Abstract
As delphinid populations become increasingly exposed to human activities we rely on our capacity to produce accurate abundance estimates upon which to base management decisions. This study applied mark–recapture methods following the Robust Design to estimate abundance, demographic parameters, and temporary emigration rates of an Indo-Pacific bottlenose dolphin (Tursiops aduncus) population off Bunbury, Western Australia. Boat-based photo-identification surveys were conducted year-round over three consecutive years along pre-determined transect lines to create a consistent sampling effort throughout the study period and area. The best fitting capture–recapture model showed a population with a seasonal Markovian temporary emigration with time varying survival and capture probabilities. Abundance estimates were seasonally dependent with consistently lower numbers obtained during winter and higher during summer and autumn across the three-year study period. Specifically, abundance estimates for all adults and juveniles (combined) varied from a low of 63 (95% CI 59 to 73) in winter of 2007 to a high of 139 (95% CI 134 to148) in autumn of 2009. Temporary emigration rates (γ') for animals absent in the previous period ranged from 0.34 to 0.97 (mean = 0.54; ±SE 0.11) with a peak during spring. Temporary emigration rates for animals present during the previous period (γ'') were lower, ranging from 0.00 to 0.29, with a mean of 0.16 (± SE 0.04). This model yielded a mean apparent survival estimate for juveniles and adults (combined) of 0.95 (± SE 0.02) and a capture probability from 0.07 to 0.51 with a mean of 0.30 (± SE 0.04). This study demonstrates the importance of incorporating temporary emigration to accurately estimate abundance of coastal delphinids. Temporary emigration rates were high in this study, despite the large area surveyed, indicating the challenges of sampling highly mobile animals which range over large spatial areas.
Highlights
Coastal bottlenose dolphins (Tursiops spp) are one of the most studied cetacean species mainly due to their widespread distribution and ease of accessibility
Much is known about their biology, social behaviour and population dynamics [1,2,3,4,5]
It is becoming clear that many coastal bottlenose dolphin populations include individuals with varying patterns of residency and ranging size
Summary
Coastal bottlenose dolphins (Tursiops spp) are one of the most studied cetacean species mainly due to their widespread distribution and ease of accessibility. Various methods for estimating the abundance of dolphin populations have been developed and used routinely [1,6,7,8,9]. Methods should be continuously refined based on the growing understanding of dolphin biology and behaviour to ensure that abundance estimates and population parameters are reliable and accurate [10,11]. It is becoming clear that many coastal bottlenose dolphin populations include individuals with varying patterns of residency and ranging size. Individuals reside permanently in an area [12], while others transit through only occasionally [7,9,10]. The varying degrees of individual residency have a large influence on abundance estimates at any given time and need to be incorporated into study designs and data analysis through statistical models
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