Research Document - 2000/104
Cadmium in BC farmed oysters: A review of available data, potential sources, research needs and possible mitigation strategies.
By Kruzynski, G.M.
Abstract
In response to a request by the Canadian Food Inspection Agency (CFIA) in May, 2000, the Department of Fisheries and Oceans (DFO), Pacific Region undertook to investigate the potential reasons for apparently elevated Cadmium levels in BC cultured Pacific oysters Crassostrea gigas. Earlier in the year, CFIA reported that several shipments of BC farmed oysters had been rejected by the Hong Kong market for being in excess of their 2 ug Cd/g (ppm) wet weight limits. A preliminary literature search was conducted, contacts were made with shellfish growers and processors, both Government and academic researchers were consulted and the resulting information was synthesized in the form of this report.
There are no historical baseline data on Cd residues in BC cultured oysters, so Cd residue data on wild intertidal oysters collected over the period 1973-1999 by Environment Canada and DFO were used as a comparison. These were mapped and overlaid on current CFIA 2000 oyster data and indicated that in the northern reaches of the Strait of Georgia, there were already Cd residues approaching 2 ppm 27 years ago. In the absence of any obvious anthropogenic Cd inputs, it is suggested that Cd is naturally available in some surface waters where oysters are reared, whether from mineral deposits local geology or sediment transport from watersheds or the heads of fjords. The bioavailability of dissolved Cd may be enhanced by low salinities.
On the West Coast of Vancouver Island, Cd accompanying nutrients such as phosphate upwelling from deep waters, could also be a contributing factor. Among recommendations are the holding of a Cadmium and Oysters Workshop to exchange information with oceanographers, geologist, geochemists, oyster growers, toxicologists, biochemists and biologists; the objective being to define fruitful avenues of research and possible mitigative strategies. A culture materials Cd leaching experiment, additional sampling of beach vs previously suspension cultured oysters, and a 1-2 year grow-out experiment using the same genetic stock of oyster distributed to geographically distinct locations on the BC coast are also suggested.
Maps showing the relationship of BC coastal geology and existing stream sediment Cd data to oyster growing areas are provided and the suggestion is made that BC Fisheries may wish to consider local geochemistry in future oyster lease suitability approvals.
Several hypotheses on Cd pathways from both marine and terrestrial sources have been gleaned from the literature and potential applications to the BC situation are suggested.
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