Trace metals in selected fish species from lakes Awassa and Ziway, Ethiopia

Abstract

Samples of two fish species (Tilapia Oreochromis niloticus and Catfish Clarias gariepinus) were collected from six sampling stations in Lakes Awassa and Ziway, Ethiopia. The edible portion was separated and placed in a freeze-drying unit until constant mass was obtained. The calculated moisture content of the two fish species obtained from six sites in the two lakes varied from 79.3%–82.5%. An optimal procedure for mineralizing 2.0 g dried and powdered fish muscle required 7.0 hours and consumed 8.0 mL of 70% HNO₃, 2.0 mL of 98% H₂SO₄ and 10 mL of 35% H₂O₂ under reflux. The accuracy of the optimal procedure was checked by digesting a mixture of standard solutions or spiked fish samples and subsequently determining percent recovery. Recoveries varied from 98.5%–123% in a mixture of standard solutions and from 92.5%–120% in spiked fish samples. Trace metal concentrations in the two fish species were measured by flame atomic absorption spectrometer employing an external calibration graph. Concentrations of trace elements in the fishes ranged (µg element/g dry mass): Zn 23.04–30.92; Fe 19.25–59.75; Cu 1.03–2.78; Mn 1.54–4.46; Cd < 0.24; Pb < 1.66; Ni < 0.99; and Co < 0.71. The maximum levels of trace metals found in edible portion of fishes were far below the thresholds. Hence, without regard to bioavailability, O. niloticus and C. gariepinus may be considered safe for human consumption relative to the analyzed elements.

Key words/phrases: Bioindicators of lake pollution, Clarias gariepinus, Oreochromis niloticus, trace metals

SINET: Ethiopian Journal of Science Vol.26(2) 2003: 103-114