This content forms part of our issue briefing on the marine environment.
The overall health of our marine environment is good, but this largely reflects the condition of offshore waters and remote coastlines. Along our developed coasts, particularly in the southeast, ecosystems are in poor health due to:
- commercial and recreational fishing
- coastal, urban, industry and port development, including the expansion of oil and gas
- invasive pests and diseases
- algal blooms
- sediment and nutrient loads to coastal waters are excessive
- worrying levels of pesticide in water near intensive agriculture
- the legacy of poor management
Pressures are region-specific. Largely undisturbed habitats in the northwest are affected by oil and gas industry expansion, whereas development and fishing are the main threats on the populated sections of the eastern and western coastlines.
Our ocean and coastal ecosystems are used by everyone, but their management is poorly coordinated and they are consequently suffering from ‘death by a thousand cuts’. As climate change intensifies, it is even more urgent that we develop a nationally coordinated approach to our marine environment and avoid the management failurse of the past.
Some Australian marine habitats are dead as a result of heavy pollution. These include mining waste-damaged Darwin Harbour and Melville Bay in the Northern Territory.
The loss of about 30 per cent of our seagrass areas and the degradation and loss of mangroves is also concerning. Seagrass beds occur in many coastal waters and estuaries, and offshore down to 50 metres depth. They act as nurseries for the young of some crustacean and fish species including yellowfin, bream, luderick and leatherjacket.
About two-thirds of eastern Australia’s commercially caught fish and prawns depend on mangroves, but these habitas are under threat from development. Mangroves also protect shorelines from damaging storms, waves and floods. Their dense, tangled root systems help maintain water quality, filtering pollutants and trapping and stabilising sediments.
Destructive fishing techniques such as trawl fishing continue damaging key fish habitats as well as being destructive to non-target species caught as “by-catch”.
The southeast is the only region where an entire habitat type has been made extinct: the oyster reef beds that used to dominate some estuaries and small bays. This loss has affected other species and probably reduced water filtration and removal of pollutants.
In the southeast, sediment, nutrients and toxics inputs have significantly degraded ecosystem functions in the Coorong, the Derwent River and estuary and the Gippsland Lakes.
The sea floor is the largest but least-known ecosystem on Earth. Much of our knowledge of marine biodiversity is centred on fished species where records have been kept over long periods of time.
In comparison, we have limited understanding of non-fished species and their roles in maintaining healthy and resilient oceans and as such the status of our fished species is a surrogate for the status of our biodiversity.
Worryingly, many Australian fisheries are far from sustainable, despite improvements in the past three decades. Intense fishing in the past and current fishing practices mean most sought-after species are at low numbers and either not recovering or recovering very slowly. These include orange roughy, southern bluefin tuna, toothed whales, whale sharks and great white sharks.
Marine reserves help protect fish and allow their numbers to recover. More than 30 per cent of the Great Barrier Reef Marine Park is now a no-take marine reserve. Coral trout on reefs closed to fishing have recovered very quickly from earlier intensive fishing.