Mapping marine invaders

December 2, 2013 - via Australian Maritime College

The extent of sea urchin barrens along the east coast of Tasmania could soon become clearer through a new mapping technique being developed by researchers at the University of Tasmania.
Dr Alex Forrest from the Australian Maritime College and Dr Vanessa Lucieer from the Institute for Marine and Antarctic Studies are collaborating on a research project that combines their expertise in autonomous underwater vehicle (AUV) technologies and acoustic mapping to create an accurate picture of where these barrens are located.
The long-spine sea urchin, Centrostephanus rodgersii, is an invasive species which has hitched a ride in the warmer waters of the eastern Australian current and extended its range along Tasmania’s east and south coast. There is currently no detailed baseline map of the ‘barrens’ habitat it creates by overgrazing kelp and other seaweeds.
“You can’t get a handle on where the species is likely to create barrens into the future without having a map of where it is now,” Dr Lucieer said.
“These urchins eat everything on the reef and have the capacity to denude it of virtually all living forms. The barrens represent a new and stable ecological state devoid of macroalgae and many of the fish usually associated with rocky reefs, significantly impacting important fisheries like abalone and rock lobster.
“So we want to create a baseline map, and from that baseline we can start correlating other variables about the seafloor and the water column, the oceanographic processes, tides and currents, and we can start to build an ecological model of how this invasive species is living in the system and where it might move.”
Dr Lucieer and Dr Forrest recently returned from a pilot research trip on board AMC’s training vessel MV Bluefin, where they spent three days gathering data along the coastline north and south of Wineglass Bay using the AUV DSTO-Gavia. The aim of the trip was to demonstrate the technology and prove the mapping technique before applying for a larger grant through the Australian Research Council.
“The AUV uses sonar to map the urchin barrens by sending out acoustic transmissions from its sensor head which travel through the water column and bounce off the seafloor,” Dr Forrest said.
“We then use these thousands of collected depth soundings to construct maps of the seafloor and areas of potential vegetation. By classifying different bottom types, in addition to identified areas of reduced vegetation, we are then able to hypothesize where urchin barrens might be located.”
The DSTO-Gavia was deployed for just under 10km in range and run at different settings to determine the optimal parameters for capturing the data required.
The mapping of urchin barrens was previously done by divers locating them by sight and by towed underwater video. These observations were combined with broadscale seafloor mapping data from the Seamap Tasmania project. While the divers could typically cover a few hundred metres in a day, the AUV is capable of covering up to 30km during the same period.
“This new technology of the AUV acoustics allows us to map at 10 times the resolution that we could map five years ago,” Dr Lucieer said.
“We can map the details of each rock, which is what is required to identify the reefs at the scale at which the urchins are responding. We’ll have the precision and the classification to say how many barrens are there, how denuded an area is and the spatial extent of the area.
“What we want to know is the pattern of where the barrens are occurring. Why are they on some reefs and not others?”
The research trip was made possible by the loan of the DSTO-Gavia from the Defence Science and Technology Organisation. This AUV is fitted with a bathymetric mapping unit that allows acoustic measurements to be collected from the water column. Mike Bell from DSTO brought the equipment to Tasmania from Sydney and subsequently helped operate it on the Bluefin.
One of the long-term aims is to purchase a similar unit for UBC-Gavia, the AUV housed at AMC and operated by Dr Forrest, for further operations.

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