Disease management in aquaculture
Intensively cultured fish and shellfish are naturally susceptible to most bacterial, fungal and parasitic infections, particularly at times of stress. Many problems can be avoided by appropriately quarantining new stock before release into culture tanks or ponds, maintaining water quality and a stress free environment and regular disease monitoring of stock. In the event of disease outbreak, stock can sometimes be effectively treated by salt or freshwater baths, or via veterinarian prescribed treatments.
Oysters
Several oyster species are cultured in NSW. This includes the native Sydney rock oyster (Saccostrea glomerata), the introduced Pacific oyster (Crassostrea gigas), the native flat (angasi) oyster (Ostrea angas)i and the Akoya pearl oyster (Pinctada imbricate). However, the Sydney rock oyster is the mainstay of the oyster industry in NSW. In 2005-2006 oyster production in NSW was worth approximately $34 million (ABARE, 2008).
The practice of 'highway oyster farming', which involves transfer of oysters between estuaries for on-growing, was established for Sydney rock oysters during the 1960's. When oysters are moved between estuaries the oysters experience a spurt in growth. Highway oyster farming has also allowed different estuaries to specialise in different parts of the oyster production cycle, be it spat, growing or finishing. The practice of highway oyster farming has led to an increase in production. However, it increases the risk of spreading diseases, as well as exotic marine pests including the now well-established Pacific oyster. Diseases and pests that have been recorded in Sydney rock oysters include winter mortality (caused by the protozoal parasite Bonamia (formerly Mikrocytos) roughleyi), QX (Queensland unknown) disease (involving the protozoal parasite Marteilia sydneyi), mudworms and flatworms.
Disease issues also affect other oyster species. For example, one of the major impediments to the growth of the emerging flat oyster industry has been a disease known as Bonamiosis. Bonamiosos has devastated flat oyster fisheries and farming elsewhere in Australia and the rest of the world. Similarly, disease has been one factor in the dramatic decline in Japanese pearl production, which has created an opportunity for NSW to enter this industry.
- Oyster mortality information
- Oyster health and disease
- QX Oyster disease
- Selective breeding of Sydney rock oysters for fast growth and disease resistance
Freshwater Aquaculture
Silver perch
'Winter disease' is an emerging disease within the silver perch aquaculture industry, which can be responsible for mass mortality events in grow-out ponds. The disease seems to be caused by a water-borne fungus.
- Winter disease in farmed silver perch (Bidyanus bidyanus) in NSW non-technical summary
- Diagnosis, treatment & prevention of the diseases of the Silver Perch
Barramundi
NSW DPI has imposed tight restrictions on the culture of barramundi in NSW because of concern about the potential to introduce barramundi nervous necrosis virus (BNNV), also known as barramundi nodavirus, which could affect a number of species native to NSW.
The Barramundi Farming Policy has a number of provisions to reduce the risk of introducing this disease into NSW and ensure farmers receive good quality, healthy stock. These include the sterilization of effluent, as well as a specific import protocol for the importation of barramundi fingerlings from out-of-state, which involves testing of fingerlings for BNNV and other diseases.
Likewise, live barramundi must not be imported into NSW for other purposes such as the ornamental fish or food trades without a specific permit and health certification see information on Importation of live fish.
Trout
Poor hygiene is the most common cause of bacterial disease in trout hatcheries, where any disease can spread rapidly if not identified and treated. Trout are more vulnerable to disease if stressed. Temperature stress (above 19°C) is the main problem in NSW, then overcrowding and low oxygen. Common parasites include Ichthyophthirius ('Ich' or 'whitespot') and Trichodina, both protozoans.
Trout are also affected by Epizootic Haematopoietic Necrosis (EHN) virus, which can be carried by introduced redfin perch and have devastating impacts on some native species including threatened Macquarie perch. Outbreaks of trout EHN virus in farms require strict quarantine policies.
Other species
Saltwater Aquaculture
Abalone
Prawns
To protect native species and reduce the risk of diseases being introduced, currently the Australian Government does not allow any live penaeids into Australia. So far, this appears to have prevented the introduction of major Penaeid viral diseases such as Infectious Hypodermal Haematopoietic Necrosis Virus (IHHN), Yellow Head and White Spot viruses, which have caused very serious disease problems in other countries. See Biosecurity Australia (www.daff.gov.au) website for details of prawn import risk analysis.
Monodon Baculovirus (MBV) has been recorded in Australia although this appears to have been a problem in hatcheries where prawns were cultured under sub-optimal conditions. Good management practices at the hatchery level, including washing fertilised eggs or nauplii with clean seawater have been shown to be effective in controlling MBV.
Mourilyan virus (MOV), gill associated virus (GAV) and spawner isolated mortality virus have been identified in eastern Australian prawns although MOV and spawner isolated mortality virus are currently considered exotic to NSW. Some of these viruses have caused problems in prawn aquaculture in Queensland, but little is currently known about these diseases. However, the Queensland Department of Primary Industries and Commonwealth Scientific and Industrial Research Organisation (CSIRO) are working on the development of disease-free stocks in an attempt to minimise the effects of these diseases on the prawn aquaculture industry.
- Identification field guide: Gill-associated virus disease (www.daff.gov.au)
- Identification field guide: Spherical baculovirosis (www.daff.gov.au)
Nodavirus in Australian Bass
Nodaviruses have caused mortalities in a range of wild, farmed and ornamental marine fish worldwide. The first detected incidence of nodavirus in NSW occurred in Australian bass* at the Port Stephens Fisheries Centre (PSFC) Marine Fish Hatchery in 2004. This was the first time nodavirus had been detected in this species and the initial outbreak was responsible for large losses of Australian bass fingerlings at PSFC.
The detection of nodavirus in Australian bass has had considerable implications for fish stockings in NSW. In order to allow continuation of stocking activities, whilst safeguarding the aquatic environment from the potential transmission of disease, a technical working group of national aquatic health experts was convened to assist policy development.
With support from the Recreational Fishing Trust and recreational fishing groups, NSW DPI is working with commercial hatcheries to develop a clearer understanding of the prevalence of the virus in Australian bass in NSW. All hatcheries producing Australian bass for stocking into public waters undertake testing for presence of nodavirus prior to stocking permits being issued. Hatcheries are provided the required equipment and training for preparation and submission of samples.
The testing requirements have evolved each year since 2004 with the considerable advances in testing availability and capacity. In 2007, virus isolation (VI) and polymerase chain reaction (PCR) were used to test samples from both broodstock and larvae.
Further research will enhance hatchery management of the disease and improve the understanding of impacts of nodavirus in the wild. Such information will continue to be adapted into risk based biosecurity policies. *Australian bass is a euryhaline species, which means these fish are found in both freshwater and estuarine environments)
Marine finfish
Marine finfish farming is a developing industry, currently focusing on a small number of species snapper (Pagrus auratus), mulloway (Argyrosomus japonicus), yellowtail kingfish (Seriola lalandi) and silver bream (Acanthopagrus australis).
Mortalities have been reported, in both juvenile and adult fish of these species, which have been associated with organisms such as ciliated protozoan and monogenean trematodes. Heavily infested fish may lose appetite, flash or rub their bodies on tank surfaces, swim slowly, lose their flight response when attempts are made to catch them and show rapid or laboured gill movement. Heavy mortality can occur if diseases are left untreated and during weaning and at other times when fish are stressed.