Reduction of mercury levels in cultured bluefin tuna, Thunnus orientalis, using feed with relatively low mercury levels

The purpose of this study was to determine whether feeding cultured bluefin tuna with feed fish containing low levels of mercury results in a concomitant reduction in the total mercury levels in tuna. The control group was fed spotted mackerel with average mercury concentrations of 0.052 ppm, and the experimental group was fed horse mackerel in combination with Japanese sand lance, whose mercury concentrations were 0.016 ppm and 0.037 ppm (0.019 ppm in average), respectively. Mercury intake in the control and experimental groups' tunas decreased from 124 to 53 µg/kg body weight/month and from 60 to 19 µg/kg body weight/month over the duration of the 19- and 18 month rearing period, respectively. Three specimens from each group were harvested at approximately monthly intervals initially, and then less frequently as the tuna increased in size, and muscle tissue, internal organs, gill filaments, and blood samples were collected for analysis. In the control group, total mercury concentrations increased from 0.25 to 0.55 ppm in all muscle samples except the front ventral one. In contrast, total mercury concentrations in muscle tissues from the experimental group were less than 0.25 ppm, with no marked increases observed over time. The mercury concentrations of the internal organs in the experimental samples were also lower than those of the control. The study showed that feeding bluefin tuna horse mackerel supplemented with sand lance can decrease the accumulated total mercury levels lower than the maximum permitted level of mercury in fish in Japan (0.4 µg/g). This study is the first report to describe the production of bluefin tuna with such low levels of mercury.