark-recapture models following the Robust Design yielded abundance (±SE) estimates of 47 (±4.5) snubfin dolphins, and between 30 (±3.4) and 50 (±5.4) bottlenose dolphins. Insufficient numbers of humpback dolphins were encountered to estimate their abundance. After three months of effort, there was a plateau in the cumulative number of identified snubfin and bottlenose dolphins and high re-sighting rates, suggesting a closed population of relatively resident animals. There was a continued increase in the number of humpback dolphin individuals identified over time. Bottlenose and humpback dolphin sightings were distributed throughout the study area, whereas snubfin sightings were clustered within three discrete areas. This study presents the first quantitative data on inshore dolphins in north-western Australia, including the first abundance estimate of snubfins in the western portion of their range and the first abundance estimate of any north-western Australian bottlenose dolphin population. These demographic data will be combined with parallel research into genetic connectivity between adjacent populations to inform the conservation and management of inshore dolphins in the region.

Publication Type: Conference Item
Murdoch Affiliation: School of Veterinary and Life Sciences
Notes: Oral presentation
URI: http://researchrepository.murdoch.edu.au/id/eprint/20217
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