Rugosity from multibeam sonar bathymetric mapping
Habitat complexity is known to be highly correlated with C. rodgersii density (e.g. Flukes et al., 2012, Ling and Keane, 2018). We therefore expected a priori that rugosity, a measure of reef complexity, may be an important covariate to control for when analysing the presence of barrens. Our study region was mapped prior to this study in 2008/09 using multibeam acoustics as part of ongoing mapping by the National Environment and Science Research Program Marine Biodiversity Hub (Nichol et al., 2009). In order to generate a rugosity measure to use as a model covariate, a 1 metre resolution bathymetric layer from the multibeam sonar was used with the vector rugosity measure (VRM) tool in the Benthic Terrain Modeler 3.0 add-on in ArcGIS 10.6 software. This tool uses vector analysis to calculate rugosity as the variation in the three-dimensional orientation of cells within a specified neighbourhood. We used a neighbourhood of three cells (i.e. three metres) as urchins are likely to respond to complexity on smaller scales. As we expected rugosity to be an important predictor, we excluded the small amount of survey area covered by the AUV transects that did not have underlying multibeam mapping (see Fig. 1).