Accumulation de quelques metaux lourds (Cd, Cu, Pb et Zn) chez l'eperlan ( Osmerus mordax) preleve sur la rive Nord de l'Estuaire du Saint-Laurent

Abstract

Heavy metals in trace amounts are normal constituents of marine organisms. At sufficiently high concentrations, heavy metals are toxic to living organisms and so it is important to know by how much their concentration may be increased before effects on marine or estuarine populations can be detected or commercial species become unsuitable as food. A method of removing metals is by storage in a particular tissue. Several different sites for storing metals were investigated and concentrations of Cd, Cu, Pb and Zn were examined in muscle, liver and gonads of the smelt ( Osmerus mordax) from the North shore of the St Lawrence estuary. Copper and zinc are constituents of several enzymes and are absolutely essential for normal growth and development, while cadmium and lead are not known to have necessary physiological function. A modified wet digestion procedure was used to prepare biological samples for the determination of trace elements by flameless atomic absorption spectrophotometry procedure, using calibration standards made up in a matrix of similar acidity (Table 1). NBS reference material bovine liver was analyzed along with the samples and the results were within the specified tolerance (Table 2). Analyses were reported on a dry weight basis (Table 3) and the correlations with total body weight were determined by regression analysis. Copper (range 0.3–3.3 μg g −1) and zinc (range 19–38 μg g −1) in muscle fillets were found to be negatively correlated with total body weight (Fig. 1). Apparent decreasing concentrations in these two metal levels in muscle sample with increasing body weight were possibly due to factor such as dilution with growth. Growth may dilute metal concentrations in an organism if tissue is added faster than metal. Livers and gonads contained greater levels of the four metals than somatic muscle. Liver metal concentrations of Zn (range 29–108 μg g −1) and Cd (range 0.06–0.37 μg g −1) increased with total body weight. All equations fit data at P < 0.01 (Fig. 2). Positive correlations between size and metal concentrations suggest that net uptake may occur. Inessential, slowly exchanging metals such as Cd appear to reflect an uptake which tend to become a cumulative process (age dependence of concentrations). The occurrence of insignificant correlation between liver concentrations of Cu (mean value: 4 μg g −1) and environmental concentrations of this metal was consistent with equilibration. Since fish are known to possess the metal binding protein metallothionein, a sequestering agent, detoxification of these metals in fish liver may be by sequestration rather than elimination. Increasing metal concentrations in liver may represent storage of sequestered products in that organ. In the gonads, no significant relationship exists between total body weight and trace metal contents. Results of t-test indicated that females had significantly greater Cu and Zn concentrations, but no significant difference existed between males and females for Cd concentrations (Figs 3 and 4). Thus, the relation between concentration and total body weight appears to be specific as to the species, tissues analyzed and environmental conditions. The comparison of metal concentrations in fish to assess variations in contamination levels requires understanding the relationship between metal concentration and body size within each population.