Monitoring the resilience of a no-take marine reserve to a range extending species using benthic imagery

IMAS, as part of NESP Hub-related research, have been monitoring reefs on Tasmanian east coast over the past decade or more, using an Autonomous Underwater Vehicle supplied by IMOS. The vehicle takes high resolution images of the seabed at a wide range of depths, especially at depths below which we can survey via our standard dive-based monitoring programs. these images are analysed to generate biodiversity and habitat data from the shallowest reefs down to the deepest parts of east coast reef systems (in excess of 60 m). In this paper we use this data to examine the extent that urchin barrens formed by overgrazing by the extension of Centrostephanus rodgersii (Long-spined urchin) has impacted east coast reef systems, how this varies with depth, and how this has changed with time. Importantly we also contrast the response between a no-take marine reserve at Governor Island (off Bicheno) and identical habitats in adjacent fished areas. We found that the large lobster population that has built up in the MPA has helped the reserve be resilient to the urchin invasion, with barren being far less common in the reserve than outside. Despite the good news that natural populations in protected areas are able to offer significant resistance to urchin invasion, there is also the bad news that urchin barrens are continuing to increase through time on eastern Tasmanian reefs, and their cover has essentially doubled over the five year period of the surveys (2011-2016). This equates going from around 0.3% to 0.8% cover within the park, and from around 3% to around 7% outside the park over that time period (although this varied between locations surveyed). Dr Nick Perkins and colleagues used a complex modelling approach to deal with a range of spatial issues in the sampling design to ensure the significance of the patterns observed was tested properly. This design is probably the most important part of the paper from the science perspective, but the ecological results have important implications for MPA, off-reserve biodiversity and fisheries management.

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