Abundance, demographics and social organisation of bottlenose dolphins (Tursiops sp.) in Shark Bay, Western Australia

Krista Nicholson

Photo-identification methods are used extensively in cetacean research. Derived data can be used to answer many research questions but photo-identification data are not always collected to meet the assumptions of all attempted analyses. In this study, photoidentification data collected to describe social organisation were also used to estimate abundance, apparent survival and temporary emigration of bottlenose dolphins (Tursiops sp.) in the western gulf of Shark Bay, Western Australia. Data were collected during boat-based surveys conducted along ten pre-determined transect lines, as well as opportunistically, during each austral winter (April-September) between 2007 and 2011. The best fitting capture-recapture model suggested a random temporary emigration pattern where the probability of an individual being present in the study area is independent of whether or not it was present in the study area in the previous sampling period. This pattern, together with high temporary emigration rates, indicates that our study area does not cover the range of the animals frequenting this area. Abundance estimates varied between years from the lowest estimate of 115 (SE 5.2, 95% CI 105- 126) individuals in 2008 to the highest estimate of 208 (SE 17.3, 95% CI 177-245) individuals in 2010. This variability is unlikely to be reflective of fluctuations in true population size but rather reflects how individuals used the study area during each study period. The variability could also be an artefact of study design, as sampling effort varied between years. Social organisation analyses confirmed that individuals associate non-randomly. Both sexes mostly associated with individuals of the same sex, with adult males forming stronger and more temporally stable associations within their sex class than adult females. After careful consideration of the assumptions of the capture-recapture analyses used in this study it was concluded that photo-identification data, collected opportunistically for social analyses, were not ideal for demographic capture-recapture analyses. However, data appropriate for capture-recapture analyses were suitable for social analyses. It is therefore recommended that the collection of photo-identification data meet the requirements of capture-recapture analyses, so that these data can be used for multiple research objectives.

Abundance, demographics and social organisation of bottlenose dolphins (Tursiops sp.) in Shark Bay, Western Australia

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