Mercury exposure and source tracking in distinct marine-caged fish farm in southern China

Coasts of South China have experienced an unprecedented growth in its marine-caged fish industry. We analyzed mercury concentrations and stable mercury isotope ratios in fourteen fish species from two cage-cultured farms in Southern China. Total mercury concentrations of all species were lower than the human health screening values, but the human exposures through consumption of several carnivorous fish exceeded the USEPA's reference dose. Isotopic compositions in the sediment (δ²⁰²Hg: -1.45‰ to -1.23‰; Δ¹⁹⁹Hg: -0.04‰ to -0.01‰) suggested that mercury in these farms were from coal combustion and industrial inputs. Commercial food pellets and fresh fish viscera provided the major sources of methylmercury to the farmed fish and dominated their mercury isotopic signatures. Non-carnivorous fish presented lower δ²⁰²Hg and Δ¹⁹⁹Hg values than the carnivorous fish. Using a mixing model, we demonstrated that the majority of mercury in non-carnivorous species came from pellets and in carnivorous fish came from combined diets of pellets and viscera. Meanwhile, methylmercury concentrations and % methylmercury in the fish were positively correlated with δ²⁰²Hg values but not with Δ¹⁹⁹Hg values, mainly because fish eating similar feeds maintained similar Δ¹⁹⁹Hg values. Environmental influences of cage farming such as fish feces and uneaten viscera that continuously provide organic mercury to the environments need to be considered.