In order to prove that mercury contamination in the middle of the outer gulf was due to natural gas production, an investigation was made by comparing mercury in fish caught from the natural gas production area and the coastal area, including from the Andaman Sea. It was found that mercury in cobia (Rachycentron canadus) in the area of the natural gas production was significantly higher than the concentrations detected in cobia of the coastal areas and the Andaman Sea ( Pongplutong, 1999).