Estimating Size and Assessing Trends in a Coastal Bottlenose Dolphin Population

Abstract

We used a case study of a coastal bottlenose dolphin population to present a framework for determining the number of individuals present and assessing the likely time scale over which trends in abundance may be determined. Such a framework is appropriate for animal species that possess natural markings sufficient for individual recognition, and may be valuable in the development and implementation of management and monitoring programs for vulnerable populations.Population abundance was estimated using mark-recapture methods applied to photoidentification data. This experiment was designed to minimize violation of method assumptions so as to allow use of the most parsimonious model for analysis. The data were examined critically to investigate mark-recapture assumptions, while analytical methods and data were selected to minimize and, where necessary, account for violations. The estimated number of animals with long-lasting marks from left and right side estimates were 73 Ϯ 12 and 80 Ϯ 11 individuals, respectively (means Ϯ 1 SE). When divided by the estimated proportion of such animals in the population (0.57 Ϯ 0.043 and 0.61 Ϯ 0.035, respectively) and averaged, weighted by inverse variance, a total population size of 129 Ϯ 15 individual animals was estimated (95% CI ϭ 110-174 animals).Data on calves observed and carcasses recovered suggest that the population could be increasing or decreasing at an annual rate of up to 5%. A power analysis, undertaken to investigate the length of monitoring program required to detect changes in population abundance at a 90% level of certainty, showed that detection of a trend could only occur following Ͼ8 yr of research effort. Biennial sampling has power similar to that of annual sampling, but savings in resources are offset by the loss of data on the reproductive histories of individuals.

Abundance, survival and temporary emigration of bottlenose dolphins (Tursiops sp.) off Useless Loop in the western gulf of Shark Bay, Western Australia

“…Data were collected in consecutive years and the majority of individuals were captured, although not necessarily on transect effort, each year. This, together with the longevity of marks used for identification (Wilson et al 1999), means that mark changes were likely to be tracked successfully.…”

“…The degree of distinctiveness varies among dorsal fins of individuals and some are not sufficiently marked to be included in capture-recapture analyses, which only pertain to the distinctly marked population (Wilson et al 1999;Read et al 2003). Each individual in the catalogue was therefore given a distinctiveness score, based on the amount of information contained on the leading and trailing edges of the dorsal fin.…”

“…False negative errors, which may lead to positive bias in abundance estimates, are more likely to occur when identifying individuals from natural marks because marks can evolve over time (Yoshizaki et al 2009). However, the nicks and deformities on the dorsal fins of bottlenose dolphins are persistent (Wilson et al 1999). Thus, mark changes, if they occurred, were detected as a change for the same animal.…”

“…The photo-identification method is widely applied with capture-recapture methods to estimate abundance and other demographic parameters, such as survival and temporary emigration rates, of small cetaceans (e.g. Wilson et al 1999;Read et al 2003;Parra et al 2006;Silva et al 2009). The main advantage of using photo-identification data for capture-recapture analyses is that animals do not need to be physically captured to be marked (Hammond 1986(Hammond , 1990.…”

Estimates of abundance and apparent survival of coastal dolphins in Port Essington harbour, Northern Territory, Australia

“…To include the unmarked portions of the populations in our estimates, we estimated the proportion of distinctively marked dolphins in the population of each species (Wilson et al 1999). The (distinctively) marked proportion (M p ) of each population was estimated by dividing the number of excellent-and good-quality photographs of marked dolphins (P m ) by the total number of excellent-and good-quality photographs (P t ), as follows:…”