Image scoring
Urchins, and urchin barrens, are restricted to rocky reef (e.g., Johnson et al., 2005) and therefore we restricted our analysis to reef. Prior to analysis, all images that contained greater than 50% sand were excluded, and then each scored transect was subsetted to every fifth image. The subsetting of images was necessary as AUV images overlap and it was determined that every fifth image allowed minimal or no overlap of images for further analysis. This subset of imagery resulted in a total of 20 602 images in the final time-series of data for analysis.
Scoring followed the methods outlined in Perkins et al. (2015), where individual images were classified as barrens when greater than 50% of the image was bare-rock barren and C. rodgersii urchins were seen to be either present in the image or in an immediately preceding or succeeding image that was also more than 50% bare rock. For subsequent analysis, an urchin barren was considered either ‘present’ or ‘absent’ at the image level based on this scoring.