Estimating survival and abundance in a bottlenose dolphin population taking into account transience and temporary emigration

Abstract

Knowledge of demographic parameters of most cetacean populations is scarce because of problems associated with sampling open populations of wide-ranging animals. In recent years, capture–recapture models have been developed to address these problems. We used a photo-identification dataset collected from a population of bottlenose dolphins Tursiops truncatus between 1999 and 2004 around 2 islands of the Azores archipelago, to demonstrate the use of some of these methods. A variety of open models and Pollock’s robust design were applied to estimate population size, survival probability and emigration rates. Using only the estimates with the lowest coefficients of variation, the annual abundance of adult dolphins varied between 202 (95% CI: 148 to 277) and 334 (95% CI: 237 to 469), according to the Jolly-Seber method, and between 114 (95% CI: 85 to 152) and 288 (95% CI: 196 to 423), according to the robust design. The number of subadult individuals varied from 300 (95% CI: 232 to 387) to 434 (95% CI: 316 to 597) based on the Jolly-Seber method. The open models yielded estimates of adult survival (0.970 ± 0.029 SE) that were significantly higher than those for subadults (0.815 ± 0.083 SE). Movement patterns of dolphins in the Azores seem to follow a Markovian model, in which dolphins seen in the study area in 1 yr show higher probability of emigrating in the following year. Despite some limitations, this is the first study to model transience and temporary emigration in a dolphin population.