The Australian salmonid industry dominates commercial production in the marine environment in southern Australia. Species farmed include Atlantic salmon and rainbow trout.
Farming of salmonids is conducted predominantly in southern Tasmania and in Macquarie Harbour on Tasmania’s west coast, although it is now being attempted in South Australia (NPI, 2001), and will soon be trialed in Victoria (Gavine & McKinnon, 2001 draft).
Salmonids are produced in a semi-open system in which the fish are contained in a relatively uncontrolled environment, with control of water movement in and around the system virtually impossible (NPI, 2001).
In addition to the salmonids, a range of native fish are being cultured in southern Australia (NPI, 2001), for example yellowtail kingfish (Seriola lalandi) and snapper (Pagrus auratus), athough production is presently on a pilot scale.
In Tasmania, a recent addition to the aquaculture industry is the farming of the fat bellied seahorse Hippocampus abdominalis (NPI, 2001).
The seahorses are grown in tanks at a land-base facility in northern Tasmania, the discharge water considered unlikely to exceed national standards (NPI, 2001).
Impacts on the environment from marine finfish aquaculture are related to farm management practices, and are a result of on-farm feeding and fish husbandry practices and aspects of farm design. The extent and nature of the impacts vary with intensity of production, farm infrastructure and site location.
The repercussions for the marine system depend on the capacity of the local environment to disperse or otherwise assimilate the wastes and to withstand changes caused by aquaculture infrastructure and operations.
Nutrient discharge and Accumulation of waste:
Salmon farming operations result in the release of a number of wastes into the aquatic environment. These include uneaten fish food, fish excretory products and organic matter from net-cleaning that enter the water column and/or settle to the seabed. The major components of solid and dissolved waste are various forms of carbon, nitrogen and phosphorous (EAO, 1998; Ritz & Lewis, 1989). The effects on the food chain from this additional organic input are many and varied, the input leading to water column nutrient enrichment and accumulation of organic matter in the sediments.
In the water column, soluble nutrients can alter the species composition and density of phytoplankton, increasing the risk of toxic algal blooms (DPIF, 1997). The accumulation of organic matter on the seabed, especially in areas of poor current flow, can produce major changes in the sediment chemistry. Changes typically associated with severe organic enrichment are a reduction in sediment oxygen levels and the subsequent production and release of methane and toxic hydrogen sulphide (Pearson and Rosenberg, 1978).
Changes in sediment chemistry in turn have effects on the substrate ecosystem, and may result in major changes to the species composition of sediment flora and fauna in affected areas (e.g. Ritz et al., 1989). Notably though, research has shown that these impacts are usually limited to a small area within close proximity to the cages (Brown et al, 1987; Gowen et al, 1988).
Farm structure and operations:
A number of impacts may occur as a result of the physical farm infrastructure and operations. For example, the construction of wharf facilities and fish cage infrastructure. Benthic communities may be altered through habitat modification and disturbance such as through the effects of increased turbidity, shading and sedimentation. Possible behavioural responses of fauna may also result from disturbances in and around the farm, for example from the increased boating activity.
Disease and use of Chemotherapeutants:
Outbreak of disease is more common in farming operations than the wild as a result of higher levels of stress in fish, high stocking densities and establishment of conditions conducive to incubation of disease organisms. Aquaculture provides opportunity for amplification of disease, though notably it also facilitates early detection of outbreaks due to frequency of testing to protect valuable fish stocks.
Additionally, increased food resources near farm cages attract large concentrations of escaped and wild fishes, which may act as vectors for the transfer of disease and parasites to other native fish (Carss, 1990).
The use of chemotherapeutants, such as antibiotics, is a concern because residuals not absorbed by the fish can potentially enter the environment in uneaten feed and faeces. Information regarding the environmental effects of this is limited (NPI, 2001), and accumulation adjacent to farms is a concern (EAO, 1998).
In Tasmania in recent years antibiotics have been used irregularly in very small quantities and not at all on some farms (DPIF, 1997). This is because virtually none of the major salmonid diseases occur in Tasmania (DPIF, 1997).
There is currently an insufficient understanding of the impacts of chemotherapeutant compounds used in aquaculture (Kevin Ellard, DPIWE, per comm.), and growing concerns over potential environmental effects necessitates careful selection of compounds used (NPI, 2001).
Other Chemicals:
Chemicals are used in finfish aquaculture for a wide range of applications. Not only are they used in fish health, but also to control nuisance organisms on equipment such as nets, and to disinfect and improve water quality (NPI, 2001). The use of such chemicals raises a number of environmental concerns, and they must be registered with the National Registration Authority before use (NPI, 2001).
Antifoulants are an important part of the maintenance of nets and cages in marine farms (EAO, 1998), and are currently being used in the South-east Marine Region. Clean nets allow the unimpeded flow of water and oxygen through the net-cage, and the flushing of excreted and other waste material.
Use of copper-based antifoulants is currently the standard global practice (Dr S. Hodson, Wattyl Aquaculture, pers. comm.), and while copper is an essential trace element in fish metabolism, extensive use for antifouling raises some concern that environmental levels will increase, with resultant damage to natural or farmed organisms (Lewis & Metaxas, 1991). However, research has suggested that in areas of adequate current flow, accumulation of copper is unlikely to occur either in adjacent waters (Lewis and Metaxas, 1991) or in fish tissues (Peterson et al., 1991).
Information Sourced From:
