Species Distribution Modelling of Western Pilbara Inshore Dolphins

Daniella Hanf

This thesis presents the first insights into inshore dolphin distribution of the western Pilbara. The region is undergoing rapid coastal development, which has the potential to threaten Indo-Pacific bottlenose dolphin (Tursiops aduncus) and Australian humpback dolphin (Sousa sahulensis) populations. Understanding the distribution of these species is essential for their conservation. Species distribution models (SDMs) were developed using dolphin sightings data that were opportunistically collected during dugong aerial surveys. A geographical information system (GIS) was used to generate the training data, which consisted of the binomial presence-absence of dolphins, distances from mainland and islands, sea surface temperature (SST), ocean fronts and bathymetric derivatives. Preliminary models were developed using generalised additive model (GAM) and component-wise boosting techniques. Models could not be fit to the data using either technique. It was unclear whether this was a result of relatively few dolphin sightings across a large study area, pseudo-absences, weak environmental variables, or a combination of all of these factors. Maximum Entropy (MaxEnt) software was subsequently used as an alternative modelling technique to model the presence of dolphins, along with automatically generated background data, in order to avoid problems associated with unreliable absence data. Bottlenose and humpback dolphins were sympatric, with overlap in occurrence across the study area. Bottlenose dolphin presence was associated with the slope at the 20 m contour and waters around the Muiron Islands. This is likely to be a productive area that could be important for foraging. Humpback dolphin presence was associated with intertidal areas, including shallow coastal waters near the mainland and surrounding islands. The presence of numerous offshore islands would thus explain why humpback dolphins were recorded more than 50 km from the coastline. MaxEnt models were limited in their predictive power. Dedicated aerial surveys for inshore dolphins, using standardised techniques, are required to obtain reliable species data. In addition to increasing the sample size available for modelling, greater certainty in group size and composition could allow count, calf and mixed species group data to be modelled. Adequate species conservation needs to incorporate various ecological processes that occur at different spatial and temporal scales. Guidance is provided for undertaking boat-based studies and biopsy sampling, gathering opportunistic sightings data, and undertaking satellite telemetry research in addition to dedicated aerial surveys. Distribution modelling for inshore dolphins of northern WA is an intrinsically challenging research project, due to limited habitat data and the elusive nature of the subject species, particularly humpback dolphins. This research has obtained the first insights into the distribution of inshore dolphins in northern WA. Through lessons learnt, this research has paved the way for the development of future models to have a greater predictive ability, which will be useful for the conservation of threatened inshore dolphin species.

Species Distribution Modelling of Western Pilbara Inshore Dolphins

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