Estimating size and assessing trends in a coastal bottlenose dolphin population

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 +/- 12 and 80 +/- 11 individuals, respectively (means +/- 1 se). When divided by the estimated proportion of such animals in the population (0.57 +/- 0.043 and 0.61 +/- 0.035, respectively) and averaged, weighted by inverse variance, a total population size of 129 +/- 15 individual animals was estimated (95% CI = 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 >8 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.