Determination of the disease status of Western Australian commercial prawn stocks
1) TO REINFORCE THE TRANSLOCATION POLICY
Western Australia has approved a number of applications to farm prawns and there is increasing pressure to import postlarvae from Queensland. Also, prawn farms using local stock are being developed in areas adjacent to wild stock fisheries.
The disease risks are, therefore, two-fold: from introduced diseases and from endemic diseases which may be amplified through aquaculture hatchery and growout operations. Both situations can pose increased risks to wild stocks in adjacent waters and to the aquaculture ventures themselves.
There is almost no data available on the health status of existing prawn stocks within WA on which to base translocation decisions and disease management plans, other than the knowledge that virus diseases do occur. Whether aquaculture of prawns in WA is economically viable or not, the translocation of prawns from Queensland will result in the introduction of diseases with potential to affect both the disease status of the State and impact on the wild fishery. The prawn trawl industry was worth ca. $30 million in 1993-94.
However, the Fisheries Department of WA does not wish to hamper the development of farms by imposing unnecessary restrictions on translocation and further, under international and domestic guidelines, movements can only be prevented on disease grounds if surveys have actually confirmed a disease-free status. Absence of information is not a valid reason to prevent movements.
2) TO PROTECT THE DISEASE STATUS OF THE WILD STOCK FISHERY
Stocks of prawns off the north coast of Western Australia have not yet been exposed to introductions of prawns from other states or from overseas. They are, therefore, of great value commercially both for the production of specific pathogen free and for high-salinity stock. This potential market is being put at risk by the importation of prawn postlarvae for ongrowing with its potential to also import prawn viruses. Though steps are taken to minimise the risk of inadvertent spread of diseases to the local wild populations, overseas experience has shown that disease spread eventually occurs. We also know very little about the existing virus diseases and their impact on the wild fishery, and relationship to viruses elsewhere in Australia.
Final report
There is little published information on the disease status of the prawns on the north-west shelf, yet these prawns (Fenneropenaeus merguiensis, Metapenaeus endeavouri, Penaeus esculentus and Melicertus latisulcatus) form the basis of a commercial fishery worth in excess of A$42 million in 2001-2002. There are also stocks of P. monodon on the shelf which form an important source of broodstock for the developing aquaculture industry in Western Australia, and potentially also for the Northern Territory and Queensland.
Unfortunately, prawns are infected with a variety of viral diseases, many of which have been translocated to new areas with movements of the host prawn – mainly for aquaculture but in some cases through frozen product destined for human consumption.
There is a zoogeographic barrier at the Torres Strait so there is no reason to expect that the prawns in Queensland and New South Wales will have the same diseases as those in Western Australia. This is particularly so since the northwest shelf has had little, if any, exposure to other areas through translocations. This presents a unique opportunity to study the viruses and other diseases that may have co-evolved in the area with the prawns. This isolation is already under threat, with, for example, the movement for aquaculture purposes of Gill Associated Virus (GAV) infected post-larvae from Queensland into the Northern Territory.
Thus, there are two disease risks for which this project provides background data. The first is the importation into Western Australia of prawns from other states and from the Northern Territory. To assess adequately the disease risk posed by the imports, we need to understand the local disease status and this has been achieved. The second risk is that diseases endemic in Western Australia may pose a risk to aquaculture establishments in other States. This report provides a basis on which those states can assess the risk to their own industries.
