MERCURY LEVELS ON CARNIVOROUS FISHES OF THE MADEIRA RIVER BASIN, RONDÔNIA, BRAZIL.

Gali, P.A.S.1; Silveira, E.G.1; Bastos, W.R.1; Vergotti, M.1 and Torres, J.P.M.2

1Environmental Geochemical laboratory, Geography Department, Federal University of the Rondônia (UNIR), Brazil. BR 364 Km 9 sentido Acre, Porto Velho-Rondônia CEP: 78900-000 Email: labmerc@unir.br

2Radioisotopes Laboratory, Federal University of the Rio de Janeiro, Brazil.

During de last thirty years, the Madeira river basin at Rondônia State, has been stage of great gold mining exploitation. Huge amounts of gold have been extracted from aluvionary deposits. The goldminers use to work on floating boats and dredges using large quantities of Hg for amalgamation of the very fine gold particles found on bottom sediments.

Considering the Hg characteristics, its toxicity, bioacumullation and the deleterious effects observed upon fetuses a great debate is taking place about the possible consequence of the great input of Hg into the Amazonian environment, where several riverine communities depends on fish as a major protein source. The goldminers (^Ógarimpeiros^Ô), although very important for the regional economy, assume a villainy position, responsible for the disaster announced to come.

In order to investigate the possible Hg exposure of the fish eating population of the Madeira river basin, the levels of this metal were determinate in three carnivorous fish species that are very popular and largely consumed, the piranha (Serrasalmus rhombeus), the tucunaré (Cichla ocellaris) and the traíra (Hoplias malabaricus).

All samples collected directly at the Madeira river and at two of its main tributaries the Jaci-Paraná (a former goldmining area, now a deposit for old dredges) and the Jamari river (were there is no history of goldmining activities). The Jamari River has nowadays a very large hydroelectric power plant.

The analyzed samples consist of a dorso-lateral piece of muscle (edible tissue). The Hg determination was based on microwave digestion techniques followed by CVAAS (Perkin-Elmer FIMS-400) at the Geochemical laboratory of the UNIR.

Forty nine percent of the sample exhibit Hg levels over 0,50 µg.g-1, the maximum permissible concentration (MPC) established by the WHO. The observed range of values is between 0,14 and 0,83 µg.g-1. The piranha is the fish specie with higher concentrations; around 60% of the samples surpass the MPC. The piranha is one of the most voracious fish of the Amazon basin.

The high Hg concentration found indicates the possibility of transference of this metal to the riverine population. However, among the other fish species available for consumption, the quantitative participation of the carnivorous ones remains to be established.

Financial support: UFRJ, UNIR, PRONEX and CNPq.