September 9, 2020

A rock lobster and two long-spined sea urchins
Southern Rock Lobster, the main predator of the Long-spined Sea Urchin, was found to exist in higher densities and at larger average sizes in areas where fishing is excluded. Image: Emma Flukes

Marine monitoring has shown that Tasmania’s Governor Island Marine Reserve offers some protection against the Long-spined Sea Urchin, which is expanding its range in warming waters off eastern Tasmania and creating barrens habitat through overgrazing of kelp.

A Marine Biodiversity Hub team led by Nick Perkins of the University of Tasmania Institute for Marine and Antarctic Studies (IMAS) analysed seabed imagery collected by an autonomous underwater vehicle (AUV) inside and outside Governor Island Marine Reserve which has been closed to fishing for almost 30 years.

“The surveys were conducted between 2011 and 2016, covering areas of rocky reef in the Bicheno region where the invasive Long-spined Sea Urchin (Centrostephanus rodgersii) has been increasing in density,” Dr Perkins said.

An autonomous underwater vehicle being deployed from a  vessel
Deploying the autonomous underwater vehicle off the coast of Bicheno, eastern Tasmania. Image: IMAS

“With the AUV we collected high resolution imagery of the seafloor to depths greater than 30 metres, which are not typically sampled by methods such as SCUBA diving. GPS locations for each image are also collected and can be matched to seafloor maps created using multibeam sonar.

“This allowed us to track the changes to rocky reef ecosystems through time, gave us valuable insights into changes in biological communities across a wide range of depths and allowed us to match barrens occurrence to different levels of reef complexity."

Published in PLoS ONE, the study indicated that the absence of fishing effort for extended periods provides resilience to barren formation, although barrens still develop.

“In 2016, barrens inside the reserve covered less than one per cent of the rocky reef surveyed, while sites outside the reserve had three to 10 per cent barrens cover in that year,” Dr Perkins said. “This may be because Southern Rock Lobster, the urchin’s main predator, exist in higher densities and at larger average sizes where fishing is excluded, which results in increased urchin predation."

Urchin barrens on the rocky seafloor
Overall barren cover doubled over the five-year survey period regardless of protection, inside and outside the reserve. Image: Emma Flukes

While the study found that areas inside the reserve may be more resistant to the establishment of barrens, Dr Perkins said there was no difference in the rate of increase of barrens inside and outside the reserve over the survey period.

“The overall barren cover doubled over the five-year survey period regardless of whether areas were protected or not, which raises serious concerns around the likely longer-term trajectory of barren formation, especially as previous IMAS research shows that, once established, recovery of barrens habitat is problematic,” Dr Perkins said.

The study also found depth and surface roughness (derived from multibeam mapping) are useful predictors for the occurrence of barrens. This knowledge can be combined with detailed multibeam sonar surveys that can provide a detailed 3D understanding of seabed complexity.

“We found that using multibeam seafloor mapping in the study areas enabled us to map out the spatial variation in reef depth and surface roughness, and to use this to more accurately predict where urchin barrens are more likely to form,” Dr Perkins said.

“This technology could be used over large sections of coastline to contribute to urchin management approaches being undertaken by the Tasmanian Government, such as informing where commercial urchin harvesting may be best targeted to help control the expansion of barrens.”

The results demonstrate the utility of AUV data, and the value of mapping, monitoring and no-take reserves to local conservation management.

Related information

Fished and sponge gardens on a rocky reef
Diverse sponge gardens on rocky reefs in the Governor Island Marine Reserve. Image: Emma Flukes
Urchin barrens on the rocky seafloor
Extensive urchin barrens in the Kent Group off north-eastern Tasmania. The Long-spined Sea Urchin has expanded its range from native habitat in New South Wales over the past 40 years, in warming waters largely associated with changes to the East Australian Current.  Image: Emma Flukes