Abstract
Detecting dolphins, accurately identifying species, and estimating group size during manned aerial surveys can be challenging. We used images obtained during a manned aerial survey over 13 non-consecutive days and covering a total of 3732 km of transects, in a high-wing aircraft flown at 185 km/h and an altitude of 152 m, equipped with three Single Lens Reflex cameras, to supplement estimates of dolphins recorded by observers. We used this opportunity to assess the adequacy of the resulting images for species identification of two morphologically similar dolphin species (i.e., with similar coloration and body size): the Indo-Pacific bottlenose dolphin (Tursiops aduncus) and the Australian humpback dolphin (Sousa sahulensis) and to accurately estimate dolphin group size. We also compared dolphin group estimates and species identification made by observers during the aerial survey, with manual review of the aerial images, where images overlapped with the observers’ field of view from the aircraft. We collected and reviewed 413000 images and found that an image resolution of 1.25 cm per pixel was adequate to distinguish between the two targeted dolphin species. Our findings suggest that aerial images can improve species identification and estimates of group size > 5. The camera setup and photography parameters that we describe here, with the survey altitude and speed that were used, has the potential to be used to identify, and accurately estimate other morphologically similar small-bodied (< 3 m) species in the marine environment. This will be an important consideration for future aerial surveys of low density, threatened species, that form small and sometimes mixed species groups, including humpback and bottlenose dolphins where their distribution overlaps.
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30 September 2022
Supplementary Information was updated
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Acknowledgements
We are grateful for the dedicated team of student interns who volunteered many days searching aerial images and sighting dolphins and were integral to the success of this study. These include, Ashley Marino, Millie Crockart, Anneke Dearlove, Melissa Rankin, Meg Vaisey, Emily Fosberry, Tamara Dibb, Alex D’Cruz, Kiah Grogan and Dershini Chetty. This note benefited from discussions between Ryan Douglas the lead author and volunteers. Sheldon Whimpey of Above Photography Pty Ltd. provided technical insight on camera settings and Shem Bisluk of DBCA on image conversion. Kym Reeve led the aerial survey team and ensured data quality as well as the successful completion of the survey. The aerial survey team included the sharp eyes and bright attitudes of Daniella Hanf, Krista Nicholson, Chandra Salgado-Kent and Julian Tyne to whom we are very grateful.
Funding
This research is funded by the Chevron-operated Wheatstone LNG Project’s State Environmental Offsets Program and administered by the Department of Biodiversity, Conservation and Attractions. The Wheatstone Project is a joint venture between Australian subsidiaries of Chevron, Kuwait Foreign Petroleum Exploration Company (KUFPEC), Apache Corporation and Kyushu Electric Power Company, together with PE Wheatstone Pty Ltd (part-owned by TEPCO). This project is also supported by Woodside through the Pluto LNG Environmental Offsets Program.
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All authors contributed to the study conception and design. Data collection was by HCR, CC and MS and data analysis were performed by, MH and HCR. The first draft of the manuscript was written by HCR and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.
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All research was conducted under the Department of Biodiversity, Conservation and Attractions scientific license SC001471 and Animal Ethics permit U10/2015–2018.
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This article is a contribution to the special issue on “Individual Identification and Photographic Techniques in Mammalian Ecological and Behavioural Research – Part 1: Methods and Concepts” — Editors: Leszek Karczmarski, Stephen C.Y. Chan, Daniel I. Rubenstein, Scott Y.S. Chui and Elissa Z. Cameron
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Raudino, H.C., Cleguer, C., Hamel, M.A. et al. Species identification of morphologically similar tropical dolphins and estimating group size using aerial imagery in coastal waters. Mamm Biol 102, 829–839 (2022). https://doi.org/10.1007/s42991-021-00214-2
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DOI: https://doi.org/10.1007/s42991-021-00214-2