Attempts to restore lost shellfish reefs in Australian estuaries using recycled bivalve shells as reef cultch to attract wild oyster settlement are gaining momentum nationwide, with several pilot scale projects being undertaken in Victoria, South Australia, Western Australia, Queensland. and soon in New South Wales. Yet at this time there has been no thorough risk analysis of the biosecurity risks involved or determination of best practice biosecurity principles that should be enacted with each recycled bivalve shell commodity in order to protect existing fisheries and aquaculture industries from translocation of pests and diseases into new areas. Because the bivalve shells used for shellfish reef restoration have been recycled through the retail seafood processing and restaurant chains, they can be originally sourced from a wide range of areas throughout Australia (and potentially even overseas in the case of mussel shells (e.g. green lipped mussels from New Zealand) or cockles, e.g. those imported from SE Asia). The scope of this project will, however, be limited to evaluating the presently undetermined risks of dissemination of pests on the outside of oyster shells (e.g. seaweeds like Undaria, Caulerpa taxifolia, fan worms Sabella spallanzanii , sea squirts Cliona intestinalis etc. ) and endemic diseases (e.g. agents of POMS, QX disease, etc.) that may occur as viable microorganisms within remnant oyster tissues inside the shells of Australian native bivalves. This is because evaluation of the exotic biosecurity risks posed by imported commodities is a separate (although equally urgent) matter that should be determined by Australia's Federal Government authorities via import risk analysis. This project aims to fulfill an urgent need by undertaking a risk analysis that identifies the potential biosecurity hazards (pests and diseases) that could be introduced through recycling of domestically sourced native bivalve shells and will determine the relevant sanitising (risk mitigation) methods required to reduce the risk of introduction of each pest and disease of concern to an acceptable level (i.e. negligible risk).
The assessment indicated that heating recycled mollusc shells in water to 80°C for at least 5 minutes would meet the ALOP for all diseases (despite uncertainly for some disease agents due to lack of information, as indicated by ?), and was within the ALOP for all pests of concern. This method would have limited throughput, however, and thus may be useful only for experimental or pilot scale restoration projects. Desiccation of mollusc shells in air for a minimum of 4 months in sunlight at >20°C was also within the ALOP for all of the pests and disease agents of concern. However, if ambient weather conditions at the recycling facility do not allow the shell pile to dry out or exceed 20°C for several months of the year (such as in temperate parts of Australia), extending the desiccation period to 6 months with >1 shell turnover is recommended to provide sufficient safety margin to ensure negligible risk (annual probability of occurrence less frequent than 1 in 100 years). Desiccation of mollusc shells can be undertaken at vast scales (e.g. 1000’s of tonnes of shell per annum), at minimal cost, and thus appears suitable for shellfish restoration at environmentally meaningful scales.