Conservation status of Australian snubfin dolphin, \u3cem\u3eOrcaella heinsohni\u3c/em\u3e, and Indo-Pacific humpback dolphin, \u3cem\u3eSousa chinensis\u3c/em\u3e, in the Capricorn Coast, Central Queensland, Australia

Abstract

This study examined populations of Australian snubfin dolphins (Orcaella heinsohni) and Indo-Pacific humpback dolphins (Sousa chinensis) inhabiting the coastal waters of the Capricorn Section of the Great Barrier Reef Marine Park, Queensland, Australia. A total of 189 humpback dolphins and 54 snubfin dolphins were identified during 1,760 hours and 20,248 kms of transect surveys completed between 2006 and 2008 in the Capricorn Coast region. Humpback dolphins were found in three different sites: Port Curtis, Keppel Bay and the Northern Region of the study area. In contrast, snubfin dolphins were found only in the Keppel Bay study area. Both humpback and snubfin dolphins were found year round with no significant variation among sampling periods and between seasons. Similarly, no substantial variation was found in pod size and composition among sampling periods and seasons. Based on the distribution of resighted individuals and the pattern of associations, it was established that humpback dolphins in the Capricorn Coast were grouped into three largely geographically distinct communities, referred to as the Port Curtis, Keppel Bay and the Northern Region communities. The interaction recorded among communities was lower than the level of association within communities. However, interaction among some members of different communities and movement of individuals between sites was recorded. Analysis of molecular variance showed high gene flow among Port Curtis and Keppel Bay study sites, which corroborates results from photo-identification data and social structure analysis. Very low gene flow was found between humpback dolphin populations from the Capricorn Coast and the Great Sandy Strait, which suggests that humpback dolphins from the Capricorn Coast form a separate management unit, which is geographically and mostly genetically isolated from populations elsewhere. Interestingly, migration rates for humpback dolphins between the Northern and Southern Great Sandy Strait was low, and significant differences in microsatellite loci frequency were evident between these communities. Abundance of humpback dolphins was estimated for the entire population and by geographical area using open population models. Estimates for Port Curtis, Keppel Bay and the Northern Region indicate that about 85 (PC = 85.1, SE = 4.36, 95% CI: 76.64–93.68), 107 (KB = 107.08, SE = 4.87, 95% CI: 97.53–116.5) and 64 (NR = 64.1, SE = 4.82, 95% CI: 54.66–73.46) humpback dolphins used those areas respectively during the study. The total humpback dolphin population size in the Capricorn Coast region was estimated to be 256 (Ntot= 75.80, SE = 8.12, 95% CI = 240.4–271.9), of which 178 (Nma= 178.1, SE = 4.14, 95% CI: 169.9–186.1) were adults. In contrast, snubfin dolphins were found only in Keppel Bay, and social structure analysis did not provide any evidence of structure in the population. This population appears to be geographically and demographically isolated. Genetic evidence based on a small sample size suggests that this population is genetically isolated from populations in North Queensland. A review of sighting information indicates that the Keppel Bay snubfin dolphin population is the southernmost population of snubfin dolphins along the Queensland coast. Population estimates indicate that about 74 snubfin dolphins (SD = 74.03, SE = 4.14, 95% CI: 65.91–82.09) live year round in Keppel Bay