June 25, 2018

Scientists Scott Nichol and Neville Barrett on the TV Bluefin during mapping of the Beagle Marine Park
Hub scientists Scott Nichol and Neville Barrett are leading a three-part survey of the Beagle Marine Park. Their June mapping survey on the TV Bluefin will be followed up in July and August with biologial sampling using baited remote underwater video, towed video and an autonomous underwater vehicle collecting seabed imagery.

Scientists from Marine Biodiversity Hub partner agencies the University of Tasmania Institute for Marine and Antarctic Studies (IMAS) and Geoscience Australia (GA) are out mapping the seabed in the Beagle Marine Park. Neville Barrett of IMAS and Scott Nichol of GA took time out while sheltering from wild weather to answer questions about their Bass Strait adventure.

What areas are you mapping?

We’re mapping the seabed in the Beagle Marine Park, which bridges the Tasmanian and Victorian borders in north-eastern Bass Strait. Much of the mapping is taking place between the Hogan and Kent groups of islands – stepping stones between Wilson’s Promontory and Flinders Island – and Curtis Island further west. (The Hogan Group was the first group of islands to become isolated by rising sea levels after the last ice age.)

A map showing survey ares in the Beagle Marine Park and changes in sea level
How old are Tasmania's coastlines? The areas being mapped today by the Bluefin are where people walked and hunted just 13,000 years ago. Before that time, the islands of Tasmania, King Island and Flinders Island were land-locked. They would have formed prominent high ground on the landscape, and important refuges for hunter-gatherers. Archaeological records show that by 8000 years ago, after sea-level had stabilised, resources and populations became established on the new coastlines. These changes are examined in a study led by Alan N. Williams of The University New South Wales Climate Change Research Centre that explored the response of past Aboriginal societies to changing sea-level. The review of sea level and seafloor data highlighted the extensive inundation of Bass Strait, with flooding beginning in the west 15,000 years ago, and leaving an ever-narrowing eastern land-bridge until 11,000 years ago, when the island of Tasmania was isolated.

Why are you mapping these areas?

The motivation for mapping is twofold. First, it is to build insight into a previously unknown environment. Our aim is to map the major habitat and geomorphological features of the seabed that otherwise lie hidden below the sea-surface. By using multibeam echo sounder (MBES) swath mapping we can visualise the seabed in 3D over extensive areas, providing maps that can be appreciated by the wider community.

Secondly, the mapping will be used to plan subsequent biological surveys to further our understanding of the biodiversity values of the marine park. (These surveys will take place in July and August 2018.) Distinct features such as reef and sand provide habitat for marine life, so are logical places for us to deploy biological sampling equipment such as baited remote underwater cameras, remotely-operated vehicles and towed cameras.

Furthermore, the mapping and biological surveys will support management and monitoring by Parks Australia, and demonstrate a viable and affordable approach to inventory and monitor other Australian Marine Parks. This includes providing a basis for evaluating the effectiveness of habitat protection zoning relative to adjacent fished areas.

Two seabed maps showing advances in mapping resolution between 2003 and 2018
Fine-scale mapping in the Beagle Marine Park is providing the basis for planning upcoming biological surveys. It is bringing into focus seabed habitats favoured by marine life, such as locally rare outcropping ridge features likely to be an important substrate for sponge gardens.

What will the mapping reveal?

For much of the marine park there is very little mapping at a resolution that reveals seabed features such as outcropping reefs, or mobile bedforms (geomorphology). Historical naval mapping has outlined isolated ridges/reef outcrops between the Kent and Hogan island groups, but the detail is insufficient to describe these features adequately or to plan biological surveys.

The new mapping is at metre scale resolution horizontally, and much finer vertically so can reveal an extensive degree of detail for this region (see maps). This fine resolution allows reef to be differentiated from sediments, and low profile reef to be differentiated from more fractured/complex reef. As these habitat characteristics (and more) are important in structuring the biological communities present, the information is of great value in planning biodiversity surveys. This fine-scale information is also invaluable in interpreting the bedforms present, and the processes that may have formed these historically.

Scientists on the vessel Bluefin during a voyage to map the Beagle Marine Park
Mapping specialists from Geoscience Australia onboard the Bluefin. Justy Siwabessy works on multibeam sonar data acquisition and processing and Rachel Nanson assists with data acquistion and the interpretation of multibeam bathymetry data.

What you are hoping to find?

We are hoping to develop an adequate description of the seabed geomorphology and habitat distribution in the park, as well as furthering our understanding of the geology of Bass Strait and the nature of the land-bridge that once joined Victoria to Tasmania. Certainly land-bridge features are of interest (although not a central driver of the survey) as is any new understanding of the variety of seabed features and habitats in this region.

How is the mapping being done?

We are using a state of the art Kongsberg 2040c multibeam sonar fitted to the University of Tasmania (Australian Maritime College) Training Vessel Bluefin. This system maps a swathe of four to five times the water depth, which during this voyage is about 60 metres. The swathe is therefore 250 m wide and about 200 m per “pass” with some overlap. It’s a somewhat slow process to build up a broad area map, but we covered more than 240 square kilometres in our first three days.

Seabed map showing ridge features in a section of the Beagle Marine Park
What has been the highlight so far? After lots of fairly flat seabed, by Wednesday afternoon (20 June) we had started mapping the area containing the ridge features, and these highly detailed maps reveal insights into their possible formation. Their eroded nature and shape suggests that they may be solidified dune features but this needs more work verify. Similarly, the eroded nature of other areas of seabed suggests bedrock may outcrop more than initially thought. If the raised reefs are relict dunes, they would have formed when Bass Strait was exposed as dry land during the last ice age.

Are there any particular geological features of interest?

Scott Nichol and Neville Barrett deploy the sub-bottom profiler from the TV Bluefin
Deploying the sound velocity profiler, a device for measuring how sound velocity (and therefore the sonar pings) changes with depth and in differing water bodies.

Before survey we were aware that the area was likely to be an extensive area of soft sediment with potential for some very limited outcropping of rocky ridges of unknown origin. The mapping up to day four suggests that this is the case, although there may be more widespread reef outcrop (eroded pavement), but at very low profile. However, some of the potential low profile reef area will require future validation using additional equipment. The linear and eroded nature of the outcropping ridges suggests they may be relict dune features.

Outcropping ridge features are of particular interest as they are both rare in this region and likely to be an important substrate for sponge gardens, a key biological feature underpinning the initial planning for this marine park. Understanding their nature and spatial distribution is therefore a central focus, as is making a quantitative inventory of the proportional cover of the key habitat types in the park.

What is a relict dune?

A relict dune is a former terrestrial sand dune that has survived sea-level rise and is likely to be composed of cemented sand that allowed the dunes to harden prior to eventual inundation as sea levels rose. So some of this area would have been dune fields prior to inundation of the land bridge. Not surprising given it would have been very sandy and windy during the last few glacial periods! These are somewhat analogous to the drowned dunes that we mapped in earlier surveys of the Freycinet Marine Park, but are nowhere as near as extensive. The dunes we see here are much sharper, in some places exposed and rocky.

Mapping the SS Bulli wreck

A seabed map showing the outline of the steamship Bulli which was wrecked off Erith Island in 1877
What have been the biggest challenges? The main challenge for the mapping team has been the weather, with a four-day delay at the start of the survey due to gale force winds and further time hiding out behind Deal Island due to unworkable sea conditions. But this is mere inconvenience compared with the ordeal of the Bulli, a steamship that hit a submerged rock while rounding Erith Island during rough seas in June 1877. (Fortunately the 27 crew found safety at the nearby Deal Island lighthouse.) The Bulli has since become a dive site and its position has just been mapped by the Bluefin team.

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