Aquaculture Innovation 1.1
Reduction of Pacific oyster mortality by improving farming & processing techniques in South Australia.
| ID Number | Project Number | Manager | Start Date | End Date | Total Project Funds ($) |
| R&D - 5645 | 2003/208 | Li, Xiaoxu | 01-Dec-2002 | 30-Dec-2006 | 364,200 |
Outcomes Achieved to Date
The research on stress assays conducted in this project has provided a tool to determine best practice techniques in oyster farming. Once adopted, this should result in an improvement in the performance of oyster farms. A 1% reduction in oyster mortalities would save approximately $200,000 per annum for the South Australian oyster industry. In addition, this project has contributed to a better understanding of the frequency, location and possible causes of Pacific oyster mortalities in South Australia and on disease agents that occur in oysters farmed in South Australian waters. This information will be used to help reduce and manage future mortalities. Finally, the survey of oyster processors has provided valuable information regarding the quality of South Australian oysters and the most important traits with respect to their marketing. This has highlighted to the industry where improvements need to be made to increase the saleability of oysters and to expend into new markets.
Non-Technical Summary
Ever since mortalities in Pacific oysters (Crassostrea gigas) were first recorded in South Australia (SA) about 10 years ago, oyster farmers have continually refined farming methods in an attempt to increase oyster survival, although limited improvements have been made. In some regions unusual mortalities occurred nearly every year prior to 2002, especially during the summer periods when ambient temperatures have remained high for several days. The purpose of this project was to investigate oyster mortalities and to minimise their occurrence by developing low stress management strategies for the oyster industry.
In April 2005 it was decided at a Project Steering Committee meeting (comprising SARDI and SAORC representatives, but subsequently involving FRDC) that the project should be terminated due to:
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no unusual oyster mortalities having occurred in South Australia during the two years since the project’s commencement in June 2003;
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further refinement of the stress assays would be required prior to their application; and
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oyster industry representatives pursuing a best practice husbandry manual for the farming of Pacific oysters in SA waters through other means (industry bench mark workshops).
At that time objectives 1, 3, 5 and 8 had been achieved:
1. Design and conduct an industry survey to assess the location, time and prevalence of oyster mortality.
3. Design and conduct an oyster health survey to investigate the prevalence of oyster pathogens in stock in SA coastal waters.
5. Identify quantitatively the stress levels created by different equipment and different farming and processing activities.
8. Communicate advice to farmers.
Objectives 2, 4, 6, 7 and 9 were not achieved and were subsequently cancelled:
2. Design and conduct on farm experiments to investigate the correlation between oyster mortalities and environmental and biological factors.
4. Reduce the number of possible causative factors by analysing the data from the industry survey, on farm experiments and oyster health survey.
6. Identify quantitatively and qualitatively the likely pathogens responsible for oyster mortality.
7. Develop best practice advice for farmers.
9. Develop a best practice husbandry manual for the farming of Pacific oysters in SA water.
The 1st project objective was to design and conduct an industry survey to assess the location, time and prevalence of oyster mortalities during the last five years from 1998 to 2003. Two industry surveys were conducted to gather information relating to mortality, health and quality of oysters. The farm-based survey found that farmers from Coffin Bay and Cowell had not experienced any unusual mortality events whilst some Denial and Smoky Bay farmers did report oyster mortalities. Some farmers reported higher mortalities in family lines compared to standard commercial stock. Many farmers thought that mortality occurred at the spat stage. The percentage of spat farmers expected to reach saleable size ranged from 50 to 95%. Most farmers thought that low mortality occurred in adult stock with estimated losses at each grade ranging between 1-2%. From the survey it was apparent that most farmers attributed mortalities to stock management and stress related issues, including grading, air exposure, high air and water temperatures and the reproductive cycle of the oysters. The survey also revealed that most farmers did not keep detailed records that allowed determination of when and where oyster mortalities were occurring.
The main issues raised from the processor/wholesaler survey were spawning oysters during summer, mudworm infestation and non-uniformity in oyster size and condition. Five out of eight processors/wholesalers stated that meat to shell ratio was the most important quality trait. The rest ranked uniform shell size as the most important quality trait. Four processors/wholesalers reported having observed abnormalities in oysters they had received during summer months. Two processors/wholesalers observed oysters with a dark or black spot in the white of the meat and an unusual smell.
The 3rd objective of the project was to design and conduct an oyster health survey to investigate the prevalence of oyster pathogens in stock in SA coastal waters. A shellfish health survey was conducted by the PIRSA Aquaculture group. A total of 2,238 Pacific oysters from 16 sites in SA were collected for assessment. The pathological findings of the survey were:
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the detection of microcell-like cells in vesicular connective tissue;
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the abnormal hypertrophied nuclei with marginated chromatin in vesicular connective tissue cells;
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atrophy of digestive tubules;
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a viral gametocytic hypertrophy-like lesion in the gonad;
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metaplastic changes of the digestive tubule epithelium (a lesion distinct from tubule atrophy);
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diapedesis through the gut epithelium;
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parasites and symbionts; and
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rickettsia-like organisms.
The health survey has helped to produce a pathology-based overview of the disease agents that currently occur in SA. This information will further help the SA oyster industry to determine the relationship between oyster mortalities and pathogens.
The 5th project objective was to quantitatively measure the stress levels created by different oyster farming equipment and different farming and processing activities. Three techniques were developed and evaluated to measure stress as part of six student projects (five Masters projects from Dalian Fisheries University, China, and one Honours project from Flinders University, South Australia). The three techniques evaluated were: 1) measurement of catecholamine levels in oyster blood, 2) measurement of phenoloxidase activity in the blood and 3) assessment of lysosomal membrane integrity using neutral red staining. Results from laboratory and field experiments showed, based on the methodology used, that the phenoloxidase assay was not a reliable method for detecting stress responses in Pacific oysters. Measurement of catecholamine activity was suitable for single stressors while the neutral red retention assay could be applied to both single and multiple stressor analysis.
In this study the major factors identified in the industry survey as contributing to oyster mortalities or poor performance were assessed using either the neutral red retention (NRR) assay, catecholamine analysis or both. Significant differences were found when animals were subjected to:
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slow and rapid temperature changes;
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exposure to different air temperatures for different durations;
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recovery after different air exposure treatments;
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starvation or food deprivation;
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gonad development;
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spawning and post spawning recovery;
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grading for different durations;
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grading with different types of graders; and
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recovery after grading.
In addition, significant differences were also found between animals of different sizes in response to some of the above-mentioned treatments using the NRR assay.
It is therefore reasonable to assume that these assays could be used as a tool to evaluate the stresses induced by different farming practices and thus identify those which result in the least stress, leading to an improvement in oyster performance on farms.
The 8th project objective was to communicate advice to farmers. Results that could be used to improve the oyster farming practices were immediately circulated to the Project Steering Committee and the South Australian Oyster Research Council (SAORC). The oyster growers were directly informed through four articles in the “SAORC Newsletters” and two presentations at SAORC annual meetings. Five articles in scientific journals have also been published at this time.