Total mercury concentration in biota of the Gulf of Thailand are shown in Table 7. In the coastal area, almost all mercury concentration in fish were lower than 0.2 μg/g wet. These concentrations could be regarded as a natural background of mercury in fish in general. Nevertheless fishes in the off shore area, in the vicinity of natural gas platforms, exhibited higher mercury concentrations. These fishes were caught and analyzed recently (ARRI, 1998). Between 5% and 10% of fish at Erawan and Funan platforms had mercury concentrations higher than 0.5 μg/g. This concentration is the maximum permissible concentration in fish set by the FAO. The biological magnification of mercury was mentioned in several reports. Fish of higher trophic levels bore higher residue than those in the lower trophic levels. This suggests that mercury might be concentrated in the same manner as organic compounds such as organochlorine compounds, i.e. passed through and amplified along the food chain.