Mercury, cadmium, zinc and copper concentrations and stable isotope ratios of carbon and nitrogen in tiger sharks (Galeocerdo cuvier) culled off Ishigaki Island, Japan

Abstract

We analyzed Hg, Cd, Zn and Cu in the liver as well as Hg and stable isotope ratios of carbon and nitrogen (δ13C and δ15N) in muscle from tiger sharks (Galeocerdo cuvier) in Japan. The Hg concentration in the muscle increased slightly and proportionally with increases in body length, but the Hg concentration in the liver increased markedly after maturation (exceeding 2.7m precaudal length). The Hg concentration in the liver of mature shark was higher than that in the muscle. The Cd concentration in the liver increased with increases in body length. On the other hand, the Zn and Cu concentrations in the liver decreased during the growth stage, but thereafter increased with increased Cd burden due to growth. The marked increase in hepatic Hg in mature sharks may be explained by the continuous intake of Hg via food, slower growth and Hg–Se complex formation. High concentrations of Zn and Cu in the liver of immature sharks and concomitant increases in Zn and Cu with the Cd burden in the liver of mature sharks may be explained by the physiological requirements of Zn and Cu during the growth stage, the induction of metallothionein synthesis due to the Cd burden and the subsequent binding of these metals to metallothionein. The δ15N and δ13C values decreased with increases in body length, suggesting a shift from coastal feeding to pelagic feeding with shark growth. The Hg and Cd concentrations tended to be negatively correlated with the δ15N and δ13C values as a result of the increase in Hg and Cd accumulation due to the growth and the decreases in δ15N and δ13C values due to the sift of feeding area.