This survey is a study of fish commercialized in Forto Velho, Rondônia, Brazil from 1934 until 1989. The area studied is the Madeira river in a region of extensive gold extraction activity and therefore subjected to mercury contamination. The results showed that from the total fish yield, the fruiteating species ranged from 39.3% (in 1984) to 49.5% (in 1989). Among them, tambaqui (Colossoma macropomum) is the most captured mass. 22.9% (in 1939) to 32.5 (in 1936) of the species are detritivore and Curimata (Prochilodus nigricans) is the most important. Only 10.7% (in 1935) to 15.1%. (in 1934) are carnivore (Tucunare-Cichla spp and Pourada-Brachyplatystoma flavicans). The paper also deals with some of the ecological factors which can interfere with mercury accumulation in the food chain.