Mapping the seabed

by , | Jun 30, 2024 | Underwater worlds

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During the last glacial period, much of Bass Strait was a vast sandy and windswept plain. In the eastern region, this formed a continuous land bridge between what is now Lutruwita/Tasmania and Victoria, including the prominent landscape features of Yiruk/Wamoon/Wilsons Promontory in Victoria, Hogan Island, Tayaritja/the Furneaux Group of Islands (including Flinders Island) and Cape Portland in Lutruwita/Tasmania.

While the islands themselves would have been rocky hills providing significant refuge, large stretches of this land bridge were open sandy plains with very little shelter from the elements for the Old People of the time.

However, recent seabed mapping has revealed the presence of a hidden reef system that would once have been a long rocky ridge offering protection from freezing ice age winds for people travelling between the peaks that are now Deal Island and Hogan Island.

Nowadays the area around Hogan Island marks the state boundary between Victoria and Lutruwita/Tasmania. But around 15,000 years ago, at the end of the ice-age when sea-levels had begun to rise, it marked the lowest lying part of what was by now the last slender remnant of the land bridge. If people wanted to cross between Victoria and Lutruwita/Tasmania by foot, they had to use this route, passing by the hills of Hogan Island and Deal Island on their way to Tasmania’s northeast. When the land bridge was sundered by the sea, this land flooded first, separating Lutruwita/Tasmania and the Bass Strait islands from the rest of Australia.

The submerged land bridge crossing section between Hogan Island and Deal Island now forms part of the Beagle Marine Park, a marine protected area in the eastern Bass Strait.

Hogan Island in the Bass Strait. Photograph by IMAS.  CC BY-NC-ND 4.0.

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About the Authors

  • Associate Professor Neville Barrett is a marine ecologist at the Institute of Marine and Antarctic Studies at the University of Tasmania. He is actively working to understand the natural values within the Australian Marine Park network and to establish ongoing monitoring programs to inform ongoing sustainable management.

  • Dr Jacquomo Monk is a marine conservation ecologist and specialist in underwater imagery at the Institute for Marine and Antarctic Studies at the University of Tasmania. Alongside Associate Professor Neville Barrett, he leads a research team monitoring and mapping the deepwater ecosystems around temperate Australia.

With thanks to

​This research was undertaken for the Marine Biodiversity Hub and Marine and Coastal Hub, collaborative partnerships supported through funding from the Australian Government’s National Environmental Science Program (NESP). With thanks to the Institute for Marine and Antarctic Studies (IMAS), Geoscience Australia, Australian Centre for Field Robotics (ACFR), the Australian Integrated Marine Observing System (IMOS), and Parks Australia.

Map of the Beagle Park showing the period of lower sea levels during the last ice age when the area was a land bridge between Lutruwita/Tasmania and Victoria. The areas boxed in white indicate where seafloor mapping was undertaken in 2018. Original map source showing sea level rise by Alan N Williams, UNSW. Courtesy of IMAS. CC BY-NC-ND 4.0.

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Beagle Marine Park lies in the Bass Strait between Victoria’s Yiruk/Wamoon/Wilsons Promontory and Tasmania’s Flinders Island in Tayaritja/the Furneaux Group of Islands. Map courtesy of Parks Australia. CC BY-NC-ND 4.0.
In 2018, while undertaking surveys of the Beagle Marine Park with Geoscience Australia using multibeam sonar to make three dimensional maps of the seabed, we recorded an underwater reef system which in the time of the land bridge would have formed a low-lying ridge on the otherwise sandy flats between Hogan and Deal Island.

The reef is now 60 metres below the surface of Bass Strait and is home to a diverse array of marine species. But it was once a series of sand dunes. The signature parabolic U-shapes of windswept dunes are part of the reef and still clearly preserved. Over the ages the dunes turned into a limestone-like rock in a process called calcification, where sand slowly turns into stone after being exposed to freshwater. When these hardened dunes eroded, they formed a series of ledges and caves and boulder fields extending over 15 kilometres, to a width of approximately 300 metres and a height of around 5 metres. They likely provided a welcome respite from the elements for those on the Bassian Plain in the time of the land bridge.

Now lying in the central region of the Beagle Marine Park in the eastern entrance of Bass Strait, this reef system is like an oasis in a desert for marine species that need a hard rock on which to attach and grow without being swept away by the strong tidal currents that scour this area as water rushes in and out of Bass Strait.

While the sandy seafloor is utilised by marine life, it is relatively sparsely populated because it is so exposed to the currents. This is why it is like a desert. But the reef, like an oasis, is covered in life. It boasts a colourful array of sponges, ascidians –animals also known as sea squirts which permanently attach themselves to the reef, coral-like bryozoans, and corals, including large fan-like gorgonian corals which bend majestically as the current flows. The coral and sponge gardens draw a diversity of fish, shellfish and other marine life attracted to the shelter and food. In winter the reef also appears to provide shelter for a large group of Port Jackson sharks, which are grey blunt-headed seafloor-dwellers with prominent eyebrow ridges and brown stripes. They presumably migrate here to feed on a range of shellfish species on the adjacent sandy seabed at this time.

A Port Jackson shark rests in a rocky reef sponge garden on the Beagle Marine Park seafloor. Underwater image captured using an Automated Underwater Vehicle (AUV). Image by IMAS and IMOS. CC BY-NC-ND 4.0.

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Low lying calcified ridges, showing the typical U-shaped parabolas of sand dunes, lie on the Beagle Marine Park seafloor. Seabed mapped using multibeam sonar.  Image by Rachel Nanson, Geoscience Australia.  CC BY-NC-ND 4.0.

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The 2018 Beagle Marine Park mapping survey utilised multibeam sonar to find the seabed. Pictured are monitoring devices on the survey vessel Bluefin. Photograph by IMAS.  CC BY-NC-ND 4.0.
Rocky reef habitat in Beagle Marine Park as photographed by the Autonomous Underwater Vehicle (AUV). Photograph by IMAS and IMOS. CC BY-NC-ND 4.0.

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The underwater reefs are now filled with diverse marine life, like oases in the otherwise bare sandy seafloor. Image captured by the Baited Remote Underwater Video System (BRUVS). Image by IMAS.  CC BY-NC-ND 4.0.

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Rocky reef habitat in Beagle Marine Park. Photograph by IMAS and IMOS. CC BY-NC-ND 4.0.

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Associate Professor Neville Barrett next to the Autonomous Underwater Vehicle (AUV) used to photograph the seafloor. Photograph by IMAS. CC BY-NC-ND 4.0.
Weather, rain and waves on the Bass Strait on the survey vessel Bluefin. Photograph by IMAS. CC BY-NC-ND 4.0.
How do we know about the underwater reef and the diverse unique life it harbours? The multibeam sonar process in which we first recorded the reef involved sending sound pulses from a research ship above the water down to the seabed below. This enabled us to create a 3D map showing the contours of the seafloor. Once the map was created, we sent follow up surveys to explore the reef further. We sent underwater robot submarines, known as Autonomous Underwater Vehicles (AUVs) to travel through the water and record the life around the reef. We also sent video camera systems, called Baited Remote Underwater Video Systems (BRUVS), to the seafloor to record what animals were in the area of the reef. They visually described the biodiversity of both the reef system and the adjacent sandy seabed, including the cover of attached animals like sponges as well as the more mobile fishes and sharks.

Our survey was the first time the underwater world of Beagle Park had been mapped in detail. The data and imagery we collected can now act as a benchmark, or point in time reference, so that the health and diversity of the marine environment can be monitored again in the future.

The AUV imagery hosted on SQUIDLE+ and a selection of BRUVS footage, in addition to the seabed mapping, is now available to be seen on the public-access website Seamap Australia. This allows everyone with an interest in the ocean and marine parks the opportunity to explore the unique seabed and marine life of the Beagle Marine Park and share our appreciation of the values of this special place.

Seafloor sediments in Beagle Marine Park. Photographed using the Autonomous Underwater Vehicle (AUV). Photograph by IMAS and IMOS. CC BY-NC-ND 4.0.
Two technicians adjust the Autonomous Underwater Vehicle (AUV) before launching it into the Bass Strait at sunrise. Photograph by IMAS. CC BY-NC-ND 4.0.
Rocky reef habitat in Beagle Marine Park. Photograph by IMAS and IMOS. CC BY-NC-ND 4.0.
A distant island in the Kent Group of Islands, with Deal Island in the foreground. Photograph by IMAS. CC BY-NC-ND 4.0.

Copyright information

Released under a Creative Commons Attribution Non Commercial No Derivatives 4.0 International (CC BY-NC-ND 4.0) license.

This story is subject to disclaimers, copyright restrictions, and cultural clearances. Copyright © Neville Barrett, Jacquomo Monk, 2024.

Further reading

Barrett N, Monk J, Nichol S, Falster S, Carroll A, Siwabessy J, Deane A, Nanson R, Picard K, Dando N,  Hulls J & Evans H (2021) Beagle Marine Park Post Survey Report: South-east Marine Parks Network, Report to the National Environmental Science Program, Marine Biodiversity Hub, University of Tasmania, accessed 30 June 2024.

Friedman, A (2024) SQUIDLE+ Map, Understanding Marine Imaging Facility (UMI) & Integrated Marine Observing System (IMOS), accessed 30 June 2024.

Nanson R, Huang Z, McNeil M, Carroll A (2023) Seabed morphology and geomorphology of the Beagle Marine Park, south-eastern Australia – Version 1, Geoscience Australia, Canberra, https://dx.doi.org/10.26186/147976

Nichol S, Barrett N, Beaman, R, Siwabessy J, Spinoccia M (2019) Beagle Marine Park bathymetry survey 2018 (GA0364), Geoscience Australia, Canberra, http://dx.doi.org/10.26186/5d4cea0bdecb4

Parks Australia (2024) Beagle Marine Park, Parks Australia, accessed 30 June 2024.

Parks Australia & Australian Institute of Marine Science (2024) Beagle Marine Park, Australian Marine Parks Science Atlas, Parks Australia, accessed 30 June 2024.

Seamap Australia (2024) Data Report, South-east Region: Beagle, Institute for Marine & Antarctic Studies (IMAS), University of Tasmania, accessed 30 June 2024.

Seamap Australia (2024) Seamap, Institute for Marine & Antarctic Studies (IMAS), University of Tasmania, accessed 30 June 2024.

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