Mercury and stable isotope signatures in caged marine fish and fish feeds

Total mercury (THg) and methylmercury (MeHg) concentrations were determined in four species of marine caged carnivorous fish, one species of herbivorous fish and three types of fish feeds (dried pellet feed, forage fish and fish viscera), collected from five cage sites in the rural areas along Fujian coastline, China. For the carnivorous fish, the concentrations of THg and MeHg ranged from 0.03 to 0.31μg/g and from 0.02 to 0.30μg/g on wet weight basis, respectively. The concentrations were lower for the herbivorous fish with both within the range of 0.01-0.03μg/g. Out of the three tested fish feeds, tuna viscera contained the highest level of mercury (0.20μg/g THg and 0.13μg/g MeHg), with pellet feed containing the lowest level (0.05μg/g THg and 0.01μg/g MeHg). The calculated trophic transfer factor of MeHg was the highest (12-64) for fish fed on pellet feeds, and was the lowest for fish fed on tuna viscera. A significant relationship was found between Hg concentrations in caged fish and in fish feeds, thus Hg was primarily accumulated from the diet. Furthermore, the stable isotope δ ¹⁵N was positively correlated with the Hg concentration in two caged sites, indicating that δ ¹⁵N may be a suitable tool for tracking mercury in caged fish. We conclude that fish farming may be a good way of reducing the human exposure to Hg because mercury levels can be carefully controlled in such farming systems.