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Oceans Facts and Figures

A report commissioned by Environment Australia October 1997

Commonwealth of Australia
ISBN 0 642 54548 0

2. Habitats and inhabitants


Australia is one of only 12 megadiverse nations in which the ocean, like the land environment, provides a comprehensive range of habitats. These contain a wide array of species. Some are even considered `living fossils' particularly on the southern marine platform which has remained fairly stable over about 40 million years and which provides a good fossil record for many marine animals, including, for example, some bivalves, that are extinct elsewhere.

The patterns of diversity in the oceans are similar to those on the land, with greater numbers of species in the wet tropics and fewer in the temperate zones. For example, on coastal land, mangrove species decline from 30 in Queensland to none in Tasmania. In the adjacent ocean, the number of seagrass species go from 12 in the north to between 1 and 4 in the south.

For biologists, the great diversity of seaweeds found in Australian waters is unusual, reaching a total of 1 100 in the southern waters. This can be compared with 168 species in the Canadian Arctic and 750 in tropical West Africa.

A key to patterns of biodiversity in the ocean is water depth. For example, in the shallowest zones around the world, there can be found about 8 000 species of seaweed and this number gradually drops away as the waters get deeper, darker and colder. The richness and diversity of marine life, particularly light-dependent plant life such as seaweed and phytoplankton, is greatest in shallow water where exposure to full sunlight occurs. This is an important factor given the importance of these species as a basic food source in the marine environment.

The entire water column provides some form of habitable environment with different types of creatures finding niches at different levels, although, in deeper water, limiting factors such as decline in oxygen and increase in pressure come into play.

Scientific knowledge of deep sea communities is far from complete, however, there are some indications that the range of bottom-living fauna of Australian waters may be far richer than elsewhere. For example, recent samples show that in an area of 10 square metres in Bass Strait there was a massive 800 different species.

Box 2 - biodiversity

Australian waters contain some of the greatest marine biodiversity on the planet. This includes:

  • an extremely diverse fish fauna of over 4 000 species (representing almost 20 per cent of species recorded worldwide);
  • around 43 different species of whales, porpoises and dolphins;
  • a rich temperate marine environment, including rocky reef systems, giant kelp forests (up to 30 metres high) and a diverse range of marine species80 to 90 per cent of which are found nowhere else in the world;
  • a very high biodiversity of seagrasses (which are critical fish habitats) including 30 of the world's 58 species;
  • a great diversity of coral species, the largest area of coral reefs of any nation, and the largest coral reef system in the world, the Great Barrier Reef, which extends over 2 500 kilometres.

Coastal zones

The coast is where the land and the sea come together to produce a vast array of specialised animal and plant life. It is also a place where people flock for recreation, such as fishing, surfing and swimming. What is generally known as `the coast' ranges from the open coast with rocky headlands and sandy beaches, to more sheltered areas including bays and estuaries with muddy or sandy tidal flats. The entire length of the Australian coast, including nearby islands, is 69630 kms.

The shape of the Australian continent resulted from the break up of the Gondwanaland supercontinent and a series of mountain building episodes over many millions of years. Sea-level changes during this period etched the broad outline of the continental shelf. Rivers and ice moved sediment towards the edge of the continent, creating thick layers, the surface of which has been gradually moulded by the ocean. The present sea level is relatively high compared with the last several thousand years.

Overall, one of the greatest threats to the coastal environment is poor land management and the spread of human activities: urban sprawl and coastal development in general. There has been a general decline in the quality of coastal and marine water due largely to increased sedimentation and nutrient levels. This has often been a result of poor catchment management, sewer discharges or urban runoff. Some of the more serious implications are the die-back of seagrasses in temperate Australia and threats to inshore corals in the wet tropics.

It is well known that one of the great problems facing Australia's inland waterways is soil erosion. What is also becoming well known is that soil erosion causes problems downstream, in estuaries, bays and coastal waters, often with far-reaching impacts on marine life. As a result, integrated catchment management is now seen as being as important to the marine environment as it is to the land.

Problems of marine pollution are usually linked to major centres of human population. For example, the main source of oil pollution in marine environments is from urban runoff. This source can also include more toxic pollutants, for example, heavy metals such as lead, mercury and zinc.

Other threats to the coastal environment include over-harvesting of molluscs, crustaceans and sea urchins for food and bait, the loss of wildlife habitat and damage by introduced organisms. In some coastal areas measures have been taken to provide protection through conservation reserves. However, in many areas, management of intertidal areas is subject to overlapping responsibilities and a lack of coordination amongst management agencies. An additional issue is the lack of comprehensive information on the effects of human uses of these areas. The dynamic nature of the coastal zone often makes it difficult to separate impacts on the marine, coastal and terrestrial environments.


Estuaries are the meeting place of fresh and salt waters and are important fish habitats. Australia has 783 major estuaries of which 415 are tropical, 170 subtropical and 198 temperate. Many estuaries and sheltered bays have been the focus of urban and industrial development and are also important for recreation. Most river catchments in eastern and southern Australia have been extensively cleared resulting in land erosion and increased sediment loads in rivers. When deposited downstream the sediment can have a serious impact in estuaries, bays and adjacent coastal waters, smothering seabed organisms and blocking shipping channels. In Victoria and New South Wales, most estuaries located near major population centres are nutrient enriched.

Many artificial barriers along Australian rivers, such as for irrigation and flood mitigation, have greatly reduced the normal flow of rivers reducing their natural flushing process. Higher sediment loads can also be associated with elevated nutrient levels causing eutrophication, which is the excessive growth of algae. Where rivers drain through disturbed acid soils, such as in northern New South Wales and southern Queensland, estuaries may become periodically acidic which leads to the formation of chemical compounds toxic to fish. Poor water quality and loss of habitat have also caused a decline in estuarine fisheries. Fisheries are thought to be threatened in 21 per cent of estuaries in New South Wales and 23 per cent in Victoria.

Saltmarshes, mangroves and seagrass beds

Australia has the highest biodiversity of seagrasses in the world with 30 of the 58 species existing worldwide present in our marine environment. Australia also possesses the largest areas of temperate seagrass and one of the largest areas of tropical seagrass. Seagrass is highly productive and provides important nurseries for fish and protection for shorelines. Increased sedimentation and nutrients from catchments have been linked with massive seagrass die-back. It is estimated that 450 sq km of the beds have been damaged directly by human activities. Also, over recent years floods and cyclones have damaged about 1 000 sq km of seagrass beds. Once lost, seagrass does not readily recover and attempts to replant seagrasses in temperate Australia have been unsuccessful. The loss of seagrass can have serious implications.

Australia has the world's third largest mangrove area including a high diversity of 39 different species. The greatest diversity of mangroves occurs in the wet tropics. Mangroves are tree and shrub species that are adapted to the periodically inundated and salty conditions within the tidal zone. Mangrove forests are very productive ecosystems and of major ecological and economic importance. Clearing and development continue to be a major threat while only 8 per cent of mangroves are in protected areas.

Australia has about 14 000 square km of saltmarshes which are intertidal communities dominated by herbs and low shrubs, often associated with estuaries. A threat to saltmarshes in developed areas is land reclamation. Extensive areas have been filled for ports, marinas, canal estates and urban and industrial sites. The total loss of saltmarshes in Australia is highly significant as habitat although it has not been great in area, with most loss concentrated in the south-east where the initial area was small.


Australia has the largest area of coal reefs of any nation and the largest coral reef system, the Great Barrier Reef (GBR), which extends over 2 500 kms.

Coral reefs are among the most productive, diverse and complex ecosystems in the world. They are also under global threat. It is estimated that around 70 per cent of all the world's coral reefs are degraded in some way. This decline in the world's coral reefs has made the conservation of Australia's reefs of growing importance. Coral reefs are well represented in marine protected areas.

Although most reefs are still in a relatively good condition, recent comparisons of several sites in the GBR show major changes have occurred to some of the reef flats over the last 40 to 100 years. The impact of human activity on the GBR requires further research and study. Parts of the reef have also suffered from periodic outbreaks of crown-of-thorns starfish.

Australia has a unique, though poorly understood, area of submerged rocky reefs in temperate zones. These are well known for their commercial and recreational value, for fishing and diving, but there is inadequate scientific data on these areas and few are protected. Temperate reefs along Australia's southern coastline have a very high species diversity of algae and invertebrates and 80 per cent to 90 per cent of species are endemic.

Sea floor

The deep sea floor remains one of the greatest unexplored territories on Earth. Australia has around 2.5 million square kilometres of continental shelf, half of which is about 50 metres deep, and at least 2 million square kilometres of continental slope, which drops from 150 metres to 4 000 metres. Sea life in these deep sea areas has been subject to very little scientific study and the impact of human activities on sea floor communities is even less well understood.

Pastures of the sea

The minute algae that make up the floating `pastures' of the sea are the food base of the oceans. These phytoplankton depend upon upwellings for nutrition and while Australian waters have no major upwelling systems compared to those off, for example, Peru, California and north-west Africa, enrichments of a lesser kind do occur. Nutrient discharges from the land, from domestic and industrial sources, can also increase algal biomass, but this can also dramatically alter the species composition with potentially serious impacts on the entire food chain. There are now growing concerns that the deterioration of Australia's major rivers and estuaries near large coastal cities is affecting phytoplankton and food chains.

Sea dwellers

Australia has a rich marine fauna, particularly in the north which is close to the centre of one of the most diverse marine provinces on earth, the tropical Indo-Pacific region.

As a whole Australian waters contain one of the largest fish faunas of any nation with 4 000 species recorded out of 22 000 known worldwide. This is expected to grow to 4 500 species with more study. The fish population is divided into four major groups: tropical, eastern and western warm temperates, and the cool temperate faunas with some broad areas of intermixing. In contrast to the diverse northern fish faunas there is less diversity in the temperate zones but more endemic species.

In northern Australia there are over 350 described species of reef-forming corals and a huge variety of invertebrates including over 4000 species of the larger molluscs. The exact number of marine invertebrate species is unknown, but could well exceed 100 000. While no Australian invertebrate or fish species is known to have become extinct in recent times, some species are threatened.

Around eight species of baleen whales and 35 species of toothed whales, porpoises and dolphins are found in Australian waters. The population numbers are not known precisely and until recent years the major impact on numbers was from hunting. In both the northern hemisphere and Australia the inshore-calving right whales were the first to be depleted. Blue and sperm whales were then hunted in the southern ocean until stocks collapsed and then attention turned to the fin, humpback, sei and minke. Most nations have now stopped commercial whaling and Australia ceased in 1978. The blue, humpback and southern right whales are on the Commonwealth's Endangered Species list.

The tropical dugong is the only fully herbivorous marine mammal and the only sea-cow found in Australia. Northern Australia has the largest number in the world, with an estimated 80 000. While some populations of dugong are strong, further research and information about dugong populations in northern Australia are still required. Further south, in the Great Barrier Reef region, dugong populations are considered critically endangered. Some Torres Strait Islanders and some northern Aboriginal communities catch dugongs for food. The Torres Strait take is estimated at 1 000 per year.

The saltwater crocodile is found in Australia and a number of other countries in South-east Asia. However, everywhere other than Australia it has almost disappeared. Most saltwater crocodiles occur in the Northern Territory, where the original size of the population was 100 000. In 1993 the upper estimate was 60 000 although the number has increased in more recent years due to protection and farming.

Pinnipeds - sea lions and seals - found in Australian waters include the Australian sea lion, which is endemic and can reach up to a massive 400 kilograms in body weight. The New Zealand fur seal is also found in Australian waters as well as the Australian fur seal, which is also found in South Africa.

Determining the exact distribution and numbers of pinnipeds is difficult because the proportion of the population ashore, or on the sea surface, is not often known. Also, they usually occur in remote locations. Over-hunting caused a major drop in seal numbers in the past but the population is now recovering. The major threats to pinnipeds all over the world are fisheries, oil pollution and entanglement in nets and other human-made objects. Disturbances to breeding colonies are also increasing as people become more mobile and interested in wildlife.

Sea turtles are among the largest living reptiles and are exceptionally long-lived. There are only seven species worldwide and six are found in Australian waters. These include the green turtle, hawksbill turtle and the loggerhead turtle. All are protected except for hunting by Indigenous communities. Sea turtles have long been important to coastal and island people in the tropics as a source of eggs, meat and shells and as clan totems. However, the development of large-scale commercial trade, elsewhere in the world, in tortoise shell, meat, eggs and leather has put severe pressure on stocks all around the world and turtles are now seriously threatened. Because some species are highly migratory, it is necessary to have regional approaches to their management.

Australia's ocean territories are also home to a rich variety of seabirds with 100 species, 76 of which breed and may sped their whole lives in the region. The unsustainable harvesting of seabirds has now ceased and stable populations are returning, although fourteen species and subspecies are endangered or vulnerable.

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