Intensively cultured fish and shellfish are naturally susceptible to 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.
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 oyster (Ostrea angasi) and the Akoya pearl oyster (Pinctada imbricata). However, the Sydney rock oyster is the mainstay of the oyster industry in NSW. In 2009-2010 oyster production in NSW was worth approximately $42 million (I&I NSW, 2011).
The practice of 'highway oyster farming', which involves transfer of oysters between estuaries for on-growing, was established for Sydney rock oysters during the 1960s. 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: spat collection, growing or finishing. Although not as common as it once was, the practice of highway oyster farming can lead to an increase in production. However, it can also increase the risk of spreading diseases, as well as exotic marine pests including the now well-established introduced (noxious) Pacific oyster. Diseases that have been recorded in Sydney rock oysters include Winter Mortality (research is currently working to determine the causative agent of winter mortality, however it was previously thought to be caused by the protozoal parasite Bonamia [formerly Mikrocytos roughleyi]) and QX (Queensland unknown) disease (involving the protozoal parasite Marteilia sydneyi). Pests known to impact oyster production include mudworms and flatworms.
Disease issues have also affected other oyster species. For example, Pacific oyster mortality syndrome (POMS) has significantly impacted Pacific oyster cultivation around the world and in 2010/11 was detected in wild and cultured Pacific oysters in Georges River/Botany Bay and Port Jackson/Sydney Harbour. Disease has also contributed to the dramatic decline in pearl production in Japan, which has created an opportunity for NSW to enter this industry.
'Winter disease' or 'winter saprolegniosis' is a disease of silver perch which can be responsible for mass mortality events in grow-out ponds. The causative agent of winter disease is a water-borne fungus, Saprolegnia parasitica, and most outbreaks commence at water temperatures below 16°C. Detection can be difficult in characteristically turbid ponds, however heavily-infected fish often swim slowly near the surface of the water (Landos et al., 2007).
NSW Department of Primary Industries (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 Protocol for Health Certification of Barramundi Fingerlings for aquaculture, prior to entry into NSW 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 sterilisation of effluent, as well as a specific import protocolprotocol for the importation of barramundi fingerlings from out-of-state, which involves testing fingerlings for BNNV and other diseases.
Likewise, live barramundi cannot be imported live 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).
Poor hygiene is the most common cause of bacterial disease in trout hatcheries, where disease can spread rapidly if not identified and treated. Like other species, trout are more vulnerable to disease if stressed. Temperature stress (above 19°C) can be a problem in NSW, as well as 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 can have devastating impacts on some native species including threatened Macquarie perch.
To protect native species and reduce the risk of diseases being introduced, currently the Australian Government does not allow any live penaeids to be imported into Australia. So far, this appears to have prevented the introduction of major Penaeid viral diseases such as Yellow Head and White Spot viruses, which have caused very serious disease problems in other countries. See Biosecurity Australia (www.agriculture.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 and 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, Fisheries Queensland 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.
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 Institute (PSFI) Marine Fish Hatchery in 2004. This was the first time nodavirus had been detected in this species and the initial outbreak caused large losses of Australian bass fingerlings at PSFI.
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 private and government 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 are required to undertake testing for presence of nodavirus prior to stocking permits being issued. Hatcheries are provided with the required equipment and training for preparation and submission of samples. The testing requirements for hatcheries have evolved since 2004 with considerable advances in testing availability and capacity. Since 2007, a real time polymerase chain reaction (PCR) has been used to test larvae for the presence of nodavirus.
In addition to this testing program, researchers are undertaking a comprehensive survey of wild Australian bass populations to assess the presence/prevalence of nodavirus in wild populations to better inform future management decisions.
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 farming is a developing industry in NSW. Currently production and research focuses on a small number of species mulloway (Argyrosomus japonicus), yellowtail kingfish (Seriola lalandi) and southern bluefin tuna (Thunnus maccoyii). Mortalities have been reported, in both juvenile and adult marine finfish, 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, particularly at times of stress.