Molluscs

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Molluscs that may need to be destroyed include bivalves (e.g. oysters, mussels) and gastropods (e.g. abalone). Anaesthetic agents cannot be used with bivalves as they can close their shells to prevent penetration of the anaesthetic agent.

4.3.1 Wild molluscs (open systems)

Mollusc species commonly growing in these systems in Australia include:
  • Sydney rock oyster (Saccostrea glomerata), pearl oyster (Pinctada maxima), abalone (Haliotis spp.)—in salt/brackish water

  • freshwater mussel (Velesunio angasi)—in fresh water.
Methods of destruction
Under most circumstances, if a decision is made to destroy wild molluscs in open systems, the most practical method of destruction will be to use commercial harvesting techniques to collect the molluscs before destroying them. Harvesting methods include removal by divers of individual molluscs from the sea floor (e.g. pearl oyster), or from the reef structure by using a flat-bladed knife (e.g. abalone).

It is recommended that any molluscs harvested by commercial methods be processed in a manner similar to standard practices for the commercial capture technique.

Unmarketable molluscs may be destroyed either by exposing them to sufficient heat to cause the two shells to open (in the case of bivalve molluscs) or by leaving them out of water for a sufficient period to cause death. The exact period will depend on the ambient air temperature; the warmer the temperature, the sooner the mollusc will die.

Non-operculate gastropod molluscs (e.g. abalone) can be destroyed by placing them in a concentrated solution of anaesthetic (isoeugenol or benzocaine).

If the molluscs are located in a body of water where a chemical could be effectively contained, it may be possible to destroy them using a chemical such as copper sulphate. Chlorine and copper sulphate have been used successfully to eradicate pest bivalve species in harbours in northern Australia (see Ferguson 2000). Tests indicated that these agents worked best if chlorine was applied first, followed by copper sulphate. Treatment using heat (60 °C for 30 minutes), chlorine (24 mg/L for 90 hours), detergent (1% v/v for 7 hours) or copper sulphate (1 mg/L for 38 hours) will kill bivalves (Bax et al. 2002). These options must be carefully considered in light of any adverse environmental effects that may result from use of the chemicals.
Epidemiological considerations
It is likely that commercial wild capture techniques will not capture all molluscs in the area and will only reduce the mollusc population size.
Molluscs harvested in this way may be used for human consumption provided that they are of marketable size. Commercially, some molluscs (e.g. abalone) are sent to markets live. Whether this is acceptable will need to be considered, given the potential for disease spread. If it is unacceptable, molluscs will need to be destroyed before being sent to markets.

4.3.2 Farmed molluscs in racks or cages (semi-open systems)

Mollusc species commonly growing in racks or cages in Australia include Sydney rock oyster (Saccostrea glomerata), Pacific oyster (Crassostrea gigas), pearl oyster (Pinctada maxima), abalone (Haliotis spp.) and mussel (Mytilus spp., Perna spp.) in salt/brackish water.

Freshwater farming of molluscs is not known to occur in Australia.
Methods of destruction
If molluscs are for human consumption and are of market size, the preferred method of destruction of molluscs grown on or in rack or cage systems in the ocean or estuaries is to remove them using routine harvesting practices.

Alternatively, molluscs destined for human consumption may be killed by freezing. How this will affect the viability of the disease agent (if present) will depend on the susceptibility of the agent to freezing.

If molluscs are not for human consumption and/or not of market size, the preferred method of destruction is to remove them using routine harvesting practices and then to destroy them by leaving them out of water for a sufficient time to cause death. The exact period will depend on the ambient air temperature; the warmer the temperature, the sooner the mollusc will die.

Non-operculate gastropod molluscs (e.g. abalone) can be destroyed by placing them in a concentrated solution of anaesthetic (isoeugenol or benzocaine).
Chemical methods could also be used to destroy molluscs not destined for human consumption (see Section 4.3.1).

Epidemiological considerations

If the molluscs are loose in cages, they can be taken out of the water and deposited in water-retaining containers or bins on the deck of a boat. Panels, racks or ropes may be treated in the same way. The practicality of this will depend on the size of the boat and the volume of racks or ropes to be removed.

Some bivalve molluscs (e.g. Pacific and Sydney rock oysters, mussels) are sent to markets live. Whether this is acceptable will need to be considered, given the potential for disease spread. If it is unacceptable, the molluscs will need to be destroyed, either by exposing them to sufficient heat to cause the two shells to open (in the case of bivalve molluscs) or by leaving them out of water for a sufficient period to cause death. The exact period will depend on the ambient air temperature; the warmer the temperature, the sooner the mollusc will die.

4.3.3 Farmed molluscs in pump-ashore systems (semi-closed systems)

Mollusc species commonly growing in pump-ashore systems in Australia include abalone (Haliotis spp.) in salt/brackish water.

Freshwater farming of molluscs is not known to occur in Australia.

Methods of destruction

Molluscs may be destroyed by normal processing (freezing or canning).

If removing molluscs from tanks is not acceptable (or the molluscs are not of market size), gastropod molluscs can be euthanised in situ by the addition of lethal amounts of isoeugenol or benzocaine to the water. The required concentration will depend in part on how rapidly the animals need to be destroyed.

Alternatively, sufficient anaesthetic can be added to the tank to sedate the animals, which can then be removed and placed into a more concentrated solution of anaesthetic.

Chemical methods could also be used to destroy molluscs not destined for human consumption (see Section 4.3.1).

Epidemiological considerations

If molluscs are for human consumption and are of market size, the preferred method of destruction is to remove them from the tanks using routine harvesting practices.

Ideally, molluscs will be processed on site. If this is not possible, they can be forwarded to an off-site processing facility, provided that this is acceptable with regard to the potential for disease spread.

Fresh chilled and live molluscs are sometimes sent to markets. The acceptability of this will also need to be considered with regard to the potential for disease spread.

4.3.4 Farmed molluscs in recirculation systems (closed systems)

Mollusc species commonly produced or growing in recirculation systems in Australia include abalone (Haliotis spp.), pearl oyster (Pinctada maxima), Pacific oyster (Crassostrea gigas) and Sydney rock oyster (Saccostrea glomerata) in salt/brackish water.

Freshwater farming of molluscs is not known to occur in Australia.

Methods of destruction

Mollusc species being grown in closed systems are likely to be the broodstock, egg and larval stages of the species.

For small numbers of larger molluscs (e.g. broodstock abalone), cooking is an appropriate method of destruction.

Destruction can be achieved by removing animals from tanks and placing them into a concentrated bath of anaesthetic.

Egg and larval stages of molluscs can be destroyed using sodium hypochlorite at 100 ppm for at least 10 minutes.

Chemical methods could also be used to destroy immature molluscs (see Section 4.3.1).



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