Comparative Susceptibility of Freshwater Fish Species to Metals from Ultramafic Soils

Soils derived from ultramafic rocks have a mineralogy rich in metals like Co, Cr, Fe and Ni, and the process of extracting these metals from soil generates highly metallic waste. When disposed near of water bodies these residues pose a risk on the aquatic environment. Two fish species are used in ecotoxicity tests worldwide: Danio rerio (zebrafish), an Asian species, and Oreochromis niloticus (tilapia), an African one. However, Hyphessobrycon eques (mato grosso), a native of South America and found in Brazilian rivers, is as yet little used in ecotoxicity tests. The aims of this study were to evaluate the acute toxicity of the metals Co, Cr, Fe and Ni to the fish D. rerio, O. niloticus and H. eques, and compare its susceptibility. Tests were conducted following the recommendations of the Brazilian Association for Technical Standardization (ABNT). Stock solutions were prepared with the analytical reagents CoCl2.6H2O; K2Cr2O7; FeCl3.6H2O e NiCl2.6H2O. The quantification of metals was carried out in the stock solutions by ICP-AES. The dilution water used was synthetic softwater pH 7.3±0.1, hardness 43 mg L-1; DO 95% and temperature 25±1°C. To perform static tests 10 fish from each species were exposed for 96 hours, in duplicate, to different concentrations of the metals tested. The measured 96-hr initial lethal concentrations LC(I)50 of metals Co, Cr, Fe and Ni, calculated by the Trimmed Spearman Karber method, were 35.7, 35.9, 5.6 and 15.0 mg L-1 for D. rerio, 79.8, 32.6, 8.8 and 49.7 mg L-1 for O. niloticus and 104.1, 23.7, 8.5 and 34.5 mg L-1 for H. eques, respectively. The data showed that D. rerio has greater susceptibility to the metals tested, which justifies its increased use as a test species, except concerning Cr, for which H. eques was the most sensitive species.