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Abundance of Coastal Dolphins in Roebuck Bay, Western Australia

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Abundance of Coastal Dolphins in Roebuck Bay, Western Australia Cetacean Research Unit School of Veterinary and Life Sciences Murdoch University Murdoch WA 6150 www.mucru.org Abundance of coastal dolphins in Roebuck Bay, Western Australia Report to WWF-Australia February 2014 Alexander M. Brown, Lars Bejder, Kenneth H. Pollock and Simon J. Allen Please cite this document as: Brown, A.M., Bejder, L., Pollock, K.H. & Allen, S.J. (2014). Abundance of coastal dolphins in Roebuck Bay, Western Australia. Report to WWF-Australia. Murdoch University Cetacean Research Unit, Murdoch University, Western Australia, 25pp. All photographs © WWF-Australia/MUCRU Table of Contents Non-technical summary ................................................................................................................................. 1 Background ................................................................................................................................................... 2 Objectives .................................................................................................................................................. 3 Methods ........................................................................................................................................................ 3 Ethics statement ........................................................................................................................................ 3 Study area.................................................................................................................................................. 3 Data collection ........................................................................................................................................... 3 Distribution of sightings, group sizes and encounter rates .......................................................................... 5 Grading and scoring of photographic-identification images ........................................................................ 6 Proportion of distinctly marked individuals in the population .................................................................... 7 Identification and resight rates .................................................................................................................. 7 Abundance estimates ................................................................................................................................ 7 Potential movements of animals between study sites ................................................................................ 8 Results ........................................................................................................................................................... 8 Effort ......................................................................................................................................................... 8 Distribution of dolphin sightings and group sizes ........................................................................................ 9 Encounter rates ....................................................................................................................................... 11 Identification and resight rates ................................................................................................................ 12 Abundance estimates .............................................................................................................................. 13 Potential movements of animals between study sites .............................................................................. 14 Local and indigenous engagement and data dissemination ...................................................................... 14 Discussion .................................................................................................................................................... 15 Abundance of dolphins in Roebuck Bay .................................................................................................... 15 Conservation and management implications ............................................................................................ 16 Recommendations for future research ..................................................................................................... 17 Acknowledgements ..................................................................................................................................... 17 Literature cited ............................................................................................................................................ 18 Appendix 1 - Encounter rate maps of all species .......................................................................................... 21 Snubfin dolphins ...................................................................................................................................... 21 Bottlenose dolphins ................................................................................................................................. 21 Humpback dolphins ................................................................................................................................. 22 Appendix 2 - Abundance estimate model outputs ........................................................................................ 23 Appendix 3 - Dugong sightings ..................................................................................................................... 25 Non-technical summary The abundance of populations and species of animals is a key consideration in assessing their conservation status and determining suitable management strategies. For dolphins, abundance estimates and other baseline data are lacking throughout the majority of tropical northern Australian waters. This lack of data is impeding assessment of their conservation status and the development of appropriate management measures in the face of numerous anthropogenic threats. Three species of dolphin regularly occur in the near-shore waters of tropical northern Australia - the Australian snubfin dolphin ( Orcaella heinsohni , ‘snubfin dolphin’ hereafter), the Indo-Pacific humpback dolphin (Sousa sp., ‘humpback dolphin’ hereafter) and the Indo-Pacific bottlenose dolphin ( Tursiops aduncus , ‘bottlenose dolphin’ hereafter). Murdoch University’s Cetacean Research Unit (MUCRU) is conducting research on these three species at several locations across north-western Australia. In 2013, MUCRU researchers were awarded funds by WWF-Australia to add to existing data on inshore dolphins in the Kimberley region by collecting data on the abundance of coastal dolphins in Roebuck Bay, with a focus on the snubfin dolphin. In collaboration with Nyamba Buru Yawuru Pty Ltd., five weeks of survey effort took place in October- November 2013. A study area of approximately 100 km 2 was surveyed several times from a small research vessel following specific transect routes of around 60 km length. Photographic-identification was conducted on all groups of dolphins encountered, resulting in a database of individual animals, identifiable by unique patterns of notches on their dorsal fins. A total of 19 days of survey effort was completed, with the research team navigating over 400 km of transect. Over this period, 74 groups of snubfin dolphins were observed, along with four groups of bottlenose dolphins and one group of humpback dolphins. Group sizes of snubfin dolphins ranged from one to 16, with a mean of 4.4 individuals. Encounter rates of snubfin dolphins per transect ranged from 0.19 - 2.69 dolphins per km 2 survey effort, with a mean of 1.61 dolphins per km 2 survey effort. Encounter rates of snubfin dolphins were highest within the Inner Anchorage and over the extensive shallow flats on the eastern side of Roebuck Bay. The mean encounter rates of bottlenose and humpback dolphins were low at ≤ 0.1 dolphins per km 2 surveyed. We photographically-identified a total of 100 different snubfin individuals (excluding calves) with distinctive markings on their dorsal fins. New animals continued to be identified throughout the study period, suggesting that only a subset of the total number of animals which may use the study area were observed during the study period. An open population model produced the best abundance estimate of 137 snubfin dolphins (excluding calves) using the study area over the five-week study period. This estimate had a narrow confidence interval (95% confidence intervals of 117-162; with standard error of 11.6) implying that the estimate is reliable. At over 130 animals, this is the largest reported abundance of snubfin dolphins in Australia to date and should be considered of regional and, indeed, national significance. Despite this relative magnitude, it is small by conservation standards and sensitive to change. Given the abundance estimate presented here and our current understanding of snubfin dolphin distribution and genetic structure in the region, decision makers and resource management agencies should be prioritising measures to minimise anthropogenic threats to this population in the current planning of a marine protected area. We also recommend that further research be carried out at different times of the year and across a broader area, in order to investigate potential seasonal changes in abundance and to examine the relative importance of different areas within Roebuck Bay. Consideration should also be given to more regular monitoring in areas of importance to dolphins using local
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