Population Viability Analysis (PVA) for Three Species of Coastal Dolphin in Darwin Harbour

Three dolphin species inhabit the coastal waters of the Darwin region: the Australian snubfin dolphin (Orcaella heinsohni), Australian humpback dolphin (Sousa sahulensis), and the bottlenose dolphin (Tursiops sp.). Populations of all three species were monitored from 2011-2015 and 2016-2019 as part of the approvals process and voluntary offsets relating to the INPEX-operated Ichthys liquified natural gas project. A population viability analysis (PVA) was conducted for all three species with the aim of identifying knowledge gaps in our understanding of their life-history parameters and directing future information requirements to enable assessment of population trends Population viability analysis is particularly useful when multi-decadal quantitative datasets are unavailable, and is thus a powerful tool in conservation management of long-lived species with low rates of reproduction. The primary objectives of this study were threefold: (1) build a base model for PVA of all three species of coastal dolphins, (2) utilise sensitivity testing to determine which vital rates are most responsible for driving changes in population growth rates, and (3) identify knowledge gaps in our understanding of local population dynamics and life-history parameters that can potentially be targeted in future research and/or monitoring programs. We conducted PVAs for each species of dolphin in the software program VORTEX, predicting trends over the next 100 years, and using 1000 iterations to encompass a range of stochastic variation. Sensitivity tests were then used to investigate the degree to which five different vital rate parameters (immigration, emigration, calf mortality, non-calf mortality, and birth rate) influenced the population growth rate of each species. Base PVA scenarios using demographic rates from the Darwin Region Coastal Dolphin Monitoring Program, supplemented with estimates from the published literature, projected population declines for all three species, with projected decline being least severe for the Australian humpback dolphin and most severe for the bottlenose dolphin. Sensitivity testing suggested that rates of immigration, emigration, and non-calf mortality were most influential in determining the population growth rate.
We highlight several challenges in constructing and interpreting PVAs for coastal dolphin populations and highly mobile species generally. A key issue across all three species is the need to understand population boundaries and the connectivity of Darwin Harbour region dolphins to populations outside the Darwin Harbour region. The core PVA output is an extinction probability for each species, which was predicted as negligible for the Australian humpback dolphin, moderate for the Australian snubfin dolphin and high for the bottlenose dolphin. However, information on population connectivity (which could be provided with genetic analysis) and demographic rates of dolphins in the broader ‘meta-population’ outside the Darwin Harbour region is required to refine the models and accurately interpret PVA projections of future population trends. Further, snubfin and humpback dolphins are relatively unstudied and there is a need for better estimates of vital rate parameters and their temporal variability for these species. Overall, the PVA has provided a framework to use estimates from the monitoring program to project population scenarios for coastal dolphins and has highlighted several critical areas where research is required to better understand the broader spatial and temporal dynamics of dolphin populations in the Darwin region.