Effect of calcium and magnesium hardness on acute copper toxicity to Indian major carp, Labeo rohita (Hamilton) and catfish, Channa punctatus (Bloch)

Four experiments were conducted to evaluate the effects of calcium and magnesium hardness on the acute toxicity of copper sulphate to Indian major carp, rohu (Labeo rohita, Hamilton) fingerlings and juvenile catfish (Channa punctatus, Bloch) in medium alkalinity experiments. A preliminary bioassay determined the 96 h LC50 of copper sulphate to be 0.56 mg L-1 for L. rohita fingerlings and 11.78 mg L-1 for juvenile C. punctatus placed in water with calcium hardness and total alkalinity set at 100 mg L-1 CaCO3. In the first experiment, rohu were exposed to 0.56 mg L-1 copper sulphate in environments where calcium hardness was varied from 50 to 350 mg L-1 CaCO3 and total alkalinity was 100 mg L-1 CaCO3. As calcium hardness increased, copper-induced rohu mortalities decreased significantly from 90% at 50 mg L-1 CaCO3 to 7% at 350 mg L-1 CaCO3. In the second experiment, rohu were exposed to 0.56 mg L-1 copper sulphate in environments where magnesium hardness was varied from 50 to 350 mg L-1 CaCO3 with total alkalinity set at 100 mg L-1 CaCO3. Hundred percent mortality was observed in magnesium-based hardness treatments. In the third experiment, catfish were exposed to 11.78 mg L-1 copper sulphate in environments where calcium hardness was varied from 50 to 400 mg L-1 and total alkalinity was 100 mg L-1 CaCO3. As calcium hardness increased, copper-induced catfish mortalities decreased significantly from 90% at 50 mg L-1 CaCO3 to 4% at 400 mg L-1 CaCO3. In the fourth experiment, catfish were exposed to 11.78 mg L-1 copper sulphate in environments where magnesium hardness was varied from 50 to 400 mg L-1 CaCO3, with total alkalinity set at 100 mg L-1 CaCO3. In this case, 100% mortality was also observed in magnesium-based treatments. Mortality rates in magnesium hardness treatments were consistent with those in the second experiment. These data suggest a calcium-specific mechanism with respect to acute copper toxicity both in rohu and catfish.