Spatial vulnerability assessment of anchor damage within the Great Barrier Reef World Heritage Area, Australia

The coral reefs and seagrass habitats in the Great Barrier Reef World Heritage Area (GBRWHA) are vulnerable to physical disturbances, including the anchoring of vessels. Both the anchor being deployed and retrieved, as well as the movement of the attaching rode, can cause damage to corals and seagrasses. Understanding the contributing processes that influence the deployment of anchors can assist with managing anchor damage in the GBRWHA, particularly in the context of climate change. Providing a spatial description of the vulnerability, rather than just a list of factors, requires the incorporation of social, geophysical and ecological factors. An integrated GIS-Bayesian Belief Network was utilised to combine 19 spatial datasets, 6 spatial models and expert opinion. The base scale was set to match the 250 m lattice interval of the Great Barrier Reef digital elevation model. With approximately 5 million data points the model was able to spatially describe the likelihood of damage from anchor deployment across the GBRWHA. While only 19% of the GBRWHA is considered susceptible to anchor damage, the assessment indicates that coral reefs and seagrass meadows adjacent to population centres and in particular close to islands are highly vulnerable. Comparisons with coral reef health surveys (Eye on the Reef Program) and detailed anchorage records from a scientific research vessel indicate the model is robust despite extensive use of disparate spatial data and expert opinion. The effect of each node in the Bayesian Belief Network on the anchor vulnerability beliefs was measured by standard variance reduction and this found that anchor site familiarity and accessibility were the dominant influences aside from the presence of sensitive habitat. Visualisation of the model outputs, including the intermediate stages, provided additional qualitative evaluation. Enhancing the vulnerability assessment to describe every location in the GBRWHA will contribute to the development of policy and governance mechanisms whilst supporting focused monitoring of sites vulnerable to anchor damage.