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.