Methylmercury in Fish as a Tool for Understanding the Amazon Mercury Contamination Helena do Amaral Kehrig* and Olaf Malm Laborato ´rio de Radioiso ´topos, IBCCF, CCS, UFRJ, Rio de Janeiro—RJ, CEP 21949-900, Brazil In order to evaluate aquatic environmental mercury (Hg) and methylmercury (MeHg) con- tamination, a wide variety of fish species were sampled from some tributaries of the Brazilian Amazon river system, the Balbina reservoir and the Pantanal watersheds. These water bodies present different mercury inputs and biogeo- chemical characteristics. Amazon fish, which are the main pathway of MeHg to the local popula- tion, are the most important protein source for them, and fishing is a significant economic activity throughout these regions. MeHg in fish samples (164) were analysed with an efficient extraction technique and measured by GC– ECD. Analytical quality was checked through intercomparisons between two laboratories with local samples and a certified standard from IAEA. MeHg concentrations of carnivorous, omnivorous, detritivorous and herbivorous spe- cies ranged from 0.1 to 1.25 mg kg 1 wet wt and the mean percentages of MeHg to total mercury were usually higher than 80%. Carnivorous (piscivorous) fish, which represented 74% of all samples, effectively showed higher MeHg con- centrations as well as a higher MeHg/total mercury ratio in muscle tissue than fish from lower trophic levels. In general, MeHg concen- trations in carnivorous fish were higher in places close to goldmining activities, the Madeira river and the Tapajo ´s river near Itaituba city. The MeHg/total mercury ratios in fish were higher in non-impacted areas and with smaller amounts of suspended particulate materials (Negro river, Balbina reservoir and Pantanal watershed). No MeHg seasonal variability was observed in Serrasalmus rhombeus (carnivorous fish) from the Madeira river basin during the year. High variability in MeHg levels was observed in muscle of the same carnivorous species fish with similar weights sampled at the same place and by similar nets. Copyright # 1999 John Wiley & Sons, Ltd. Keywords: methylmercury; mercury; fish; gold- mining; Brazilian Amazon; river basin; reser- voir Received 6 October 1998; accepted 25 March 1999 INTRODUCTION In the Brazilian Amazon region, informal gold production, which uses large amounts of metallic mercury [Hg(0)] to amalgamate the fine gold particles, is one important source of environmental mercury pollution; it is the major health risk for occupationally exposed persons and is a potential risk for the general population. Amazon fish are the most important protein source for the local riverine populations and fishing is a significant economic activity throughout the region. The local riverine populations, which have a high daily ingestion of fish, are exposed to methylmercury (MeHg) through consumption of contaminated fish from goldmining activities in rivers. 1 Methylmercury is formed from inorganic mer- cury by methylation, preferably under anaerobic conditions, 2 and is then bioaccumulated in biota and biomagnified up through the aquatic food chain. 3 In particular, factors such as high bacterial activity under slightly acid conditions in the presence of high concentrations of dissolved organic carbon are very important for methylation. 4 Furthermore, temporary or permanent flooding of vegetation and soil may also increase mercury mobilization and consequently the concentrations in fish. 5 The Amazon aquatic ecosystems present APPLIED ORGANOMETALLIC CHEMISTRY Appl. Organometal. Chem. 13, 689–696 (1999) Copyright # 1999 John Wiley & Sons, Ltd. CCC 0268–2605/99/100689–08 $17.50 * Correspondence to: Helena do Amaral Kehrig, Laborato ´rio de Radioiso ´topos, IBCCF, CCS, UFRJ, Rio de Janeiro—RJ, CEP 21949-900, Brazil. E-mail: kehrig@biof.ufrj.br