Mark-Recapture Analysis of the Critically Endangered Eastern Taiwan Strait Population of Indo-Pacific Humpback Dolphins (Sousa Chinensis): Implications for Conservation

Abstract

Accurate and precise estimates of abundance and survival rates are important for assessing the conservation status of cetacean populations. Mark-recapture analysis of photo-identification data of the critically endangered eastern Taiwan Strait (ETS) population of Indo-Pacific humpback dolphin, Sousa chinensis (Osbeck, 1765), was conducted on data collected between 2007 and 2010 to refine a preliminary, and the only available, abundance estimate for this isolated population (n = 99; CV = 51.6%), as well as to provide survival rates. About 14,000 good quality photographs (about 2100-6300 yr-1) were used to estimate both parameters for marked animals under Pollock's Robust Design model. The total population size (NT) was determined by correcting for the proportion of the population possessing long-lasting marks (?). The annual point estimates were lower, varying from 54 to 74, and had much better precision (CV varied from 4% to 13%) than previous estimates, suggesting that mark-recapture is a suitable method for estimating abundance of this population. These estimates also further supported the precarious state of the ETS population under another criterion of the IUCN Red List of Threatened Species. As expected for long-lived mammals, annual apparent survival rate was high at 0.985 (95% CI = 0.832-0.998). Continuing to monitor the ETS population of humpback dolphins with such high precision and accuracy will allow examination of the population's trends over time and to better understand its future persistence.