Conclusions
We demonstrate that a time-series of imagery from an AUV was able to
detect a strong and consistent protection effect on the presence of
urchin barrens in a study region on the east coast of Tasmania. Although
the NTR has lower rates of barrens presence we also found the odds of
barrens presence increasing at approximately 31.9% per year in both NTR
and reference sites. Therefore, there is currently strong evidence for
the resistance of the NTR to barrens formation, but importantly there is
also no evidence of recovery. While barrens cover is currently
relatively low inside the NTR, a slowing of the rate of increase or a
recovery of kelp habitat may be necessary to prevent an eventual phase
shift to persistent barrens. With ongoing climate change related warming
and incursion of urchins into new areas, monitoring the increase in
barrens is critical to informing adaptive management and developing
innovative solutions. To support these efforts, we also report that
measures of reef complexity and depth from multibeam sonar mapping
provide important predictors for the occurrence of barrens. Therefore,
detailed seabed mapping and spatial analysis may help in spatial
management when targeting efforts to reduce urchin numbers via
mechanisms such as spatially targeted fishing incentives for urchin
divers. Finally, we advocate the use of spatio-temporal models such as
those presented here when assessing time-series of benthic imagery due
to their flexible nature and ability to incorporate spatial and temporal
correlation inherent in imagery data.