Reclamation of wastewater for polyculture of freshwater fish: bioaccumulation of trace metals in fish

Abstract

The main objective of the present study is to investigate the bioaccumulation of trace metals (Zn, Cu, Cd, Cr, Pb, Ni) in the polycultured fish. A series of eight ponds (Au Tau pond system) were constructed at Au Tau Fisheries Office, Hong Kong Agriculture and Fisheries Department. Polluted river water was pre-treated in the first four ponds by providing sedimentation (S1, S2) and aeration (A1, A2), the effluent from A2 was introduced into the remaining four ponds (C1–C4) for fish culture. Fries of six species of freshwater fish including silver carp, big head, grass carp, tilapia, common carp and black bass were stocked in the ponds (C1–C4) during 11/93–2/94 and 1/4 of normal fish food (peanut cake) was used in the fish ponds. Bimonthly sampling of fish was conducted from 3/94–1/95 and trace metal concentrations in fish flesh and viscera were examined. In addition, the distribution of trace metals in different components of the fish pond ecosystem was also investigated.The results showed that bioaccumulation of trace metals occurred in the organisms at the lower levels of the foodweb and no biomagnification was detected at the upper levels, except for Cu concentrations in mosquito fish. The results revealed that metals in viscera were allometric, negatively correlating with body length (p<0.05) (excluding Zn in the viscera of common carp), which suggested that metal metabolism in fish played an important role in metal accumulation in fish viscera. Comparisons of trace metal concentrations in the viscera of the fish were therefore conducted by Student's t-test with body size as the covariant and the results suggested that higher metal contents in the food led to higher metal accumulation in the viscera. Zn accumulation in common carp was different from that in other species, with the highest Zn accumulation in viscera and in flesh. More than 60% of variations of Zn concentrations in fish flesh could be explained by Zn concentrations in fish viscera and fish body length according to the regression analysis (p<0.001) (common carp excluded) but no such relationships were detected for other metals (Cu, Cd, Cr). Significantly higher concentrations of trace metals were obtained in the viscera of the polycultured fish than those in the flesh, which suggested that viscera played an important role in storing trace metals in fish. When compared with trace metal contents reported in other studies, the present study demonstrated similar values and the fish (flesh) cultivated in the treated wastewater seemed to be safe for human consumption, in terms of trace metal concentrations.