Metal concentrations in oyster, Crassostrea gigas, were investigated using 155 samples collected seasonally during 1996-2001 from the Chi-ku, Tai-shi, and Tapeng Bay areas in southwestern Taiwan. Seasonal variations as well as site difference were identified in the metal concentrations. The metal concentrations of oyster in southwestern Taiwan indicated a typical seasonal pattern of "Winter-spring maximum" and "Summer minimum" Among the three sites, Zn, Cu and Cd contents were found to be the highest in the March sample from Tai-shi, whereas Hg level was found to be the highest in the January sample from Chi-ku. The overall mean concentrations of Zn, Cu, Cd and Hg sampled from the three sites were 860±375, 267±193, 0.954±0.484 and 0.097±0.056 mg/kg dry wt., respectively, representing the baseline metal concentrations of oyster in Taiwan. In comparison with the 1970's survey, except the 1.8-fold increase in Cu, the other three elements were within the same ranges. After the transformation of the dry-weight-base data into flesh-weight-base data based on a ratio of 6.8 to1, the mean metal levels, for the most part, closely agreed with many international food standards. However, extraordinarily high level of Cu (1115 mg/kg dry wt.) in oyster were found occasionally in wintertime. Therefore, it is strongly suggested that regulations with respect to food safety standards of metal concentrations need to be established as soon as possible.

Keywords: baseline, zinc, copper, cadmium, mercury, summer minimum, cultured oyster, Chi-ku, Tai-shi, Tapeng Bay, Taiwan.