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Satisfying model assumptions for capture-recapture estimates of bottlenose-dolphins (Tursiops sp.) in the western gulf of Shark Bay, Australia

Nicholson, K., Pollock, K., Krützen, M., Allen, S. and Bejder, L. (2011) Satisfying model assumptions for capture-recapture estimates of bottlenose-dolphins (Tursiops sp.) in the western gulf of Shark Bay, Australia. In: 19th Biennial Conference on the Biology of Marine Mammals, 27 November - 2 December, Tampa, Florida.


Estimates of population abundance and other demographic parameters can inform effective management and conservation of a species. To date, many cetacean population size estimates have been conducted peripheral to the main aims of the research. Field studies not designed to meet the requirements of capture-recapture analyses can result in violation of the assumptions of abundance estimation models and biases in parameter estimates. The data used for this project were collected as part of a long-term study assessing factors that influence tool use (foraging with a protective marine sponge over the rostrum) in bottlenose dolphins (Tursiops sp.). Boat-based photo-identification surveys were conducted in each austral winter (April-September) from 2007 to 2010 along ten pre-determined transect lines, each sampled 25 times, covering an area of approximately 230km2. A total of 600 dolphin groups were encountered, resulting in the identification of 313 distinctive adult dolphins. Pollock’s Robust Design was chosen as the appropriate modelling approach. With this design abundance, survival and temporary emigration rates can be estimated using a combination of closed and open population models. In this study, the temporary emigration is estimated to investigate movement patterns in and out of the study area. Preliminary results suggest that a random temporary emigration model best fits our data, indicating that individuals are moving in and out of the study area and the probability of encountering an individual is independent of whether or not it was encountered in the study area in prior sampling periods. Our results have both empirical and applied value for future study, allowing us to (1) better estimate the proportion of the population that engages in tool use, and (2) assess whether or not future increases in anthropogenic activity affects population size. In addition, our research provides an opportunity to discuss study design modification to satisfy capture-recapture model assumptions.

Item Type: Conference Item
Murdoch Affiliation(s): Centre for Fish and Fisheries Research
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