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.