State of the Environment 2011 Committee. Australia state of the environment 2011.
Independent report to the Australian Government Minister for Sustainability, Environment, Water, Population and Communities.
Canberra: DSEWPaC, 2011.
Australia’s sheltered coastal waters are increasingly being considered as providing important opportunities for aquaculture. The main species being farmed are Atlantic salmon, southern bluefin tuna, rock oysters, pearl oysters, mussels, prawns and abalone. These species are farmed in land-based and sea-based facilities, both of which have a range of environmental risks. There are four main areas of environmental concern: the potential for spread of diseases and parasites, the impacts of the facilities and supporting infrastructure, the interaction with wildlife, and the source and sustainability of wild stocks (if required) and feed. Key issues of environmental concern are diseases that can be harboured in, and spread from, both types of facilities; treatment and impacts of wastes, particularly feed and faeces; intensification of infrastructure in sensitive habitats; and effects on species that may become dependent on the structures or waste discharges.
In Australian waters, evidence indicates that both land-based and sea-based aquaculture has been the source of a number of major outbreaks of diseases in wild populations. The resulting impacts have been ecologically significant and will leave a lasting imprint on some of the affected ecosystems. In addition to disease outbreaks, there have been issues associated with use of chemicals, and impacts on threatened species such as sharks and seals. This is consistent with overseas experience of aquaculture impacts.60 However, given appropriate levels of management and verification, the impacts of aquaculture facilities can be constrained to a minimal and acceptable level, bringing the aquaculture industry in Australia into line with other modern farming practices to produce wealth from the ocean with minimal environmental degradation.
Australia has a long history of aquaculture in the estuaries of the east coast. The Sydney rock oyster (shell) was harvested for use as lime in cement production in Sydney in the 1800s, but this quickly depleted the local oyster beds. The earliest marine farming operations of oysters were subsequently established by Thomas Holt in Gwawley Bay (Georges River) in 1872, in response to the depletion of wild oysters. The industry was heavily focused on the Hawkesbury River in its early years, but declining water conditions and high levels of diseases have now almost eliminated production from this estuary. Oyster farming in New South Wales has now diversified to include the Pacific and flat oyster, on selected sites held under some 3200 aquaculture leases, with a total current area of approximately 4300 hectares.61 The main oyster-producing areas are located away from urban areas. Commercial production in New South Wales occurs in 41 estuaries between Eden in the south and the Tweed River in the north, although Wallis Lake (on the north coast) is now the main Sydney rock oyster-producing area.
In the first 75 years of the New South Wales oyster industry, production of the endemic Sydney rock oyster grew to about 60 million oysters per year. In the subsequent 25 years, production increased to about 175 million oysters per year, peaking in 1977, and then trended downwards to the current 70 million oysters per year—less than half the production of the industry at its peak. Disease and environmental issues remain significant problems for this industry.
The statewide reduction in production is related to the impact of land-based sources of pollution (from urbanised areas and agriculture), and to an extensive and diverse set of waterborne diseases in farmed oysters, including viral and bacterial infections, protozoa and flatworms.62 These accelerating issues have resulted in a much greater emphasis on the development of land management in river catchments that recognises the need for high water quality in oyster-growing areas.
Disease issues in the oyster industry are also concerns for wild oyster populations. They include the potential transmission of diseases between the estuaries, related to industry practices, and possible maintenance of diseases in the wild population that might otherwise naturally dissipate to background levels. Oysters (wild and farmed) have an important role in estuaries, filtering water and feeding on plankton and other fine debris to clarify the water. Although the role of the intensive aquaculture system in transporting and spreading disease among the wild population or to other molluscs is unclear, these are important ecological impact issues for these estuaries and coastal waters. Also of concern is the spread of the Pacific oyster—this species is endemic to Japan and farmed in several states, and has developed many naturalised populations along the east coast. The ecological impact of this introduced species is uncertain, but is likely to be significant. Where its populations have become established, it is likely to compete with native species (including the Sydney rock oyster) for space and food, and possibly has impacts on a range of other sedentary species that also inhabit the estuaries of New South Wales.
Abalone aquaculture is a recent initiative, mainly undertaken in Tasmania, Victoria and South Australia, where the most substantial natural populations of abalone also occur. In 2008–09, around 640 tonnes of abalone were produced from the aquaculture facilities in these states.
Two species are farmed—greenlip abalone (Haliotis laevigata) and blacklip abalone (H. rubra)—as well as a hybrid of these species, in land-based and sea-based farming systems. The two systems have very different siting and infrastructure requirements, and a different range of associated environmental risks. For example, in land-based tank systems, the growing abalone are fed on an artificial diet, require large volumes of fresh sea water and produce a large volume of wastewater. In sea-based systems, the growing abalone are fed on natural macroalgae (which may be harvested locally by hand), require only modest current flows of high-quality sea water and produce little waste. However, in both cases, high densities of individuals can lead to the risk of outbreaks of diseases that can very quickly (within days) become difficult to treat and control (Box 6.8).
In 2010, wild abalone populations in Victoria suffered from an outbreak of the lethal abalone virus known as abalone viral ganglioneuritis (AVG). AVG is a herpes-like virus that causes inflammation of the nervous tissues in the abalone, interfering with its ability to properly adhere to surfaces or feed. An AVG outbreak has also recently been identified in Tasmanian farmed abalone, although it is suspected to be of a different origin from the strain in Victoria.63-64
AVG was first reported in Australia in December 2005, when several abalone aquaculture farms near Portland and Port Fairy in western Victoria experienced unusually high levels of abalone deaths. It is suspected that a discharge from one of these farms where AVG was first detected permitted the virus to escape and infect wild abalone nearby. Since then, the virus has caused substantial deaths in wild abalone populations and continues to spread eastwards along the coastal waters of Victoria to Cape Otway. The persistence of AVG in wild abalone populations now threatens the vigour of these populations in Victorian waters, and may also affect the fishery for wild abalone. The broader ecological impacts of this disease outbreak are as yet unknown, but are likely to be regionally significant, given the important role that abalone play in the benthic ecology of reef systems across the southern Australian shores, from New South Wales to Western Australia.
At-sea cages for salmon culture are the fastest growing Australian aquaculture industry. The salmon farming industry is now Australia’s single most valuable seafood production sector, overtaking the wild-catch fishery for western rock lobster, which has been in decline for a number of years. Australia’s total production of caged salmonids—around 30 000 tonnes of salmon and trout, mainly Atlantic salmon from Tasmanian waters—was valued at $323 million in 2008–09, while the Western Rock Lobster Fishery production was valued at less than $200 million. However, salmon farming is not without environmental impact, and there are many areas of major uncertainty, particularly surrounding the use of chemicals to treat disease outbreaks.65 Disease outbreaks destroy the farmed stock, can easily escape into wild populations66 and are the subject of intense management in marine fish-farming systems (see Box 6.9). Entrainment of wild species on cage facilities is also a major global issue, attracting fish to the locality of the cages for access to uneaten feed pellets and other waste materials from the cages.67
Despite these and other issues, the careful siting and management of caged fish facilities can result in acceptably low impacts and risks. For example, the Australian Conservation Foundation has accepted the barramundi sea cage farm at Cone Bay, Kimberley, for recommendation within its sustainable seafood program, after an independent ecological assessment found that the impacts of these key factors were acceptably low.68
Perhaps the worst fish kill in a wild population recorded from human causes is the massive series of pilchard kills that repeatedly occurred across temperate Australian waters (New South Wales to Western Australia) in 1995 and 1998–99. After a single event in 1999, at three Western Australian locations, 28 000 tonnes of pilchards were estimated to have been killed.66,69. The fish kill episodes were observed across more than 4000 kilometres of temperate Australian coastline. Although there has been no attempt to estimate the total mortality of pilchards, mass fish mortalities of this scale are of national and probably global importance. The most likely source of the virus thought to be responsible is the frozen, but otherwise unprocessed, food used for tuna aquaculture sea cages on the Eyre Peninsula in South Australia.66 Food for aquaculture purposes is now more systematically managed to reduce the risk of such disease importations. However, the virus that affected the pilchards is probably now well established in Australian marine ecosystems and likely to have low-level but ongoing impacts on the pilchard population and species that depend on this fish, including seabirds such as terns and penguins.70
The main aquaculture system based on lines is Australia’s tropical pearl farming industry. This is a lucrative business that harvests natural tropical pearl oysters, seeds them with ‘nuclei’ of carbonate material, and then grows the oysters attached to long lines or dropper lines in at-sea facilities. A similar system is used to culture mussels in various bays and gulfs of temperate Australia. Such line systems, provided they are well designed and managed, are thought to have only limited environmental impacts on surrounding waters and seabeds; however, their extensive spatial scale can have other impacts, depending on the location of the facilities. The leases for pearl culture, for example, can spread across large areas, restricting access for other marine users (such as recreational fishers and boaters, and Indigenous people wishing to access their sea country71); dolphins and their calves, which avoid transit through the facilities; and whales, which are at risk of entanglement.72
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