Marine Protected Areas
For school projects
Department of the Environment and Heritage, February 2003
The Great Australian Bight is a very large bight (or bend in the coast that forms an open bay) on the southern edge of the Australian continent. Aboriginals have lived on its shores since time immemorial. Europeans first explored it in the nineteenth century, with whalers, sealers, farmers and government surveyors entering the region following initial surveys by Matthew Flinders (1774-1814) and Edward Eyre (1815-1901). The Marine Park is in the middle of the Great Australian Bight.
Like other areas of seabed near our coastline, the bottom of the Great Australian Bight is part of the continental shelf (the shallow seabed next to the land) of Australia. This picture or diagram shows the names of the main parts of the edge of a continent like Australia. It is an imaginary side-on slice or section through a typical part of the continental shelf. In reality, the seabed is not as smooth and does not slope as steeply as it appears in this diagram.
The shallow continental shelf in the Bight is very wide. In some places the shelf break, where the water is around 200 metres deep, is over 100 nautical miles (or around 190 kilometres) away from the coastline. Beyond the shelf break the seabed slopes down gradually to the abyssal plain, where the water is around 4 000 metres (4 kilometres) deep.
The seabed in the Bight includes the following interesting features:
The seabed of the Bight is made of layers of sediment like mud and sand. Deep below the seabed the sediment has been pressed into sedimentary rocks. These sedimentary rocks can tell us about how the climate has changed in the past and how the southern edge of Sahul (the ancestor of the Australian continent) formed by rifting from eastern Gondwana, now called Antarctica. At the moment, engineers are trying to work out if these rocks are trapping layers of petroleum. They think there may be oil or gas under the Ceduna Terrace.
The Great Australian Bight is an unusual environment. The coastline is aligned with the equator, meaning that it runs mostly from east to west instead of from north to south. It is the longest ice-free coastline running in this direction in the Southern Hemisphere.
Little rain falls on the Nullarbor Plain and no major rivers flow into the Bight. Very little sediment has washed from the land onto the continental shelf. Instead the sediments on the seabed build up mainly from particles such as seashells settling out of the seawater. This has helped to protect and build up layers of sedimentary rocks that can tell us about what the climate was like in the past. For example, scientists can tell that Antarctica first started to freeze tens of millions of years ago, after Sahul had separated completely from Gondwana, allowing an ocean current called the Antarctic Circumpolar Current to form.
In many parts of central Australia it is possible to find springs of underground or artesian fresh water. There are probably springs of artesian water beneath the Great Australian Bight.
Like the Southern Ocean, strong winds drive across the Great Australian Bight and the sea is usually very rough. In the summer hot, dry winds blow from the deserts of central Australia over the Bunda Cliffs, which form the northern coastline of the Bight. They help evaporate more water from the sea surface, increasing the amount of salt in the surface waters. Often the winds blow from the right direction to force cold water to well up from the bottom to the surface near the Eyre Peninsula, which forms the eastern coastline of the Bight. This brings nutrients near the surface where marine plants can grow and support food chains.
The Bight is a place where different types of seawater mix. Strictly speaking it is part of the south-east Indian Ocean, but most people think of it as being part of the Southern Ocean. Strong winds affect the patterns of flow in the Bight, which are also influenced by several important currents. Near the coastline the main current is close to the surface and flows from west to east. During the winter it extends the Leeuwin Current, which wraps around south-western Australia and brings warm tropical waters from the Timor Sea to the northern part of the Bight. Sometimes tropical animals like turtles are washed into the Bight. Further offshore the main current is deeper and flows from east to west. This current is called the Flinders Current. It brings cooler waters and nutrients into the southern part of the Bight. The cool waters are channelled upwards along underwater canyons and carry nutrients onto the southern edge of the continental shelf where they encourage plants and animals to grow. In turn these plants and animals can support food chains. In addition, large eddies and gyres are common and cold surface water from the Southern Ocean mixes with the waters in the Bight.
Many large animals live in or visit the Great Australian Bight. The most famous are the southern right whale, the Australian sea lion, the great white shark, and the southern bluefin tuna.
Scientists think that around 600 to 800 southern right whales regularly visit the southern coast of Australia. They are not sure whether this population mixes with other populations that visit the southern coasts of Africa and South America. In the past every winter hundreds of southern right whales used to visit protected bays in New South Wales, Victoria, Tasmania, South Australian and Western Australia to give birth to calves and to breed. However this made it easy to hunt them and whaling reduced their numbers. Now the main calving sites are in South Australia and Western Australia.
Each winter around 40 to 50 but sometimes as many as 100 southern right whales swim from the Southern Ocean into the large bay at the northern end of the Bight, which is called the Head of the Bight. There the females give birth to calves, producing around a third of all southern right whale calves born in Australian coastal waters each year. Some of these whales spend the whole winter season at the Head of the Bight. They often move close to the shore and rest at the surface. The shallow water may help to protect the new-born calves from predators like the white shark. Adult whales may be attracted to shallow areas where there are springs of fresh water on the seabed, which may help to kill parasites. Visitors can easily see the southern right whales from a viewing platform on the cliffs at the Head of the Bight.
There are colonies of the Australian sea lion along the Bunda Cliffs and islands in the Great Australian Bight, including the largest mainland colony. These colonies are sheltered places where the sea lions come ashore to rest and to breed. The Australian sea lion is Australia's only endemic (which means "occurring nowhere else in the world") pinniped (which means animals like seals and sea lions). Around 700 Australian sea lions may live in the Great Australian Bight, or about 6 per cent of the total number of Australian sea lions. The Australian sea lions that live in the Bight help to bridge the gap between those that live on the east coast and those that live on the west coast. However scientists think that Australian sea lions usually do not travel from one region to another.
Seals and occasionally whales are an important food for the white shark, which is sometimes called the great white shark or white pointer. White sharks are among the largest predators in the oceans, growing to lengths of at least 6 metres. They are warm-blooded and may live for a long time. Little is known about their behaviour or how many there are, but scientists think they are becoming rare. Within the Great Australian Bight sometimes it is possible to see several of them at one time in areas such as Nuyts Archipelago, or near a whale carcass. Although they can travel long distances it is likely that some sharks return to the same seal colonies each year to feed. Some scientists think that there are fewer large adults than there used to be. White sharks used to be caught for sport and are sometimes accidentally caught in fishing nets.
The southern bluefin tuna is a large fish that travels long distances during its life cycle. Young tuna gather in schools in the shallow waters off southern Australia. Many visit the Great Australian Bight during summer. Southern bluefin tuna is good eating. Fishermen use nets towed behind trawlers to trap the young tuna. They tow the nets back to Boston Bay near Port Lincoln and put the tuna into underwater cages called fish farms. They feed the tuna on other fish they have caught. When the tuna are large enough they are sold, often to overseas markets such as Japan, where they fetch high prices.
Many kinds of seabirds, fish and marine mammals live in or visit the Great Australian Bight. Blue whales, humpback whales and sperm whales travel through the Bight. New Zealand fur seals feed in the Bight, as do dolphins and other toothed whales. Squid, octopus and cuttlefish are important food for these animals.
There are also many other species in the Bight that are important as food for humans, especially southern rock lobster, king crab, abalone, school shark, gummy shark, deepwater flathead, Bight redfish and orange roughy. Every year school and gummy sharks travel into the shallow waters at the northern end of the Bight to give birth to pups. Orange roughy is a deep-sea fish that scientists think can live for a very long time.
The seabed of the continental shelf in the Great Australian Bight is home to a wide variety of plants and animals. Many of them are found nowhere else in the world. In fact, the Great Australian Bight has some of the highest levels of marine diversity (which means the variety of plants and animals) and endemism (which means "occurring nowhere else in the world") in Australia. There are large numbers of species of red algae (sea weed), ascidians (sea squirts), bryozoans (similar to corals), molluscs (shellfish) and echinoderms (sea urchins and sea stars). Scientists find new species almost every time they set up a voyage to sample the seabed in the Bight.
Scientists have developed maps to show the locations of different types of ecosystems in Australia's marine environment. One set of maps is called the "Interim Marine and Coastal Regionalisation for Australia". The map for the continental shelf in the Great Australian Bight includes three of these ecosystems.
Australian laws protect many of the animals of the Great Australian Bight. They list the southern right whale and Australian sea lion as "endangered" species, and the great white shark as a "vulnerable" species. It is against the law to kill or collect these animals unless you have special permission.
Fisheries managers protect the wild stocks of fish that provide us with important food and income. Southern bluefin tuna and orange roughy fisheries managers set quotas that limit how many fish can be taken. Recently fisheries managers decided to stop people from catching school shark in coastal waters to allow populations of these fish to build up again.
The Commonwealth and South Australian Governments have established the Great Australian Bight Marine Park to protect the important habitats of the Great Australian Bight.
You can find information about the Great Australian Bight and especially the Marine Park at these websites: