Demography and socio-ecology of Indo-Pacific humpback dolphin (Sousa chinensis) metapopulation in the Pearl River estuary

Abstract

Indo-Pacific humpback dolphins (Sousa chinensis) inhabiting the Pearl River Estuary (PRE) region are affected by multitude of anthropogenic impacts. Although early research and conservation efforts date back two decades, much of the dolphin population ecology remained poorly understood. Studies that would rigorously assess their population connectivity, socio-demographic structure and viability were long overdue. The research described in this thesis undertook the effort of furnishing these much needed data.

Individual photo-identification boat-based surveys were initiated in Hong Kong in 2010 and, subsequently, expanded across the entire PRE region through 2016. Dolphins were photographically identified using their individually distinctive features, and their individual sighting histories were subsequently analysed with socio-spatial, behavioural, and mark-recapture population modelling techniques. Viability analyses were performed using a suite of demographic parameters.

Multiple quantitative measures indicate that the dolphins inhabiting PRE region form three socially distinct and spatially discrete subpopulations, in the Eastern, Middle, and Western PRE (EPRE, MPRE, and WPRE, respectively). Individual site fidelity is weak in the short-term (days/weeks), but moderate-to-strong in the long-term (years). In other words, individual movement across few tens of kilometres is common, across several tens of kilometres infrequent, and across few hundreds of kilometres is unlikely. While all humpback dolphins in the PRE exhibit similar general pattern of spatio-temporally restricted dispersal, individuals of the three distinct subpopulations differ in the magnitude of dispersal. Although dolphin groups are fluid and inter-individual associations are short-lasting, there is a notable social structure across the PRE region with three well-discernible social communities. The spatial ranges of the communities are identical to that of subpopulations, and it seems that the ranging/foraging strategies of individuals determine their potential affiliates and hence the broader social dynamics (and social structure) of the PRE metapopulation.

Mark-recapture analyses indicate that there are 2059 individuals in the PRE metapopulation, with 735, 445, and 879 dolphins in EPRE, MPRE, and WPRE, respectively. Adult survival rates in EPRE, MPRE, and WPRE were estimated at 0.946, 0.852, and 0.941 respectively, all of which are below the threshold of long-term population persistence. In EPRE, at least 368 dolphins use Hong Kong waters as part of their range, and these individuals exhibit higher survival rate (0.980), underscoring the importance of Hong Kong coastal habitat within EPRE waters.

Although most individuals travel across only limited spatial scales, there is low-level connectivity across the metapopulation that prevents the demographic independence of individual subpopulations. The underlying mechanism shaping the metapopulation structure is likely related to habitat heterogeneity and is affected by man-induced fragmentation of the coastal seascape. Although the metapopulation dynamics have stabilising effect on population persistence, viability analyses indicate that the PRE metapopulation may decline by ~87% in a lifespan of three generations, 3.17% per annum, and its conservation status meets the IUCN criteria of Critically Endangered. MPRE subpopulation is of particular conservation concern because of its considerable probability of going extinct, which could trigger major population fragmentation. Consequently, a better understanding of the metapopulation dynamics of the PRE dolphins is crucial to any future conservation actions.