Biodiversity Theme Report

Australia State of the Environment Report 2001 (Theme Report)
Prepared by: Dr Jann Williams, RMIT University, Authors
Published by CSIRO on behalf of the Department of the Environment and Heritage, 2001
ISBN 0 643 06749 3

Biodiversity Issues and Challenges (continued)

Disturbance Regimes and Biodiversity (continued)

Clearing, Fragmentation, Degradation of Native Vegetation or Marine Habitat (continued)

Biodiversity effects of clearing, fragmenting and degrading marine habitat [BD Indicator 2.1]
  • Effects on marine and coastal biodiversity [BD Indicators 2.1 and 2.2]
  • Seamounts
  • -->

    Australia's marine environment extends from the shores and wetlands along the coastline to the abyssal deep, and from the coral reefs of Torres Strait in the north to the pack ice of the antarctic continent in the south. The warm tropical waters off northern Australia are only 1000 km from the equator while the nation's coolest waters are some 3700 km to the south (Zann 1995). From east to west, the coastline of the Australian continent spans a distance of almost 4000 km, from Cape Byron to Shark Bay. The marine environments include extensive, well-developed and specialised reef systems, giant kelp forests, seagrass meadows, mangrove ecosystems and near-shore systems, and sand and mud-bottomed habitats that cover much of the continental shelf.

    Effects on marine and coastal biodiversity [BD Indicators 2.1 and 2.2]

    Several human activities destroy or significantly modify and degrade marine habitat and its associated biodiversity. These threatening activities and processes include eutrophication and pollution from elevated levels of nutrients, dumping of wastes including ballast water, overfishing and collecting, incidental bycatch, introduction of exotic organisms, loss of habitat, bioaccumulation of noxious materials like heavy metals, pesticides, herbicides, siltation, tourism (Figure 20), oil spills, downstream effects of dams and dykes and fishing litter (see Pollution sources on the Great Barrier Reef catchment) (see also Zann 1995). The destruction of habitat on the seabed has many parallels to habitat clearance on land, with sedimentation on the continental shelf being considered one of the most important factors in the decline of marine biodiversity (see also Estuaries, Impacts of pollution on biodiversity and the Coasts and Oceans Theme Report).

    Figure 20: Day trip access and range of the Great Barrier Reef World Heritage area.
    As boats become faster, the range of day trip operators increases (see legend). Processes are being put in place to manage this increase in access.

     Day trip access and range of the Great Barrier Reef World Heritage area

    Source: Wachenfeld et al. (1998)

    Estuaries: Estuaries are the meeting place of fresh and salt waters. Naturally rich in nutrients, estuaries are ecologically highly productive, and provide habitat for fish, migratory birds and other species. In Australia, estuaries and sheltered bays have also been the focus of urban and industrial development and recreation.

    Australia has 783 major estuaries: 415 in the tropics, 170 in the subtropics, and 198 in temperate areas (Zann 1995). A recent national evaluation has shown that the geographical extent of relatively undisturbed estuaries continues to decline. The long arid coastlines in the south-west and west have few estuaries. Most river catchments in eastern and southern Australia have been extensively cleared (Table 11). This has resulted in land erosion, sedimentation of rivers, and increased sedimentation and levels of nutrients downstream in estuaries, bays and adjacent coastal waters. High sediment levels in the water reduce light penetration, which affects rates of photosynthesis. When sediments settle they can also smother seabed organisms. Sedimentation of estuaries and shipping channels causes shoaling and alters currents. Sedimentation is a major problem in ports and shipping channels, necessitating regular dredging. This resuspends sediments, creating further environmental problems (see also Zann 1995).

    Table 11: Overview of Australia's estuaries showing the percentage of estuaries in a range of categories, in each state and the Northern Territory
    State Uncleared catchments Excellent water quality High fisheries value High conservation value Threats to conservation value Adequate state of knowledge
    NSW 24.7 0.0 24.7 16.0 21.0 100.0
    Vic. 22.9 14.3 14.3 48.6 22.9 31.4
    Qld 55.4 48.5 18.9 21.8 2.3 11.1
    WA 86.2 83.4 7.6 7.6 2.8 7.6
    SA 0.0 0.0 6.7 20.0 6.7 13.3
    Tas. 27.0 25.4 0.0 6.3 4.8 6.3
    NT 99.3 97.1 17.5 22.6 0.7 5.1
    Total 60.8 54.2 15.2 18.6 5.2 19.2

    Source: Zann (1995).

    Pollution sources on the Great Barrier Reef catchment

    Human population densities in north Queensland are low and concentrated along the coastline. Only five cities have populations exceeding 40 000. Grazing of cattle for beef production is the largest single land use (77%) in the catchment with cropping, mainly of sugar cane (3%), and urban and residential development (3%) considerably smaller in areal extent. Other significant land uses include mining (coal and metalliferous) and cotton cropping.

    Beef cattle numbers are about 4 500 000 with the highest numbers in the Fitzroy catchment. The sugar cane cultivation area has increased steadily over the last 100 years with a total of 390 000 ha reached by 1997. Other industries with significant expanding land use (and fertiliser use) are cotton (mostly in the Fitzroy catchment) and horticulture (in many catchments), particularly bananas.

    Overall, 66% of the estimated nutrient and sediment flux is estimated to come from grazing lands, with 8% from cropping lands and 26% from 'pristine' areas. The total run-off flux of sediment is estimated to be four times the amount prior to European settlement.

    Water quality: Where rivers drain disturbed acid soils, such as in northern New South Wales and southern Queensland, estuaries may become acidic periodically. This increases levels of dissolved aluminium and iron, which form compounds very toxic to fish. As a result, fish diseases such as 'red-spot' disease and fish kills are increasingly common in estuaries. At least 64% of estuaries in New South Wales and 22% in Victoria are considered to have poor water quality. Elevated nutrients in river systems may cause eutrophication, and the excessive growth of algae can smother other organisms and deplete oxygen levels. Catchments in the tropical north are less affected by human activities, but may carry heavy sediment loads resulting from soil erosion (see Zann 1995). Unnaturally elevated sediments and nutrients have led to major declines in seagrass beds in temperate Australia. Poor water quality and loss of habitat have also caused a decline in estuarine fisheries. For example, fisheries are thought to be threatened in 21% of estuaries in New South Wales and 23% in Victoria. However, eutrophication of some estuaries has enhanced their value for oyster aquaculture (Barratt et al. 2001).

    Coastal lakes and lagoons: Of great concern in south-east and south-west Australia is the declining water quality and eutrophication of coastal lakes and lagoons and the effects these conditions have on biodiversity. Settings of particular concern are those that are insufficiently flushed by the sea, such as Tuggerah Lakes and Lake Macquarie (NSW), Gippsland Lakes (Vic.) and the Peel-Harvey system (WA). As coastal lakes are largely restricted to the densely inhabited south-east coastal strip, a significant proportion of Australia's coastal lakes have been degraded (Zann 1995). Conservation of these areas is essential as they may support habitat and feeding grounds for water and shore birds including migratory birds such as Whimbrel, Eastern Curlew, Terns and Sandpipers.

    Intertidal shores: The intertidal shores are the meeting place of the land and sea. Shores are periodically immersed by sea and exposed to air, and thus subject to extremes in salinity. They are often enriched by land nutrients and have a high biological productivity. Shores have a high diversity of very specialised animals and plants, and may require special conservation measures. Australia's shores include open coasts with rocky headlands, cliffs and sandy beaches; and sheltered coasts, bays and estuaries with muddy and sandy tidal flats. Intertidal rocky shore habitats are often limited in area. They are also potentially vulnerable to human impacts. Threats to shore communities include over harvesting of molluscs, crustaceans and sea urchins for food and bait, trampling by fishers and other visitors, oil slicks and other pollutants which float on the sea surface, and loss of habitat. In the more populous south-east, south and south-west of Australia, significant areas of shores around coastal cities and towns have been reclaimed or alienated by sea walls, port development, industry, housing and tourism, and recreational facilities (SoE 1996).

    Coastal disturbance: A summary (Zeller 2000) of disturbances in Queensland resulting from human structures and activities (e.g. water diversion, dam building, disturbance of spawning habitats, increase in the number of sewage treatment plants, gravel and sand extraction, and port, resort and real estate developments) showed that during the 1990s, disturbance of fisheries habitats has generally increased as human activities have expanded and intensified along the coastline. For example, waste water generated by about 2.4 million people is treated by sewage treatment plants in the coastal zone of Queensland. In excess of 90% of the treatment plants located within 50 km of the coastline discharge into coastal freshwaters, estuaries or inshore marine waters. These discharges have been treated to a 'secondary stage' where most suspended solids and bacteria have been removed. However, few of these discharges have their nutrients removed.

    Commercial channel dredging for sand and gravel occurred from around the turn of the 1900s in the Brisbane River estuary and probably in excess of one million cubic metres were removed during the 1990s. In 1996, 390 000 m3 of sand was approved for extraction from northern Moreton Bay. From 1990 to 1996, about 2467 ha of tidal land supporting marine plants were reported as being disturbed by human activities, particularly for port expansion, tourist developments and marina/boat harbours (Figure 21) (Zeller 2000).

    Figure 21: Types of marine plants authorised for disturbance in Queensland marine bioregions in 1996.
    The total area of marine plants authorised for disturbance in 1996 was 3630 ha.

     Types of marine plants authorised for disturbance in Queensland marine bioregions in 1996

    Source: Zeller (1998)

    Salt marshes: Australia has some 13 595 square kilometres of salt marshes (Zann 1995). Coastal salt marshes are intertidal plant communities dominated by herbs and low shrubs, and are often associated with estuaries. Salt marshes are highly productive, and support key habitats for many organisms including migratory species such as the rare Orange Bellied Parrot (Neophema chrysogaster) in Victoria. A major threat to salt marshes in built-up areas is land reclamation. Extensive areas have been filled for ports, marinas, canal estates and urban and industrial sites. Other threats include degradation by rubbish dumps, off-road vehicles, invasion by weeds (particularly by introduced cord grass, pampas grass, para grass and rushes), periodic surges associated with low-pressure systems and drainage for mosquito and sandfly control. Human developments that result in the total loss of salt marshes have typically occurred in south-east Australia, where biodiversity and endemism are highest.

    Mangroves: Mangroves are tree and shrub species that are adapted to the periodically inundated and salty conditions between the tides. Mangrove forests are very productive ecosystems and are of major ecological and economic importance (Figure 22) (see also Zann 1995). They provide habitats and nurseries for many fish, form a buffer for estuaries from sediments and for coastlines from storm waves, are natural nutrient filters, and are critical habitats for many birds and other wildlife. Australia has the third largest area of mangroves in the world, and has some of the most diverse communities, but these are poorly protected from habitat destruction and degradation (Table 12).

    Figure 22: Mangrove plant richness around the Australian coastline showing many more species in the wet tropics compared with temperate Australia.
    Species diversity also varies within the tropics, with a decline in diversity of mangrove species from wet tropical environments to the arid tropics.

     Mangrove plant richness around the Australian coastline

    Source: Duke (1992)

    Table 12: Status of mangrove forests in Australia
    State Area (km2) No. reserves Area reserved (km2) Reserved (%)
    Queensland 4 602 54 600 13
    Tropics 4 117      
    Subtropics 485      
    New South Wales 99 8 5 5
    Victoria 12 2 3 23
    Tasmania 0 0 0  
    South Australia 211 7 113 56
    Western Australia 2 517 A A A
    Tropics 2 507      
    Subtropics 10      
    Northern Territory 4 119 2 282 7
    Total 11 558 73 3 520  

    AWestern Australia protects all mangroves.

    Source: after Zann and Kailola (1995).

    Mangrove ecosystems, in general, exhibit a decline in the diversity of species from the tropics to temperate zones. Similarly, within the tropics, there is a decline in diversity of mangrove species from wet tropical environments to the arid tropics. At a finer scale again, mangroves in the Great Barrier Reef show a general decrease in diversity with increasing latitude and a cross-shelf decrease from the mainland coast to the islands.

    Overall losses of mangroves in Australia are small compared with those from other countries. However, locally significant losses have occurred around Australian coastal cities and towns. For example, about 20% of mangrove ecosystems have been cleared in Moreton Bay, near Brisbane, for coastal development. Elsewhere, in areas such as the Hawkesbury and Parramatta Rivers in Sydney, it is thought that accelerated silting associated with poor land management has led to significant expansion of mangroves. The major threats to mangrove ecosystems include continued local clearing and development, and the effects of various human-induced catchment alterations. Only some 8% of Australia's mangrove communities are in protected areas (Table 12). Protection may be given through other avenues (e.g. in Queensland, mangroves are protected under Fisheries legislation).

    Seagrasses: Some of the world's largest seagrass habitats occur in Australian waters. Seagrasses are marine flowering plants and 30 of the world's 58 species are found in Australia (SoE 1996). Seagrass beds are ecologically important because of their high productivity, their ability to trap and stabilise sediments, their importance as fisheries habitats, and as the habitat for important species such as Dugongs and turtles. Australia has the highest biodiversity of seagrasses in the world, the largest areas of temperate seagrass and one of the largest areas of tropical seagrass. Anthropogenic changes that are degrading these habitats and threatening biodiversity include eutrophication, heavy metals and toxins, changes in hydrology, sediment run-off, mining and dredging, trawling, moorings and boat propellers and introduced species (Zann 1995).

    Australia's unique temperate seagrass beds appear to be under particular threat. Increased sedimentation and nutrients from catchments have been linked with massive dieback of seagrasses in many areas. New South Wales has lost half of the Zostera seagrass in its estuaries. In Victoria, around 85% of the total biomass of seagrass in Western Port Bay has been lost. In Tasmania, there have been declines in the Hobart and D'Entrecasteaux region, Triabunna and St Helens on the east coast, and Tamar, Port Sorell and Duck Bay on the north. In South Australia's Gulf of St Vincent, around 5000 ha of seagrass has been lost, and this trend is continuing (Government of South Australia, Environment Protection Agency 2000). Some regrowth of seagrass is occurring in the Port Adelaide area, but its survival is uncertain. In Western Australia, around 97% of seagrass in Cockburn Sound have been lost. A serious loss of tropical seagrasses has occurred in Hervey Bay, Qld, causing major mortality of Dugongs.

    Long-term monitoring of seagrass beds by the CSIRO in a study area located in the north-west Torres Strait revealed a 30% loss of seagrass between 1989 and 1993 (Long et al. 1997), and this trend apparently continues. An estimated 1119 square kilometres of seagrass were lost in the north-east region of the study area. Combined, the net losses that have been quantified in this area alone represented a 10% reduction in the estimated total area of seagrass in the Torres Strait (Long et al. 1997). Once lost, seagrasses do not readily recover and this has serious implications for the species these ecosystems support.

    Coral and rocky reefs: Coral reefs are among the most productive, diverse and complex ecosystems in the world and support a significant proportion of the planet's marine biodiversity (Zann 1995). Coral reefs are also under global threat. A global assessment of reefs (Wilkinson 2000) shows continuing decline with 27% of the world's reefs having been effectively lost, with the largest single cause being the massive, climate-related coral bleaching event of 1998, which destroyed about 16% of the world's coral reefs in nine months. Climate change adds a new dimension to threats to reef ecosystems since changes in ocean temperature, ocean currents and flows of nutrients, disturbances regimes and sea level rise have the potential to destroy many reef ecosystems. Because of the general decline in many coral reefs, the biodiversity conservation values of Australia's reefs are of growing importance, and the management of reef ecosystems is attracting even greater attention (Zann 1995; Wachenfeld et al. 1998).

    Australia has the largest area of coral reefs of any nation and the largest coral reef complex, the Great Barrier Reef (Zann 1995; Wachenfeld et al. 1998). Major areas of coral reefs are also present in Torres Strait, the Coral Sea Territories, and central and northern Western Australia. The Oceanic Shoals Bioregion, which consists mainly of submerged reefs, is also a very significant coral reef province, with many unexplored reefs on the Sahul Shelf and along the edge of the Timor Trough. The Tasman Sea reefs (Elizabeth and Middleton Reefs and the Lord Howe Island fringing reef) are the highest in latitude in the world, and thrive in conditions otherwise marginal for coral growth. High latitude reefs, such as the Abrolhos Islands, are also found on the west coast of the continent.

    Of the reefs that have been well studied, a striking feature of coral reefs is the natural variation which can exist in reefs in close proximity to each other (Figure 23) (Zann 1995; Wachenfeld et al. 1998).

    Figure 23: Changes in coral cover at Heron Island, Qld.
    A 30-year study of four different coral reef zones at the island showed variation in coral cover to be as great as 80%. Most declines in coral cover were due to cyclones (indicated by arrows).

     Changes in coral cover at Heron Island, Queensland

    Source: Connell et al. (1997)

    General issues affecting Australia's coral reefs include effects of sediments, agricultural chemicals and nutrients, effects of fishing and tourism, the threats of oil spills, and negative changes in habitats as a result of enhanced climate variability and climate change (Zann 1995; Wachenfeld et al. 1998). Specific threats include elevated nutrients in the inner Great Barrier Reef (see Pollution sources on the Great Barrier Reef catchment), outbreaks of Crown-of-Thorns Starfish (Acanthaster planci) in the outer central and northern Great Barrier Reef and Tasman reefs (see the Coasts and Oceans Report), damage from the passage of tropical cyclones, and outbreaks of coral-eating Drupella snails in Ningaloo Reef, WA. Wachenfeld et al. (1998) summarised the major environmental attributes on the Great Barrier Reef and the state, pressure and responses associated with them (Table 13). Coral reefs are relatively well represented in Marine Protected Areas in Australia, although only a small area is protected in zones where fishing is prohibited.

    Table 13: The state, pressure and response model associated with various environmental attributes of the Great Barrier Reef 
    Attribute State Pressure Response
    Water quality Status fairly well known
    Limited trend data but no obvious adverse trends
    Adjacent land use and associated nutrient and sediment run-off during flood events
    Loss of freshwater wetlands
    Direct inputs of pollutants prohibited or strictly regulated. Collaborative arrangements with state government agencies being developed to reduce indirect inputs through run-off. Comprehensive research and monitoring programs in place.
    Mangroves Status well known
    Some medium-term trend data available
    No obvious adverse trends
    Principle pressure is clearing for coastal development Mangroves protected by legislation from damage and removal. Further work on cumulative effects needed.
    Island plants Status fairly well knownNo information on trends Historical effects from plant introductions and grazing, some ongoing Plants on most islands protected from direct damage or removal. Ongoing monitoring needed.
    Seagrasses Status fairly well known
    Some information on trendsNo obvious adverse trends
    Some potential pressures from coastal run-off and trawling, but few major effects documented Trawling prohibited by Marine Park zoning plans in nearly half of mapped seagrass area. Trawling prohibited in additional areas by coastal strip closures. Offshore beds less protected by effects not documented.
    Macroalgae Status poorly known
    No information on trends
    Anecdotal reports of increased abundance due to human effects
    Potential effects on near-shore algae from increased nutrients in run-off Status information being collected. Management needs uncertain. Trend monitoring needed.
    Corals Status fairly well known
    No evidence of any major declines directly attributed to human effects
    Some recent disturbances from Crown-of-thorns Starfish and bleaching
    Potential pressure on inshore corals from increased sediments and nutrients in run-off, but no major effects documented Comprehensive research and monitoring programs in place. Most major direct pressures regulated or prohibited.
    Crown-of-thorns Starfish Status fairly well known
    Current outbreak in northern Great Barrier Reef
    Cause of outbreaks uncertain
    Role of human activities in causing outbreaks is uncertain Comprehensive research and monitoring programs in place. Control measures developed for significant sites. Need for further action not clear, given uncertainty over causal factors.
    Fishes Status of commercial species and common reef fishes fairly well known
    No evidence of any major declines caused by human activities
    Commercial, recreational and Indigenous fishers
    Heavy fishing pressure in some areas
    Comprehensive research and monitoring programs in place. Variety of management measures to restrict and regulate fishing effort.
    Birds Status fairly well known
    Trends known for some sites where some species have declined
    Human disturbance from visitation
    Habitat loss and deterioration
    Some sensitive nesting sites closed to visitors. Research and monitoring programs in place at some sites. More information needed on status for many areas. Need for further action uncertain due to lack of trend data.
    Marine turtles Status well known for two species
    Significant decline for one species, indications of decline for two others and no indication of decline for a fourth
    No information on status of two other species, but both rarely seen
    Bycatch in trawl and shark nets
    Hunting both locally and in other countries
    Predation of eggs and young by feral animals
    Habitat removal and disturbance
    Important nesting sites protected. Efforts under way to reduce bycatch in trawls. Need for international agreement to protect turtles. Ongoing monitoring.
    Sea snakes Status information needed Bycatch in trawl nets Management requirements uncertain. Processing of sea snake skins no longer allowed.
    Dugongs Status and trends fairly well known
    Decline in southern Great Barrier Reef population
    Bycatch in mesh and shark nets
    Boat strike
    Indigenous hunting
    Dugong protection areas established. Voluntary cessation of traditional hunting by most Indigenous communities south of Cooktown. Traditional hunting south of Cooktown no longer permitted. Comprehensive research and monitoring programs in place.
    Whales and dolphins Status and trends for Humpback Whale fairly well known
    No information on other whales or dolphins, but inshore species possibly in decline
    Whale watching of Baleen Whales, particularly Humpback and Dwarf MinkeInshore dolphins caught as bycatch in mesh nets Whale-watching guidelines developed. Monitoring and protection measures for inshore dolphins needed.
    Inter-reef and lagoon benthos Status poorly known
    Likely substantial effects in areas of high intensity of trawling
    Trawling
    Nearshore communities potentially affected by increased sediments and nutrients in run-off
    Some progress towards understanding responses and recovery. Trawling prohibited by Marine Park zoning plans in over half of inter-reef area and about 10% of lagoon area. Management plans being developed which aim to reduce fishing effort.

    Source: Wachenfeld et al. (1998).

    Because of the general decline in many of the world's coral reefs, tourism values of Australia's reefs are growing. Tourism on the Great Barrier Reef is actively managed, which should keep habitat destruction and degradation to a minimum. However, the annual increase in visitation rates and increased mobility of visitors to coral reefs as a result of new and improved forms of transport (see Figure 20) is likely to have some effect. The increased mobility of tourists is an important management issue for all marine environments, including shallow waters amenable to underwater tourism (Wachenfeld et al. 1998).

    Natural variation of biodiversity in coral reefs

    Results from three, separate long-term studies of corals have emphasised variability of reefs through time. In one 30-year study on Heron Island in the southern Great Barrier Reef, coral cover was found to vary between 0 and 80% in different patterns depending on the site.

    In another study covering six reefs in the central and northern Great Barrier Reef between 1980 and 1995, substantial changes occurred in the numbers of areas dominated by corals, bare substrate or other organisms, but by the end of the study, the proportions were similar to those at the beginning. However, this overall similarity in average conditions masked that some reefs had improved while others had degraded. At one inshore reef, an area once dominated by macroalgae was replaced by a coral-dominated community.

    Unexplored ecosystems: Seamounts and terrestrial cliffs

    Many ecosystems in Australia remain virtually unexplored because they are inaccessible, or because the species they support are cryptic or inconspicuous. Two such areas are oceanic seamounts, and the cliffs of the terrestrial environment.

    In June 2000, CSIRO reported that hundreds of new species have been discovered on seamounts in the Coral Sea and the Tasman Sea between New Caledonia and Tasmania. The south-west Pacific contains the greatest density of seamounts and seamount ridge systems in the world. Each seamount is an ecological island and many different species have evolved independently in each area. The communities on seamounts are dominated by corals, sponges, sea fans and other organisms that filter their prey from the strong currents characteristic of this environment. There are about 30 000 seamounts in the world's oceans, but most species previously known from this deep ocean environment come from sampling only five of these. The study of 25 seamounts uncovered 850 species, more than previously reported from all studies of seamounts in the past 125 years. About one-third of these species are new to science and are likely to be restricted to the seamount environment.

    Cliff environments are a unique combination of substrate and physical conditions that support unique biotas that depend only partly on mass effects from adjacent communities. Many are relatively undisturbed by human activities. Larson et al. (2000) reported on the substantial undersampling of cliff faces in terrestrial environments. The discovery of the Wollemi Pine (Wollemia nobilis) in a sandstone gorge in the Blue Mountains is symptomatic of this sampling bias. This very conspicuous species remained undiscovered close to Australia's largest human population for 200 years because there have been no studies of the vascular or non-vascular flora of the associated cliffs.

    There is little doubt that many new species and unique communities remain to be discovered in cliff environments, on oceanic seamounts and in many other unique ecosystems.

    Despite their high conservation and economic values, Australia's temperate reefs are inadequately studied scientifically, and relatively few are protected. Although very little is known of the effects of human activities on temperate reef habitats, it is often assumed that outside metropolitan and industrial areas, they are relatively unaffected (Zann 1995).

    The continental shelf and sea floor communities: Continental Australia has around 2.5 million square kilometres of geomorphic continental shelf, half of which is less than 50 m deep. The continental slope, which drops from a depth of 150 m to 4000 m, has an area of at least two million square kilometres (Zann 1995). An extensive continental shelf is also found adjacent to the AAT and there is much to be discovered about the biodiversity in this area.

    Little is known of the effects of humans on sea floor communities, although seamount ecosystems have attracted study (see Seamounts and Unexplored ecosystems). The rate of sedimentation on the sea floor has greatly increased since European colonisation of Australia by factors of ten to 100, and even more in some areas. Dumping of wastes on the sea floor is controlled under international conventions (e.g. the London Convention) and various Commonwealth and state/territory legislation. However, dredging and dumping of wastes may still cause localised disturbances around ports.

    Trawling [BD Indicator 2.2] : Some shelf and slope fish species have been severely overfished, and trawl nets may dislodge attached species such as sponges and modify the habitat and food chains. Barratt et al. (2001) reported that the South East Trawl Fishery is increasing in intensity of trawling, and that the Twofold Shelf Region may be the most disturbed of the Interim Marine and Coastal Regionalisation for Australia (IMCRA) regions in south-east Australia. Trawling intensity and the area in the Northern Prawn Trawl fishery decreased substantially during the 1990s, but appear to be slowly increasing.

    Trawling is of serious concern for certain marine biodiversity and recommendations have been made to list prawn trawling as a threatening process to turtles. For example, about 3% of the Loggerhead Turtle (Caretta caretta) population that forages in the Northern Prawn Fishery is killed annually as a result of this fishing activity (Barratt et al. 2001). Trawling can also have considerable effect on invertebrate benthos, although these organisms are often ignored in bycatch studies, which focus on retained bycatch. Work on fish trawling in Western Australia, however, has indicated that each pass of the trawl removed 16% of the large (>20 cm) macrobenthos, with multiple passes effectively denuding the substrate. This is a marine form of habitat fragmentation. Trawling should, therefore, be considered for its effect on the entire range of species, not just target and non-target fish caught.

    Trawling is prohibited in some areas (e.g. in parts of the Great Barrier Reef Marine Park). Changes in the design of trawl nets used to catch prawns should reduce the chances of catching unintended species such as cod and turtles (Figure 24). This will help to minimise unintended changes in species composition and trophic interactions in reef ecosystems.

    Figure 24: A bycatch reduction device.
    The trawl net in the upper diagram has no bycatch reduction devices fitted. All animals that enter the net are caught in the cod end, including prawns, turtles and unwanted fish species. The trawl net in the lower diagram has two types of bycatch reduction devices fitted. The turtle excluder device prevents turtles from swimming into the cod end, forcing them out through a flap on the trawl net. Fish have a tendency to swim against the current and may swim out through the top of the trawl net. Prawns, however, are still caught in the cod end.

     A bycatch reduction device

    Source: Wachenfeld et al. (1998)

    Hydrocarbon exploration: Levels of hydrocarbon exploration of the seabed and production activity tends to fluctuate markedly annually, and there has been a relatively small increase in both types of activity since the early 1990s. Petroleum industry activities and facilities occupy a small surface area of the seabed and are concentrated almost entirely in the region of the Gippsland Shelf and Carnarvon Shelf. Six rigs were in operation at September 1999 and these drilled almost 100 wells with a total length of over 280 km. Seismic surveys and drilling affect the biodiversity of benthic marine environments. Seismic surveying involves the introduction of a high-energy acoustic pulse into the earth's crust, such as the release of a high pressure air blast into the water, dynamite, electromechanical vibrations and steam injection. Sonic echoes from the sea floor are then received by a cable, up to one nautical mile long (1852 m), which is filled with hydrophones and is dragged by the surveying ship. In 1998, the total length of 2-D (a single line or track) and 3-D (systematic grid coverage of a discrete area) surveys exceeded 900 line kilometres (Figure 25).

    Figure 25: Number and length of 3-D seismic surveys per year in the Twofold Shelf and Otway IMCRA regions.

     Number and length of 3-D seismic surveys per year in the Twofold Shelf and Otway IMCRA regions

    Source: Barratt et al. (2001)

    Seamounts

    Seamounts are remnants of extinct volcanoes, typically cone-shaped, 20 to 500 m high and several kilometres across at their base. About 70 seamounts arise from water depths of between 1000 and 2000 m on the continental slope, between 50 and 170 km off southern Tasmania (Tasmanian Seamounts). This field of seamounts is a distinctive geological feature not known elsewhere on the continental margin of Australia. They support a distinct benthic (bottom-dwelling) community of animals, many of which are native to the Tasmanian Seamounts and do not occur elsewhere. A high proportion of species (24-43%) new to science has been discovered, including at least eight new genera. The species in the area appear to be representative of the seamount fauna of the Seamount Region (see Unexplored ecosystems) and have been protected recently by the Commonwealth government (Environment Australia 2000). Generally, however, most of Australia's sea floor is not actively managed.