Abundance of Indo-Pacific bottlenose dolphins (Tursiops aduncus) in the Shoalwater Islands Marine Park, Western Australia

Zijing Jiang

Effective conservation management of coastal dolphins relies on robust estimates of population abundance. This study provides the first quantitative abundance estimates of Indo-Pacific bottlenose dolphins (Tursiops aduncus) in the Shoalwater Islands Marine Park, Western Australia. Boat-based photo-identification surveys were conducted along systematic transects between the austral summer of 2016 and spring of 2017. Pollock's closed robust design was applied to capture-recapture data derived from photographs of distinctly marked adults and juveniles. Photographs were graded for quality and dorsal fins for distinctiveness to minimise heterogeneity in capture probabilities and misidentification. A total of 101 independent individuals were identified from 52 on-transect and 65 off-transect surveys. Data collected during the winter of both years were excluded due to no individuals being sighted on some transects, leading to an insufficient number of sampling occasions (i.e. transects) for the modelling approach. Final analyses were based on 58 distinctly marked individuals with 91 captures. Thirty-five distinctly marked individuals (60%) were captured only once. The proportion of distinctly marked individuals was estimated at 0.87 (SE = 0.035). The best-fitting model assumed no temporary emigration, constant apparent survival and capture probabilities that varied among sampling occasions. Estimated number of distinctly marked individuals using the marine park ranged from 35 (SE = 18) to 131 (SE = 48), corresponding to total estimates of 41 (SE = 21) to 150 (SE = 56) individuals after accounting for unmarked dolphins. Capture probabilities were low, ranging from 0.01 to 0.08 among secondary periods with seasonal effective capture probabilities 0.10-0.18. Apparent survival was estimated at 0.97 (SE = 0.082), but was poorly estimated, with wide confidence interval, and should therefore be interpreted cautiously. Temporary emigration could not be estimated reliably from the available data. Despite substantial uncertainty in the model-derived estimates, this study demonstrates that the marine park is used by dolphins beyond the previously identified social community of 51 individuals. A total of 101 independent dolphins were directly identified within the marine park over the two-year study period, indicating substantial use of the area by both members of this social community and additional dolphins. The low capture probabilities and broad confidence intervals largely reflect the use of survey data originally designed for a much larger geographic area rather than specifically for the marine park. These findings provide an initial quantitative baseline for dolphin use of the Shoalwater Islands Marine Park and highlight the need for a dedicated monitoring design to improve abundance estimates, assess demographic parameters and detect population changes over time.

Abundance of Indo-Pacific bottlenose dolphins (Tursiops aduncus) in the Shoalwater Islands Marine Park, Western Australia

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