Indo-Pacific humpback dolphins face extirpation in Shantou waters

Abstract

The Indo-Pacific humpback dolphin is an inshore species occurring in productive estuarine and coastal areas of the Indo-Pacific. Since the end of the 20th century, coastal region of China has experienced tremendous economic expansion, exposing this dolphin species to a wide range of human activities. Despite frequent voices of concern in the past three decades calling for research and conservation attention, protracted debates were not followed by constructive conservation actions due to the lack of or inconsistent use of demographic data. The current study examined 13-year sighting and photo-identification data collected for a small and little known population of humpback dolphins off the coast of Shantou (ST) in the northern South China Sea. Compared to conspecific populations inhabiting large or medium-sized estuarine systems, the ST dolphins have much larger home range (mean = 181.3 km2, S.D. = 87.7) which, however, has been shrinking recently due to coastal reclamations and expanding aquaculture farming. Although at least two dolphin births were recorded between 2010 and 2022, there was no successful recruitment of a non-calf individual. Meanwhile, the population shrank by 29.4 %, with the most updated population size of only 12 individuals, estimated in 2022. Based on the current demographic trend, we predict that the ST humpback dolphins will be extirpated within one or two decades. Furthermore, as the dolphins are aging and becoming less reproductively viable, there is little chance to reverse the declining trend even without additional stressors. Moreover, we noticed prevailing vertebral column deformities amongst the ST dolphins, possibly a sign of inbreeding depression of a small population. The present study exemplifies a typical case of a small, isolated and locally endangered dolphin population that is approaching local extinction without efficient monitoring program and conservation action. We emphasize that without solution-based research and efficient conservation planning, other humpback dolphin populations in China might soon follow in the steps of ST humpback dolphins.