Cadmium and zinc in Greenshell® mussels, Perna Canaliculus, from Pelorus Sound, Marlborough, New Zealand

Pelorus Sound, Marlborough is the current centre of aquaculture in New Zealand. The main mariculture species of the area is the Greenshell® mussel, Perna canaliculus. To ensure that mussels grown in Marlborough can be exported to international food markets, the mussels' trace metal levels have to be within limits set by food safety regulations. The levels of potentially toxic trace metals in mussels grown in Marlborough Sound were measured and found to be below the food safety limits set by the Australia and New Zealand Food Safety Council. Although all the levels tested showed some variability, those for cadmium (Cd) were the most variable. Detailed analysis of the variation of Cd and zinc (Zn) levels in mussels from Pelorus Sound was carried out, allowing a comparison of trends in the levels of these two metals. Distance along Pelorus Sound and water depth explained most of the variation in the level of Cd in mussels. The distance from Pelorus River mouth was positively correlated with the level of Cd in mussels within inner Pelorus Sound. Zinc levels, though also variable between different farming locations, showed no relationship between Zn level and distance from Pelorus River mouth. The level of Zn was also dependent on the sex of the mussels, with the levels in female mussels being significantly higher; no such relationship was found for Cd. Investigation of seasonal variability in Cd and Zn levels in mussels showed that both metal levels varied only slightly between months. Total metal content of the mussels, however, steadily increased as the tissue weight increased. Analysis of Cd and Zn levels within the tissue of mussels showed that the metals have a different distribution within tissue types. Highest Cd concentrations were in the gills and digestive tract, whereas Zn was distributed more evenly, with little differences between tissue types. The comparison of Cd and Zn levels between a pure bred family line of mussels with mussels grown from wild-caught spat indicated a significant difference for both metals. Compared to P. canaliculus, significantly higher levels of Cd and Zn were found in Mytilus galloprovincialis. However, due to the presence of symbionts in M. galloprovincialis it was not possible to determine whether the difference in Cd and Zn levels was species related. The level of Cd in Pelorus River water was determined, and the possible source(s) of Cd in the mussels from Pelorus Sound is discussed, along with suggestions for future research