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A recent report by the Healthy Rivers Commission to the NSW Government has recommended that areas in estuaries suitable for the cultivation and safe harvesting of oysters should be identified and protected. Sydney rock oyster production in NSW and State wide average productivity (oysters/ha/y) has consistently declined since the late 1970’s despite the introduction of new technologies. The decline is not product demand-driven. Only one of the state’s 30 production estuaries shows an increasing trend in production over the past 10 years. The impact of diseases, acid sulfate soil drainage and the introduction of Pacific oysters into the Port Stephens estuary do not account for the overall decline. One hypothesis is that oyster production is decreasing because of limited food (seston) supplies. Oysters probably make up the largest fraction of the total biomass in NSW estuaries and process most of the primary production. Spatial variation in the quantity and quality of seston could limit cultured oyster growth. Farmers have identified the need to investigate oyster production decline and to identify food conditions required for optimum oyster growth and production. We propose to examine the components of seston, their spatial distribution and origin between oyster farming areas and their dynamics over time. The study will determine the spatial distribution of oyster growth rates in farming areas in response to resource availability and environmental conditions. This information will be used to assess the adequacy of food in estuaries, the identification of areas with optimal oyster growth rates and enable establishment of benchmark oyster growth rates for comparative analysis temporally and spatially. This will contribute to improving farming strategies and to identifying farming areas for protection. Oyster growth and food sources (which are intimately related to the concept of carrying capacities) in estuaries are a high priority in the ORAC 2000-2005 strategic plan.

Objectives

1. To characterize the food supply for oysters in selected NSW estuaries (quantity and quality).

2. To determine the spatial distribution and dynamics of seston in selected NSW estuaries

3. To identify the source of matter that supports oyster production and understand the trophic interactions in the aquaculture oyster system.

4. To assess oyster growth rates in terms of seston and nutrient levels

5. To develop a predictive ecological-trophic coupled model of oyster growth and productivity

6. To predict oyster growth based on oyster density and mortality levels, and size, achieving sustainable production parameters for the oyster industry

7. To integrate the above with GIS to map areas in terms of scales of food limitations

8. To transfer the results to the NSW oyster industry

Final report

Author: Ana Rubio

Final Report • 2008-10-20

2004-224-DLD.pdf

Summary

The primary outcome of this study has been to increase the understanding of the environmental drivers that influence the southern NSW Sydney rock oyster (SRO) industry, in particular in the Clyde and Crookhaven/Shoalhaven estuaries and to identify some of the factors that limit the production of SRO. Increased amounts of nitrogen and organic carbon are delivered by increased river flows following rain events and these were found to significantly enhance oyster growth in the two south NSW estuaries. During normal and/or dry conditions, the estuaries were nitrogen-limited suppressing primary production and, potentially, oyster growth. On the other hand, during heavy rain periods, large amounts of nitrogen entered the estuaries, which then became phosphorus-limited. Optimally an intermediate level of Nitrogen:Phosphorus ratio is desired for enhancing SRO production in the south coast of NSW so that neither nutrient is limiting.

An important outcome has been to identify the diet of the SRO in the Clyde River. Prior to this study, diet preferences for the SRO were assessed only under laboratory conditions and using a narrow range of food sources limited to some specific phytoplankton species. In this study a wider range of natural food sources were used as field experiments took place at the oyster cultivation grounds where oysters are exposed to a much wider array of food sources. Through the use of carbon and nitrogen isotopic signatures it was found that seagrass’ debris, its epiphytes and seasonal filamentous green macroalgae played little part in the SRO diet in the Clyde River. However, benthic diatoms were the main contributors of the diet. In addition, the signature of mangrove debris was found to be within the isotopic SRO diet range. Consequently, resuspension processes reflecting wind, currents and water depth play an important role in making benthic food sources accessible to the oysters and thus in coupling benthic and pelagic processes.

Another outcome of this study has been the identification of oyster condition index as an useful indicator of oyster performance in terms of stocking densities in order to assess production carrying capacity levels in an area. Condition index levels were found to decrease with increasing stocking density even when there was no statistically significant trend in oyster growth. These experiments suggested that the lowest experimental stocking density in the tray experiment (1 kg / m2) produced oysters with the highest condition index, at certain times up to 16% higher than the other two experimental stocking densities (2 and 3 kg / m2). The difference in oyster performance during winter was much lower (1%) as a result of oysters spawning at the end of autumn and due to drops in temperature levels. A similar relationship between condition index and oyster density was found in floating cylinders at the Shoalhaven/Crookhaven River. At densities above 0.5 kg / per cylinder there was a consistent and significant drop in condition index. Stocking densities used in this project were lower than the typical stocking density level used in NSW cultivations except for the highest level. However, biomass gain increased with stocking density without reaching a plateau. If a plateau would have been reached this would have indicated there would not be any advantage in having higher stocking densities.

The above environmental drivers were incorporated into a computer model that combined the hydrology and nitrogen levels in the Clyde River. The model was used to investigate the consequences of changes in phytoplankton levels, as the main component of the diet, and oyster growth. The output of this model suggested that an additional food source– a carbon source – in addition to phytoplankton was needed to reach the observed growth rates and that the nutrient deliveries into the estuary from rain events played an important role in enhancing oyster growth.

In addition, a series of simple environmental indices were investigated to assess the carrying capacity of areas or estuaries. The indices chosen are easy to calculate by oyster growers and they can give an indication of when the ecological and production carrying capacity are exceeded.

The above outcomes contribute to the ecological sustainability of SRO farming by identifying an optimum level of stocking density under which growers could maximize condition index of SRO. Overall, mud flat habitats, due to the presence of large biomass of benthic diatoms mainly, have been identified as a key parameter for maximizing oyster growth due to their contribution towards the SRO food source.

PROJECT NUMBER • 2002-661

PROJECT STATUS:

COMPLETED

Aquatic Animal Health Subprogram: enhancing the emergency disease response capability of NSW and Qld Government agencies and industry bodies associated with oyster culture

AQUAPLAN was generated as a National Strategic Plan for Aquatic Animal Health in recognition of the growing importance of protecting fisheries and aquaculture industries from disease. This project allowed NSW Fisheries to begin implementing one component of the National AQUAPLAN objectives,...

ORGANISATION:

Department of Primary Industries and Regional Development (NSW)

TAGS

SPECIES

Aquatic Animal Health Subprogram: development of a disease zoning policy for marteiliosis to support sustainable production, health certification and trade in the Sydney rock oyster

Project number: 2001-214

Project Status:

Completed

Budget expenditure: $281,226.02

Principal Investigator: Rob D. Adlard

Organisation: Queensland Museum

Project start/end date: 6 Jun 2001 - 15 Jul 2005

Contact:

FRDC

TAGS

SPECIES

Need

The rock oyster industry in Australia is currently valued at around $28 million annually. The current output is about half of the industry peak in the late 1970’s. For the industry to survive in the long-term requires the ability to service what may become a premium domestic market demanding a high quality product. The expansion of the industry is likely to be available only from international export, which in turn requires compliance with international regulations on oyster health with a transparent health audit trail. The rock oyster is potentially positioned for re-emerging export success, being a unique product with an extended shelf-life relative to other oyster species (e.g. the Pacific oyster, Crassostrea gigas) and this is an opportunity that should be exploited by the industry.

The techniques of surveillance and diagnosis for molluscan pathogens required by the OIE for imported oyster products are not only stringent and accepted as the worldwide standard, but are also applicable to domestic requirements within Australia. In essence, the regulations state that appropriate diagnostic tests are applied for detecting the presence of pathogens of molluscs (microscopic identification techniques with the potential for specific molecular identification using monoclonal antibodies or DNA probes) which have been collected as part of a surveillance program within delimited coastal zones. The sample size, period and frequency are determined with reference to the cycle of infection of the particular pathogen and its prepatent period. There is an initial 2 year period of surveillance before a zone can be granted a disease-free status, with ongoing surveillance required for this status to be maintained.

The development of a zoning policy framework for marteiliosis will provide a valuable opportunity to implement and field-test Australia’s zoning policy guidelines in a practical context to assist with the development of further zoning policies for diseases of aquatic animals. Considerable interest has already been expressed in the case study by State authorities and it will be discussed at an Aquatic Animal Disease Zoning Workshop in Canberra on 23 January 2001, hosted by the National Offices of Animal and Plant Health. Furthermore, the development of the zoning policy will be of direct benefit to the oyster industry by facilitating domestic and international market access, and through identifying and protecting the remaining disease-free production areas

Objectives

1. 1. The primary objective is to implement and field-test the zoning policy framework developed under Aquaplan in a practical context and to facilitate the development of further zoning policies for other significant diseases of aquatic animals. This will be conducted using marteiliosis as a case study to develop an effective zoning policy that is consistent with internationally recognised (OIE) standards. The zoning policy will aim to:* Reduce the risk of introducing this pathogen into the remaining disease-free production areas

and* Facilitate domestic and international market access for the industry.

2. 2. The sub-objectives necessary to achieve this are to:* Identify through sampling and appropriate diagnosis marteiliosis-free and marteiliosis-endemic estuaries within oyster culture areas

* Determine the specific identity of Marteilia sp. from positive samples through ultra-structural and molecular diagnostics

* Develop a rational and effective program of surveillance for marteiliosis, based on occurrence and an assessment of risk for each oyster producing estuary

* In consultation with fisheries managers and industry, develop a coastal zoning plan for marteiliosis.

Final report

ISBN: 0-9751116-3-9

Author: Robert Adlard

Final Report • 2006-02-01 • 975.12 KB

2001-214-DLD.pdf

Summary

The edible oyster industry in Australia is currently valued at around $62.5 million annually of which rock oyster production accounts for approx 56%. For the industry to survive in the long-term requires the ability to service what may become a premium domestic market demanding a high quality product. The expansion of the industry is likely to be available only from international export, which in turn requires compliance with international regulations on oyster health with a transparent health audit trail. The rock oyster is potentially positioned for re-emerging export success, being a unique product with an extended shelf-life relative to other oyster species (e.g. the Pacific oyster, Crassostrea gigas) and this is an opportunity that should be exploited by the industry.

Within Australia, the Sydney Rock Oyster industry is subjected to periodic epizootics of disease induced by a range of parasitic organisms that produce significant mortality and morbidity of commercial oyster stocks. The most significant of these is the agent responsible for ‘QX disease’ (caused by the protistan parasite Marteilia sydneyi) affecting the Sydney rock oyster Saccostrea glomerata. Management of this disease has been based on quarantine of affected estuaries enforced through limitation on the movement of potentially infected stock. In this context, it was obvious that the oyster industry required a disease zoning policy based on scientifically defensible data to allow domestic best practice in oyster farming and to maximise market accessibility for the industry. This host/parasite system then formed the basis for a test of the zoning policy framework developed under the federal government’s ‘AQUAPLAN’.

A number of key issues related to zoning and surveillance for specific diseases were addressed through this project. Initially the design of field collection and the appropriate test to use for diagnosis were assessed to maximise, and allow quantification of, disease detection limits in the surveillance program.

1. The design of field sampling to identify disease infected oysters was critical in order to reach a statistically robust probability of disease detection. Global animal health standards (Office Internationale des Epizooties) recommend random sampling from a zone to detect a prevalence of 2% or greater disease in a population. This was fulfilled using a computer generated random selection of geographic co-ordinates under which individual oysters were sampled (Angus Cameron, AusVet).

2. The appropriate method for diagnosis of disease, another critical issue in disease surveillance programs, was assessed by comparing the sensitivity and specificity of: tissue imprints (cytology); or tissue sections (histology); or the presence of specific parasite DNA (by polymerase chain reaction - PCR). Our analysis showed clearly that PCR was the most sensitive diagnostic test followed by cytology then histology. PCR also detected the presence of sub-clinical infections which could not be unambiguously identified using either histology or cytology. Confirmatory diagnosis (following PCR) at sub-clinical levels was undertaken using DNA in situ hybridisation tests designed to stain the QX organism specifically in tissue section.

Combined surveillance results from 2001 (NSW estuaries only), 2002-03 (NSW and Queensland estuaries) and 2004 (Queensland estuaries only) demonstrated some significant departures from the geographic distribution expected for QX disease. In 2001 diagnosis was undertaken using cytology and no unexpected occurrences of the disease were observed, with positives recorded only from the Clarence River (1.5% of sample infected), Georges River (47% of sample infected). In 2002 the distribution of disease was significantly different to that expected. Initially using cytology for diagnosis there were no apparent unusual infections with Southern Moreton Bay (0.8% of sample infected), Richmond River (40.8% of sample infected), Clarence River (22% of sample infected) and Georges River (16% of sample infected) recording oysters positive for the disease. However, when PCR techniques were used for diagnosis in estuaries that had never recorded the presence of the disease agent it became obvious that the organism was more widespread than indicated by previous diagnostic testing or previous occurrences of disease outbreaks. In total 142 unexpected positives for Marteilia sydneyi were found in oysters scored as negative by cytological examination during surveillance in this project. Of these, 61 were identified in oysters sampled from estuaries with no prior record of Marteilia sydneyi. These represent oysters from Hastings River, Wallis Lake, Port Stephens, Bateman’s Bay, Tuross Lake, Narooma and Merimbula.

Further testing of these infections confirmed the identity of the QX organism and found it to be present in the oyster tissues at a sub-clinical level i.e. prior to reaching the oyster’s digestive gland where the parasite would normally produce spores. At this stage of development, pathology in the oyster is reduced and the condition factor of oysters is not seriously compromised.

In 2003 surveillance and diagnosis using PCR techniques showed a reduced impact of QX disease with Southern Moreton Bay (0.67% of sample infected), Brunswick River (1.3% of sample infected), Richmond River (13.3% of sample infected), Clarence River (6% of sample infected) and Georges River
(0.67% of sample infected).

This project has had a significant impact on our understanding of QX disease in rock oysters as it applies to management. Rather than the disease agent being limited geographically to those estuaries that experience periodic outbreaks, the agent has been identified in most rock oyster growing areas on the east coast of Australia. As such there is the potential for outbreaks of QX disease in all commercial growing areas (indeed such an outbreak occurred in 2004, with seasonal re-occurrence in 2005, in the Hawkesbury River) and that disease is likely to be regulated through a combination of the dynamics of the parasite lifecycle and the level of oyster fitness. Furthermore, in any aquatic system the environment will play an equally significant role in the outcomes of host/parasite interactions both through direct impact on stages (spores, infective stages) in the lifecycle of the parasite and indirectly through its impact on host fitness.

In the light of our new understanding of the distribution of the QX disease agent it could be argued that management through quarantine of identified QX-endemic estuaries is no longer appropriate. However, the biology of Marteilia sydneyi (dynamics of the life cycle of the parasite, interactions with alternate hosts) and its interaction with the host oyster’s immune system are incompletely understood and the precautionary principle should be upheld especially in the case of such a serious disease.

While estuaries which undergo periodic outbreak should remain closed to export of oysters for relaying live in water elsewhere, local management will focus on disease seasonality and stock rotation to avoid the high risk periods in mid to late summer. These periods should be identified with accuracy to maximise available growth periods in disease endemic areas of estuaries. The ongoing projects to develop QX disease resistant oysters (NSW DPI and collaboration with Macquarie University) should run parallel with a program of incremental addition to the biological knowledge of this pathogen. Specifically, an absence of our ability to maintain a laboratory based infection model hampers research on identifying those factors (pathogen-specific, oyster-specific and environment-specific) which promote disease.