Threats and pressures

This content forms part of our issue briefing on the marine environment.

Climate change

Climate change is the major threat looming over our oceans and coastal waters. Its key impacts come from:

  • increased ocean temperatures
  • ocean acidification
  • increased incidence of extreme weather and sea level rises

Warming seas

Australian ocean temperatures have warmed since the early 20th century by 0.7˚C. Six of the ten warmest surface temperatures on record have occurred in the past ten years. By the 2030s, water surface temperatures are predicted to be around 1˚C higher (relative to 1980-99).

Warming seas from 1900 to 2010
Figure – Annual (line) and decade (box) mean ocean surface temperature anomalies for the Australian region relative
to the 1961–1990 average. The average value for the most recent 10-year period (2002–2011) is shown in darker grey.
Source: CSIRO – State of the Climate Report 2012.

Rising ocean temperatures in Australia force species southward. This will eventually mean major decline for southern coastal species that require shallow, cool water. Further loss of seagrass meadows and algal beds are expected due to warmer water, storms and turbidity.

Higher temperatures cause coral bleaching and disease outbreaks. Coral bleaching happens when a coral expels its zooxanthellae, the marine algae that live in symbiosis with corals. This results in the death of the coral.

Coral is the base habitat for much of the marine biodiversity of a reef. Many other organisms are badly affected by coral bleaching. Butterfly fish appear to survive for no more than five years after severe coral bleaching.


Increased carbon dioxide in the atmosphere forms a weak acid in water. Some organisms such as corals, sea-shells, crustaceans and sea urchins are vulnerable to acidification because it reduces their ability to make shells.

Ocean acidification also poses a risk to marine food chains. There have already been changes detected in calcification not only of Great Barrier Reef coral, but also Southern Ocean zooplankton, which are a key link in the marine food chain that goes all the way up to some of our most highly valued fish species.

Around half the oxygen in the atmosphere is produced by oceans. Acidification is likely to affect microscopic organisms such as ‘coccolithophores’ that help make oxygen. This may well have a critical impact on the mix of carbon and oxygen in the atmosphere.

Extreme weather and sea level rises

With climate change comes increased frequency of natural disasters. Queensland’s 2011 floods destroyed three of the four major seagrass feeding grounds of the vulnerable dugong. The dugong death toll in 2011 was 1,048, compared to 555 in 2010, with most deaths attributed to starvation.

There has been substantial loss of coastal land due to sea level rises. Intrusion of salty water into coastal waterways and wetlands also threatens habitats and ecosystems.

Fishing and aquaculture

Some of the largest commercial fishing operations in tropical marine waters are those targeting prawns, scallops and slipper lobsters. Trawl fishing involves dragging a net across the seabed, creating a cloud of muddy water and ‘clear-felling’ the sea floor of animals and plants. For every kilogram of fish, up to five kilograms of other marine life including sponges, shells, crustaceans and molluscs are inadvertently caught by some trawlers.

Other destructive fishing practices such as longline fishing for tuna and gill netting for sharks are also widespread in Australia’s waters. They take a heavy toll on some of our most threatened marine species.

In 2009, 12 per cent of the species managed by the Australian Fisheries Management Authority were assessed as overfished and 10 per cent as suffering from ongoing overfishing. In addition, not enough was known about the numbers of about a third of the species being fished.

Intense fishing of a species has cascading effects. Persistent low populations:

  • make the species vulnerable to other pressures such as climate change (for example by reducing the breeding stock) and habitat loss
  • have flow-on effects for the resilience and persistence of marine biodiversity (for example by taking top-level predators out of the food chain).

Despite this, there is no nationally integrated analysis of the cumulative impacts of fishing on ecosystems. And there are no national assessments of the ecological sustainability of commercial or recreational fishing.

Coastal development and new fishing technology (such as greater access to bigger recreational fishing boats, and fish-finding sonar systems) have removed refuges that once existed in remote places or where the seabed was formerly too rugged to be fished.

The accidental capture and drowning of sea lions and dolphins is also a significant problem in net fisheries off Australia’s south coast.

Recreational fishing also places pressure on the marine environment. Recreational catches can be larger than commercial catches for many sought-after species.

Australia has a small but growing aquaculture industry, which includes Atlantic salmon farmed in cages off the Tasmanian coastline, and the fattening of young wild-caught southern bluefin tuna (known as ‘tuna ranching’) in the Spencer Gulf off the South Australian coast. Farmed fish such as salmon and tuna have high protein needs which are mostly met by feeding them wild-caught fish, depleting the stocks of these “forage” species.

The main areas of concern with respect to aquaculture are:

  • the potential for spread of diseases and parasites such as sealice from caged salmon to wild fish
  • waste including nutrient enrichment of the seabed around cages from excess feed
  • the escape of farmed fish and shellfish into the natural environment
  • the source and sustainability of wild-caught fish to stock farms and for use as feed
  • the use of chemicals, antibiotics and antifoulant paints.

Aquaculture in Australia has caused major outbreaks of disease in wild fish. These have left lasting imprints on some ecosystems.

Pests, algal blooms and diseases

Introduced marine plants and animals arrive into Australian waters on all types of ships, carried on hulls and in ballast waters and inside internal seawater pipes. They are also introduced via aquaculture, aquarium imports, marine debris and ocean currents. Once marine pests are established, eliminating them is virtually impossible. They can multiply quickly and force out native species. Some, such as toxic algae, can pose a threat to human health.

Pests in southeastern Australian waters include some species of starfish, sea urchins, plankton, algae, molluscs, crustaceans and worms. Port Phillip Bay has been described as one of the most invaded marine ecosystems in the southern hemisphere, but there are others of equal note, including the Derwent estuary. Periodic outbreaks of harmful native species also occur elsewhere around Australia, such as crown-of-thorns starfish and algal blooms in Moreton Bay near Brisbane.

Oil, gas and shipping

In Australia, oil and gas industries are concentrated in Bass Strait and northwestern Australia, with developments expanding into other regions.

Marine disturbance results from:

  • seabed structures such as wellheads, anchors and pipelines
  • hipping traffic
  • accidents and spills
  • ocean noise pollution from underwater seismic exploration
  • industrial sites, ports and processing and maintenance facilities

Dredging of the sea floor near the shoreline has serious impact. In Western Australia alone, more than 200 million cubic metres (the equivalent of nearly half the water in Sydney Harbour) of dredge spoil has recently been approved for disposal in the deeper ocean and along the coast.

The Montara spill in August 2009 was Australia’s worst offshore exploration oil accident. For 74 days, oil and gas flowed into the Timor Sea at a rate of at least 64 tonnes a day. The accident exposed inadequacies in governance, science and logistics.

The management of oil and gas expansion is poorly coordinated. Each development is considered on its own merits with little thought for cumulative impacts across a region.

The shipping industry is expanding with many new ports being built and existing ports being expanded. Much of this is supporting mining growth in Queensland, the Northern Territory and Western Australia.

Expanding ports can require dredging and major development in sensitive areas and can lead to habitat destruction, death or injury to marine species, the spread of toxins and noise pollution.

Shipping lanes traverse some of the most ecologically sensitive marine areas, and there are regular grounding and other accidents at sea that harm the marine environment. Ships frequently hit marine mammals in open waters.

As oil-industry shipping intensifies in the northwest, the ‘whale highway’ migration route of humpback whales is likely to be affected. Despite this, there are still no upper limits on the size of ships, shipping lane use, frequency of transit or seasonal constraints.

Noise from shipping and sonar used in defence exercises may also harm whales and dolphins.

Runoff and land-based pollution

Nutrients and sediment from human activity can lead to the decline of coastal water quality. Water quality is poor in a high proportion of New South Wales estuaries. This is directly linked to the fact that more than half the estuaries in New South Wales are subject to double the natural sediment and nutrient inputs and around one-third of all the catchments in the state have been cleared of more than 50 per cent of their natural vegetation.

On the Great Barrier Reef the quality of water entering the reef continues to drop. The 38 river catchments emptying into Great Barrier Reef waters deliver:

  • two to ten times more nutrients and sediment than before European settlement
  • agricultural chemicals, including fertilisers, pesticides and herbicides

This excess can cause algal blooms and the suffocation of reefs. It also increases the susceptibility of reefs to outbreaks of coral-eating crown-of-thorns starfish. These impacts are then potentially passed on further up the food chain to turtles, dugongs and predatory fish.

Marine debris

In 2010 Achim Steiner, the executive director of the United Nations Environment Programme (UNEP) said:

“If collectively we carry on using the seas and oceans as a dustbin, human beings will soon have turned the once beautiful and bountiful marine environment from a crucial life-support system into a lifeless one.”

According to the UNEP, 8 million items of litter enter the oceans and seas every day, about five million (63 per cent) of which are solid waste thrown overboard or lost from ships. More than 13,000 pieces of plastic float on every square kilometre of the world’s oceans.

The UNEP estimates that every year 100,000 turtles and marine mammals such as dolphins, whales and seals are killed by swallowing or becoming entangled in plastic.

Marine debris has a significantimpact on Australian vertebrate marine lifeand the Federal Government has developed a Threat Abatement Plan for it.

High concentrations accumulate on coasts close to cities and have also been reported at some remote areas including Cape York, Groote Eylandt, northeast Arnhem Land and the far north Great Barrier Reef.

Plastic bands or net fragments entangled around the necks of young animals restrict their ability to feed properly and, as the animal grows, strangle and kill them. Abandoned fishing gear, ropes and other debris tangled around animals can lead to infections, restricted mobility, slow amputation of limbs and death through drowning, starvation or smothering.

Swallowed litter can starve animals by making it hard to digest food, causing internal wounds and ulceration or making them more buoyant and inhibiting diving.

Marine wildlife absorb toxins by swallowing ‘microplastics’. Microplastics are small particles that come from the weathering of larger plastic items and from the use of plastics in other processes, such as the plastic ‘sand’ used to remove paint from ship hulls. Plastics are a particular threat because they are slow to break down, even more so in the ocean than on land.

Plastic debris spread toxic chemicals such as polychlorinated biphenyls (PCBs) into marine food webs. These chemicals are known to compromise immunity and cause infertility in animals even at very low levels.

In Australia, 77 species are known to be affected by marine litter. These include whales, sharks, sea birds and turtles. Because turtles have spines that face downwards in their throat, they cannot cough up swallowed litter.