Alterations in ion levels of freshwater fish Oreochromis niloticus following acute and chronic exposures to five heavy metals

Freshwater fish, Oreochromis niloticus, were individually exposed to 0, 0.1, 0.5, 1.0, and 1.5 mu g/mL of Cu2+, Cd2+, Cr6+, Ag+, and Zn2+ for 96 h (acute) and 0.05 mu g/mL concentrations of the same metals for 0, 5, 10, 20, and 30 days (chronic). Following each period, metal accumulation and ion levels (Na+, K+, Ca+2, and Mg+2) were measured in the gills, kidneys, and muscles. Except for Ag+, none of the metals killed the fish within 30 days. Silver killed all the fish within 16 days. With the exceptions of Ag+ and Cr6+, as their levels were below detection limits, metal accumulation occurred in the tissues following both acute and chronic exposures. Ion levels in the tissues were altered by metal exposure, the general tendency being a decrease in Na+ and K+ levels and an increase in Mg2+ and Ca2+ levels. Acute exposure to heavy metals seemed to be more effective in altering ion levels of the tissues than chronic exposure. Na+ was the most affected ion while Mg2+ was the least affected. Results of this study emphasize that ion levels in the tissues of O. niloticus can be altered by heavy metals, both in acute and chronic exposures. This suggests that heavy metals should be monitored carefully in ecotoxicological studies in the field due to their importance in fish physiology.