The extinction risk has been seriously underestimated for the Indo-Pacific humpback dolphins in the Pearl River Estuary

Abstract

Indo-Pacific humpback dolphins (Sousa chinensis) are among the most threatened coastal delphinids. Owing to their habitat selectivity, most extant populations are limited in size and constrained by habitat fragments, while scarce scientific evidence frequently hampers informed conservation management. To obtain reliable demographic parameters for a rigorous population risk assessment, a 5-year monitoring program was carried out in the Neilingding Sea area of the Pearl River Estuary (PRE), southeast China. Mark-recapture photo-ID data was obtained during 1287 encounters at 465 survey-days (326 and 252 in Hong Kong and mainland China, respectively). Separate datasets were cross-matched and combined for the final analyses. A total of 991 individuals were identified and catalogued (543 seen only in mainland waters, 127 only in Hong Kong, and 320 seen in both); of which 754 were seen on more than one survey (mean and maximum re-sighting rate = 11.2 and 75, respectively). To counterweigh for the heterogeneity of effort, we divided the study area into three sections: Hong Kong waters, southern PRE (sPRE) and northern PRE (nPRE). The median c-hat of 2.65 indicated no sign of data over-dispersion. The best fit Cormack-Jolly-Seber model revealed an apparent survival rate as 0.939 (95% CI: 0.902–0.963), comparable to previous estimates based on static life table analyses; while the second-best model (DQAICc=1.14) suggested a survival rate in nPRE (0.873; 95% CI=0.784–0.929) to be much lower than in sPRE–HK (0.954; 95% CI=0.916–0.976). The super population size estimate (NT) corrected for ID-ratio was 996 (95% CI: 902–1019) and 959 (95% CI: 983–1010) for marked-database and highly-marked-database, respectively. Population trend analyses indicate that over one/third of the population was lost in the past two decades; whereas population viability analysis suggest that the PRE humpback dolphins should be listed as critically endangered under the IUCN criterion A3.