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Environmental risk assessment of shellfish farming in Tasmania


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Crawford, CM 2000 , Environmental risk assessment of shellfish farming in Tasmania.

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The Terms of Reference for this report were to undertake a qualitative analysis of the likelihood and significance of identified impacts associated with shellfish culture on the Tasmanian marine environment. This included a study of the available national and international scientific literature, and relating the environmental effects observed overseas with Tasmanian shellfish production, farming practices and marine environment. The main shellfish species cultured in Tasmania are the Pacific oyster Crassostrea gigas, which was introduced into Tasmania from Japan during the period 1948-1952, and the blue mussell, Mytilus edulis.

Important differences between shellfish culture in Tasmania and overseas were noted. These included much lower levels of total shellfish production from Tasmania, and also lower levels of production per area farmed. Densities of shellfish on Tasmanian farms are generally at least an order of magnitude lower than in major shellfish producing countries overseas, largely because of the relatively low nutrient levels, and hence phytoplankton production, in Tasmanian coastal waters.

There is also a lack of traditional shellfish culture in Tasmania compared with many overseas countries where practices of restocking shellfish beds and harvesting by dredging have changed little since the 1800’s. By contrast, farm management protocols in Tasmania have developed since the 1970’s with an emphasis on efficiency and a greater awareness of environmental management. Legislative-based Management Controls, which limit the production of shellfish per hectare in Tasmania, also serve to minimise environmental impacts. However, there appear to be few effective controls on shellfish stocking densities in other countries.

Beneficial effects of shellfish farming on the Tasmanian marine environment were identified, and included increased monitoring of estuarine and coastal waters and the potential for scallop aquaculture to enhance wild scallop stocks. Improved water clarity and reduced nutrients and phytoplankton concentrations may also occur in some shellfish growing areas due to the increased filtration by cultured shellfish.

Potential detrimental effects in Tasmania include the spread of introduced pests and diseases by movement of stock around the State. Alteration to the habitat may also occur but generally with minor ecological effect, and restricted to within the lease area.

A qualitative assessment of the risks of ecological impact occurring as a result of shellfish farming activities in Tasmania was conducted based on Australian/New Zealand Standards for Risk Management, 1999. This involved identifying the likelihood and consequence of each area of risk, and developing a qualitative risk analysis matrix from which the levels of risk were identified. These risk levels were based on information available from Tasmanian studies and by comparison with effects observed overseas, taking into account total production and density of shellfish. They were also based on shellfish farming only occurring at suitable locations and with industry standard management practices.

Outcomes of the qualitative risk assessment were:
• A high risk of spread of pests and pathogens due to movements of shellfish stock around the State, ( however, it was also noted that a high risk exists from other anthropogenic activities such as commercial and recreational fishing and sea transport).
• A moderate risk within the lease area of changes to the environment resulting from habitat disturbance due to shellfish farming.
• Low risk of ecological impact due to organic enrichment.
• Low risk of reduced food resources for other filter feeders.

Item Type: Report (Technical Report)
Authors/Creators:Crawford, CM
Publisher: Tasmanian Aquaculture and Fisheries Institute
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