A comprehensive health survey of pearl oysters Pinctada maxima was undertaken across northern Australian marine waters in a collaborative project between fisheries organisations and pearl producers in Northern Territory (NT), Queensland (Qld) and Western Australia (WA). The majority of animals examined in the study represented mature animals from the wild or from pearl culture farms from NT, Qld and WA (4502 animals). The study also reports on 22 batches of 150 spat, examined after spending a minimum of 6 weeks in open water sites in WA as part of the regulatory controls in place controlling oyster movements in the State. A low number of mature and immature animals examined for disease investigations and following placement in sea cages/panels in NT were also included in the study.

The study established the occurrence, prevalence and distribution of a taxonomically diverse range of microbial, protozoan and metazoan agents associated with pearl oysters in Australian waters and, within the limits of the study, ascribed pathogenic significance to these agents. In some cases, the prevalence and distribution of agents identified in earlier studies were established. The majority of animals examined were free from infectious agents which may adversely impact upon oyster growth and pearl production. A proportion of oysters carried agents which were not considered significant pathogens. A number of microbial, protozoan and metazoan agents were identified in the shell matrix or in the tissues of the oyster which were considered to have potential to adversely impact upon the breeding, rearing and production of pearl oysters in Australian tropical waters.

Pathogenic or potentially pathogenic agents identified in mature P. maxima from clinically normal populations in the study included a papova‐like virus of the palp associated with epithelial hypertrophy and cilia loss, viral‐like inclusion bodies in the digestive gland associated with tubular degeneration, enigmatic protozoan‐like bodies associated with severe degenerative and inflammatory lesions in the digestive gland of mature oysters and a copepod associated with oesophageal occlusion and epithelial erosion. The shell matrix was also a target for potentially pathogenic boring bivalves, invasive sponges and mudworms, resulting in shell denaturation and blistering.

In the first 6 weeks of exposure of juvenile oysters to the marine environment in WA, a Haplosporidian sp. with high morbidity was detected, together with a heart apicomplexan, palp virus, rickettsiales‐like agent in the digestive gland, viral‐like inclusion bodies in the digestive gland, a copepod in the digestive gland, Ancistrocomid‐like ciliates in the alimentary tract and gills.

Sequential examination of batches of juvenile oysters up to 23 weeks after placement in the sea in panels in the NT demonstrated progressive colonisation by a range of unidentified protozoan and metazoan organisms.

Examination of diseased mature and juvenile oysters in NT associated mortalities with Vibrio spp., an enigmatic protozoan‐like agent and abnormal environmental parameters.

A number of agents showed marked differences in distribution between states and between regions within states. The establishment of a restricted geographic distribution of potentially pathogenic agents in Australian P. maxima provides a basis on which rational quarantine may be implemented to avoid introduction of deleterious agents or pests when considering translocations or introductions of oyster stocks from different regions within Australia.

The study established normal histological criteria for P. maxima and defined a range of host responses to injury. These studies provide a basis on which the normal structure of the pearl oyster may be differentiated from the structure altered by disease, thus establishing criteria for disease diagnosis in pearl oysters. The normal histological criteria and histopathological changes associated with infectious and non‐infectious conditions found in the study are to form the basis of an FRDC atlas of pearl oyster morphology and pathology.

The study included a comprehensive review of infectious and non‐infectious agents, conditions and disease states of pearl oysters Pinctada spp. This review provides an international comparative basis on which to diagnose infections and disease states in Australian oysters and also provides an international perspective if introductions from elsewhere are contemplated.

All findings of the study have been collated on a relational database which can be utilised to determine the prevalence, occurrence and distribution of all agents and conditions identified and by which correlations between variable factors and specific agents or conditions can be made. It is intended that the database be made available to interested parties.

The study provides essential baseline data on disease occurrence and prevalence and a basis for the diagnosis of infectious and non‐infectious diseases of P. maxima. Avenues for further investigation of infectious agents are suggested.

Keywords: pearl oysters, Pinctada maxima, health survey, populations, pathogenic significance, regulatory controls, serious diseases, detection

This small, but extensive, sampling survey was conducted on South Goulburn Island, located off West Arnhem Land in the Northern Territory (NT) to assess the occurrence of heavy metals (both spatially and temporally) in tropical blacklip (Saccostrea mytiloides) and milky (Saccostrea mordax) oysters. Heavy metals tested where those identified by the Australian Shellfish Quality Assurance Program

Results were used to determine whether heavy metal levels exceeded the Maximum Residue Levels (MRLs - or MLs as the more commonly used terminology) set by Food Standards Australia New Zealand (FSANZ) within the Australia New Zealand Food Standards Code (ANZFSC). The range of metals tested were chosen based on previous national residue surveys in seafood across the NT (and our preliminary screening of the study site) that indicated likely contaminants. For example, in this study mercury was not tested as the preliminary screening test done on South Goulburn Island indicated mercury to be low (0.005-0.007 mg/kg; ML 0.5mg/kg) and previous extensive heavy metal testing done by various national surveys along the NT coastline over the last few decades reported consistently low levels of mercury in various seafood products.

This sampling survey was initiated in response to an unforeseen event that arose in the early development phase of the Indigenous oyster enterprise program of the NT Government’s Aquaculture Unit. In December 2011 opportunistic samples of oyster flesh taken at two sites on Goulburn Island showed high levels of cadmium and arsenic, both at levels above the MLs for these elements. The implication of these results for Indigenous organisations planning to sell tropical oysters into Australian seafood markets was unknown at the time.

A more extensive assessment of the occurrence of heavy metals in potential growout areas was needed to assess the risk to human health and identify possible management strategies to ensure oyster product met the food safety standards set by the FSANZ. To assess the risk to human health from heavy metals in tropical oysters the following objectives were addressed:

1 Conduct a sampling survey of the spatial and temporal variability of heavy metals in tropical oysters (blacklip and milky) in the West Arnhem region.
2 Assess the implications of results on the development strategy of the oyster enterprise and the sale of tropical oysters into the Australian seafood market.
3 Employ Indigenous partners to conduct the shellfish monitoring outlined in this project to develop Indigenous capacity in fisheries sciences and an additional employment steam for Indigenous people.

The Aquaculture Unit of the Department of Primary Industry and Fisheries, the Goulburn Island Indigenous Aquaculture Team and Charles Darwin University (CDU) researchers collaborated to measure trace elements (metals) in blacklip and milky oysters collected from four sites around South Goulburn Island. Sampling (of oysters and seawater) was conducted during the dry season in September 2012, the wet season in February 2013, and again during the dry in September 2013. Samples were collected from the shore within a 24-hour period during extreme low daytime tides, flown to CDU’s Environmental Chemistry and Microbiology Unit (ECMU), where they were analysed for heavy metal content. A suite of heavy metals were analysed but of prime interest were arsenic (As) (note - FSANZ considers arsenic as a metal for the purposes of the Food Standards Code), cadmium (Cd) and lead (Pb) as MLs are set by FSANZ for these elements only. Oyster product must conform with MLs set for these metals to allow placement of product in the Australian seafood market.

The results
Ideally, oyster sampling would target market sized animals within a narrow size range (10-15 cm length), as the heavy metal content of these aniamls would be assumed to reflect heavy metal contect of harvestable animals from commercial operations. However this was not possible as the oyster sampling program conducted in this study was done on a remote island, at remote sites across the breadth of the island that were accessably only during dry weather conditions, and during a small window of opportunity when oyster beds were exposed during extreme low tides. As a result, the data is compromised due to the small sample size for some sampling sites and times. Every effort was made to meet the targeted sample size and number, but final oyster samples were limited to those that were available.. An initial collection trip failed to collect sufficient samples at most sites and so was not included in the dataset. Farmed blacklip oysters were deployed during the project to increase sample availability. Subsequent collections were sometimes done at night-time low tides to ensure all sites were sampled. It must be noted that accumulation of heavy metals may differ between oyster age classes (and size), most likely due to different exposure times. Thus the smaller size range of oysters collected in this study may be an underrepresenation of heavy metal content of marketable oysters.

Our analysis of trace elements in milky and blacklip oysters in the West Arnhem region showed that the heavy metal content of oysters differed between sites and sampling times and that the two species accumulated heavy metals differently. Farmed blacklip oysters showed different heavy metal accumulations than wild caught blacklips at some sites.

Wild harvest blacklip oysters accumulated Cd levels that exceeded the food safety standards at all sites and on each of the three sampling events (two during the wet season and one during the dry) over the 12-month survey period.
Farmed blacklip deployed for up to 12 months repeatedly exceeded Cd at only one site (site 2) for the three sampling event. There were no other exceedences of Cd by farmed blacklip at any other sites or sampling events.
Wild harvest milky oysters also exceeded Cd levels at site 2 for each of the three sampling events. They also exceeded Cd at one site (site 1b) on the first sampling event.
We also tested total arsenic in the two oyster species. Levels of total As recorded in this study suggests that the inorganic component to which the guidelines relate are not likely to have been exceeded. Further As speciation analysis would be needed to confirm this.
The lead content of oysters was below MLs for all sites and at all sampling events.