CASE NOTES

OYSTER FARMING AND INPUTS FROM VETERINARY SCIENCE

Richard Whittington and Ika Paul-Pont, Faculty of Veterinary Science, The University of Sydney, Camden

Posted Flock & Herd April 2013

Introduction

Aquaculture in Australia

The total global production of capture fisheries and aquaculture in 2010 was about 148 million tones with a total value of US$217.5 billion (FAO, 2012). Global wild fisheries catches peaked about 1990 and since then aquaculture has provided the balance, and an ever increasing share of total fisheries production. Aquaculture is one of the fastest-growing industries producing animal protein to feed a growing global population. FAO predicts that in the next decade, the total output from capture fisheries and aquaculture will be greater than that of livestock or poultry (FAO, 2012). FAO also recognized that fishing and fish farming are associated with incentives for better ecosystem management and sustainability.

Australian fisheries production was about $2.3b in 2010, 43% of which was aquaculture (ABARES, 2012b). In comparison, all livestock production was worth $6.3b (ABARES, 2012a) but the wool industry had GVP of only $1.9b in 2010 (Australian Bureau of Statistics 2012). These figures illustrate the importance of aquaculture (Table 1). The growth of this industry provides a unique opportunity for veterinary science to provide the kinds of services in research, extension and practice that it has always provided the livestock, pig and poultry sectors, and that it aspires to provide in the context of "one health".

In this paper aspects of oyster aquaculture in New South Wales will be described, together with a current example of veterinary research to tackle a key production limiting disease. Oysters comprise 23% of Australian aquaculture production, the other major components being salmon (Tasmania) and tuna (South Australia). Edible oysters are farmed in substantial quantities in QLD, NSW, TAS and SA (Table 2). According to the NSW Food Authority, there are 76 commercial shellfish growing areas in NSW between Eden in the south and Tweed Heads in the north with around 300 oyster farming businesses. An oyster farm consists of a lease over water, measured in hectares and referred to by some oyster growers as the "paddock", and a shore base with a mooring/wharf for boats, sheds and equipment.

Apart from minor production of flat oysters, there are two main commercial edible oyster species, the Sydney rock oyster and the Pacific oyster, both of which ideally are cup -shaped. Both these species breed in the wild. Traditionally, wild-caught juveniles (termed spat) comprise the bulk of the source of seed for grow-out. More recently, hatchery production of spat has increased and it will eventually take over as the dominant source of seed. Pacific oysters are more readily produced in hatcheries than Sydney rock oysters, and the former species can be engineered to genetic triploidy to improve growth characteristics. Triploidy also dramatically reduces the risk of spawning of this introduced species in the wild, enabling its culture in certain estuaries in NSW (eg Hawkesbury River). In NSW estuaries Pacific oysters reach market size in 1 year, but 2-3 years is required for the Sydney rock oyster. Selection for favourable traits has been practiced for many years. In SRO, a successful research program to breed for resistance to QX disease has led to commercial production of resistant lines in a few small hatcheries, but growers complain of inadequate supply.

Spat are grown in baskets or trays, and are mechanically graded based on size every few weeks to months depending on growth rate. Grading is also required for marketing. The shells are mechanically cleaned periodically. All this requires removal of oyster trays/baskets from the lease, transfer by boat, and work in a land-based shed. Consequently oyster farming is highly labour intensive, and by the time an oyster reaches market it has probably been handled dozens of times.

Oysters are filter feeders. All feed is natural (planktonic, algae) and so oyster farmers are completely dependent on environmental conditions, particularly nutrient loads and salinity levels in estuaries. Both are affected by run-off from urban areas and agricultural industries, and rainfall, all of which influence algal life cycles. Blooms of toxic algae and the potential for contamination of estuaries with sewage has led to a rigorous food safety program under the NSW Shellfish Program which is administered by the NSW Food Authority (www.foodauthority.nsw.gov.au). Rainfall events are monitored closely, and estuary-specific risk-based guidelines are applied to determine when the risk of human enteric pathogen contamination becomes unacceptable, leading to "closure", i.e. a ban on marketing oysters until water quality is deemed to have returned to satisfactory levels. In addition oyster tissues are tested for residues.

Oyster farmers operate in a highly regulated industry, and increasingly are adopting quality management systems and forming local groups to work collectively on the major factors which impact their production and ability to market a product. An example is the Broken Bay Oysters Association (www.brokenbayoysters.com.au). NSW Farmers represents oyster growers through its Oyster Committee (www.nswfarmers.org.au).

Table 1. The economic value of livestock and fisheries in Australia. (Source ABARES 2012 and ABS 2012)

Table 2. The economic value of oyster aquaculture in Australia. (Source ABARES 2012)

Animal health issues

There has been a disturbing pattern of emerging diseases in commercial mollusk aquaculture and wild fisheries in Australia: QX disease, Sydney rock oysters, NSW and QLD; Winter mortality Sydney rock oysters, NSW; Pacific oyster mortality syndrome, NSW; Oyster oedema disease, pearl oysters, WA. These diseases have required a succession of government/industry responses, with no clear solutions. Wild fisheries and aquaculture have both been affected and the economic impacts have been substantial or devastating. In every case the new disease has spread. It was not possible to devise an intervention strategy that would halt disease spread or ensure the recovery of the industry. A recent example of this was in 2005 with the appearance of QX disease in Sydney rock oysters in the Hawkesbury River system. Production dropped from 456,000 dozen in 2005 to 5,450 dozen in 2006 and commercial production was abandoned (Wiseman, 2009). This led to replacement of Sydney rock oysters by triploid Pacific oysters to reestablish an oyster aquaculture industry in the Hawkesbury River.

Infectious diseases in molluscs can be due to a wide variety of pathogens including protozoan and metazoan parasites, bacteria and viruses. Herpes-like viruses are of particular concern due to their economical and ecological impact on cultured marine molluscs for the past 20 years (Renault and Arzul, 2001). A syndrome of "summer mortality" of juvenile Pacific oysters had been recognised for decades in the northern hemisphere, but in 2008 epizootic mortality (80-100% losses) occurred in association with a variant of a common virus. The mutated virus was named OsHV-1 uvar. From 2008 massive mortality of C. gigas were reported in France, United Kingdom, Jersey, Ireland, Spain, Netherlands and the United States. Outbreaks were attributed to interactions between oysters, pathogens (OsHV-1 and Vibrio) and environmental parameters (Davison et al., 2005; Friedman C. S., 2005; Renault, 1994; Renault and Novoa, 2004; Schikorski et al., 2011). Mortality was worse in younger age classes (larvae > spat > juveniles > adults) and water temperature may be a trigger for disease onset.

OsHV-1 uvar was detected in Australia and New Zealand in 2010, causing a disease which was named Pacific oyster mortality syndrome (POMS). OsHV-1 uvar was detected first in the Georges River (Botany Bay) oyster farming leases at the end of 2010, where it caused near total loss of the Pacific oyster industry there. The virus was subsequently found in wild Pacific oysters in Sydney Harbour. A national survey was conducted in 2011 with no evidence being found of the virus in other estuaries in NSW or in other states.

A research program commenced in Australia with two main components, the first being hatchery production of genetically resistant lines of oysters (funded by the Seafood CRC) and a second project (funded by Fisheries Research and Development Corporation) to develop understanding of the pathobiology and epidemiology of the infection, based on the premise that the oyster industry will need to learn to live with POMS by managing husbandry.

PACIFIC OYSTER MORTALITY SYNDROME - A CURRENT PROBLEM

We investigated the epidemiology of POMS in Woolooware Bay, (part of Botany Bay) from November 2011 to April 2012 using an intervention study. C. gigas were stocked into 3 different oyster leases in Woolooware Bay and allocated into groups to evaluate growing height and age. Two heights were used: a standard growing height, and one 300mm higher than this. Oysters comprised adults (12 month old) and spat (3 month old) and were placed in plastic trays with lids. Other oysters were kept in floating baskets at each site. The sites were managed by oyster growers with the assistance of researchers and all sampling was conducted by the researchers. Oysters were placed during October 2011, before POMS had recrudesced. The level of OsHV-1 virus, Vibrio sp., mortality rates and environmental parameters such as temperature and salinity were studied in detail before, during and after the outbreak. Differences in mortality among sites, ages and growing heights were evaluated in relation to the intensity and prevalence of viral infection and the environmental data recorded during the outbreak.

The distribution of disease was clustered in time and space, and dependent on the age and growing height. The pattern of infection and disease was different on different leases suggesting that underlying environmental factors influence disease expression.

Spat were highly susceptible to the virus and all those kept in trays died regardless of growing height. In contrast, the high growing height reduced the deaths of adult oysters by half. These results were consistent at all three sites. In a second smaller experiment, spat survived in floating baskets but not in trays. Implementation of different farming practices, in this case modification of the growing height, could play a role in disease management and help reduce mortality of adult oysters during an OsHV-1 outbreak.

Overall the observations suggest that OsHV-1 is a necessary but insufficient factor in the mortality event. The mere presence of the virus in oyster tissue did not lead to mortality suggesting that other factors are required. These factors could include environmental conditions or other as yet undiscovered pathogens. Vibrio spp. Bacteria did not appear to be involved in this outbreak in Woolooware Bay.

Environmental factors may affect POMS disease events. We found a slight decrease in salinity and variable changes in water temperature just before the mortality events in Woolooware Bay. Importantly, salinity and temperature readings in the Hawkesbury River system were similar, suggesting that POMS could establish there if it spread.

During the outbreak most oysters tested positive for OsHV-1 but the infection prevalence decreased over time suggesting that surviving adults can clear the virus. Some appeared to be resistant as they survived three separate mortality events during the summer 2011-2012. This suggests possible immunological mechanisms and underlying potential for genetic resistance to the infection.

It can be concluded from this study that husbandry factors may strongly influence the survival of adult C. gigas during an outbreak of POMS. It is possible that measures to reduce the level of exposure of oysters to OsHV-1 as distinct from preventing exposure may be sufficient to prevent mortalities. Further studies are indicated to confirm the effect of growing height on mortality rates, as the present research trial was conducted during an unusually wet and cool summer. If the same results are obtained when the trial is repeated in a more typical summer, oyster growers can confidently take steps to reduce the risk of losses of valuable adult oysters should POMS spread in Australia. Further studies are also required to investigate why some oysters appear to be resistant and how some clear the virus from their organs, to precisely identify the seasonal window of infection, to confirm the mode of transmission of the virus in the environment, to evaluate the risk of transmission with equipment and by handling oysters, to understand how environmental factors combine with the virus to cause mortality, whether a certain level of viral load in the environment is needed to initiate mortalities, and to identify potential wild mollusk hosts for the virus.

Underlying this project is a broader responsibility towards the Australian community to ensure the sustainability of Australian aquatic natural resources. During the project we promoted information about oyster health in general and POMS in particular, leading for example to proposals from specific sectors of the oyster industry for voluntary restrictions on oyster movements between estuaries, and objective laboratory testing of oysters for specific pathogens prior to movement, to reduce the chance of disease spread. This will protect both wild populations of molluscs and also commercial aquaculture. We established a fully illustrated website during the project and this has proven to be very popular with industry (www.oysterhealthsydney.org).

Acknowledgements

The research described in this report was funded by the Fisheries Research and Development Corporation, the University of Sydney and the Sydney Metropolitan Catchment Management Authority.

References

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