The Health Status of Western Australian Aquaculture
Introduction
Aquaculture in Western Australia is a significant industry that is usually worth more than A$200 million annually based on the culture of pearls, marine algae, crustaceans, mussels, finfish, edible oysters and potentially abalone and prawns (For the values of individual sectors read Aqua Info 28).
Success in aquaculture depends on using healthy stock and managing aquaculture operations so that health problems are minimised. Fortunately, there are few significant health problems in this industry in WA.
Successful assessment of the health status of a State aquaculture industry is dependent on two factors:
- an adequate disease reporting system backed by a sound diagnostic capability which allows for the identification of any disease causing organisms in samples provided by industry, and
- the level of independent disease surveillance undertaken to detect unreported diseases.
The Department of Fisheries has operated a fish disease diagnostic laboratory in Western Australia since 1988. This laboratory has investigated reported disease outbreaks in the State and undertaken surveys and surveillance for some diseases of concern.
Serious aquatic animal diseases with international trade significance are listed by the Office International des Epizootes (OIE) in Paris. Countries that are members are required to report the occurrence of these diseases if they occur. In Australia, States report to the Australian Government that then sends a compiled report to the OIE. The 35 currently listed (2004) diseases are shown in Table 1, of which 10 occur in Australia and six in Western Australia.
Of the 16 fish diseases, only two are known to be present in WA (Viral encephalopathy and retinopathy and the fungal disease Epizootic Ulcerative Syndrome). One virus - Epizootic Haematopoietic Necrosis - has been tested for in Western Australia since 1995 but has not been found.
Of the seven mollusc diseases listed by the OIE, three occur in Western Australia.
Of the eight crustacean diseases listed by the OIE, only one is known to occur in WA (Spherical Baculovirosis), one probably occurs (a strain of Infectious Hypodermal and Haematopoietic Necrosis) and three are believed, based on surveillance activities, not to occur (White Spot Syndrome, Spawner-Isolated Mortality Virus and Gill Associated Virus)
In the following sections, disease incidence in individual aquaculture industries is highlighted. Individual diseases mentioned below are not OIE listed unless they are included in Table 1.
All of these diseases are considered to be harmless to consumers although they may pose serious problems for the species being farmed.
Pearling
The pearling industry is the most valuable of the aquaculture industries in Western Australia and is predominantly based on silverlip pearl oysters (Pinctada maxima), although production of other species is increasing. Unlike pearling industries overseas, there are no significant disease problems in Western Australia, even though the industry spans a wide section of coastline and involves numerous translocations of stock within the State. In the 1980s, the industry was seriously affected by vibriosis, mainly due to the bacteria Vibrio harveyi, but since then improved management methods have all but eliminated the problem. The industry remains vigilant with a complex series of hygiene and translocation protocols accompanied by separation of lease areas to allow for control of serious disease events should they occur. A species of Haplosporidium rarely affects pearl oysters but could be a problem in the future. An intracellular ciliate in the Exmouth Gulf area also causes concerns and translocation of infected batches is prohibited.
Mussels
Up to 1000 tonnes of mussels (Mytilus sp.) are harvested in Western Australian waters annually. The only disease of note is infection of female mussels with the microsporidan Steinhausia mytilovum (a proctistan). This occurs in the major production area, Cockburn Sound, but not at sites in Albany. This parasite causes unsightly puckering of the cooked female mussels and probably reduced the reproductive output of affected mussels, but is otherwise harmless.
Marron and Yabbies
There is a consistent yearly production in Western Australia of about 50 tonnes of marron (Cherax tenuimanus) and about 70-200 tonnes of yabbies (Cherax albidus), depending on rainfall. The industry has carried out surveillance for the exotic fungal disease Crayfish Plague (Aphanomyces astaci) since 1989 and the State, and Australia, remains free of this devastating disease. A breakdown in border control saw the illicit introduction of yabbies infected with the microsporidan disease Thelohania in the 1990s. The disease causes low-level mortalities in affected yabby farms in Western Australia and is managed by restricting movements of yabbies between farms in different geographical areas of the State.
Barramundi
Barramundi (Lates calcarifer) production increased very rapidly in the first few years of this century. Barramundi fry are very susceptible to the listed disease Viral Encephalopathy and Retinopathy (“Barramundi Nodavirus”) and older fish are affected by bacterial diseases, principally Streptococcus iniae and Aeromonas hydrophila. Lymphocystis has occasionally been seen in fingerlings. All of these problems are overcome by appropriate stock management methods, for example, disease testing for Nodavirus before barramundi are moved interstate for farming and, for Streptococcus iniae, avoidance of very high suspended organic matter levels in culture systems.
Abalone
Abalone aquaculture in Western Australia is based on land-based cultivation of greenlip abalone Haliotis laevigata. A national survey of abalone diseases in which Western Australia is participating has commenced. Bacterial problems are the major issue for this fledgling aquaculture industry and are overcome by the development of improved management practices. Perkinsosis has never been found in abalone from Western Australia, though the infective agent Perkinsus is known to be present.
Other species
The aquaculture of flat oysters (Ostrea angasi) in Albany was attempted in the 1990s but was abandoned due to chronic mortalities caused by Bonamia sp. Rock oysters (Saccostrea commercialis) are successfully farmed in Albany on the south coast. Edible oysters are affected by a haplosporidan on the northwest coast, which causes 80 per cent mortality in affected wild oysters and will likely prove an impediment to tropical rock oyster aquaculture.
Trout (principally rainbow trout, Oncorhynchus mykiss) are grown at the Department of Fisheries hatchery at Pemberton and at a range of production and/or tourism-based farms. Since 1995, trout at Pemberton have been tested annually for the virus Epizootic Haematopoietic Necrosis (EHNV) with negative results. Mycobacteriosis was a problem in earlier years but was overcome by reducing environmental stressors.
Prawn hatcheries have operated in Exmouth and Broome for a few years but it is too early to tell what disease issues will potentially accompany the grow-out phase. Surveys of prawns to date have indicated that the State is free of Gill Associated Virus (GAV) - the major prawn virus affecting interstate producers.
Conclusion
There are three key reasons why disease, so far, has not had a major influence on aquaculture in Western Australia. Firstly, some of the industry sectors are relatively new and disease problems may emerge in the future. Secondly, WA is relatively isolated, as is evident in the genetic differences that exist between east and west coast populations of some tropical and temperate Australian species. Finally, the major cause of disease in aquaculture internationally, particularly with crustacean and mollusc aquaculture, has been the movement of live animals for breeding or stocking purposes. In the past, this movement could have been undertaken without regard for, or in spite of, the known disease risk. In addition, diseases that were unknown have been spread to new locations and naive hosts with disastrous results. Western Australia has been fortunate that its isolation and the precautions taken by industry, particularly the pearling industry, to monitor and control farm hygiene, movements and translocations have so far protected the State from serious disease outbreaks.
Acknowledgments
Useful editorial comments were provided by Dr Rick Fletcher, David Prestipino and Jo McCrae.
Further Reading
This popular article does not attempt to provide detailed taxonomy or technical detail for these diseases. The following websites are very useful for accessing more information.
Australian Government Department of Agriculture, Fisheries and Forestry
For further information on fish health activities and procedures within the Department, visit the fish health section of this website
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