Effects of calcium and magnesium hardness on acute copper toxicity to juvenile channel catfish, Ictalurus punctatus

Two experiments were conducted to evaluate the effects of calcium or magnes ium hardness on the acute toxicity of copper sulfate to juvenile channel ca tfish (Ictalurus punctatus) in low alkalinity environments. A preliminary b ioassay determined the 48-h LC50 of copper sulfate to be 1.25 mg l(-1) for juvenile catfish placed in water with calcium hardness and total alkalinity set at 20 mg l(-1) CaCO3. In the first experiment, catfish were exposed to 1.25 mg l(-1) copper sulfate in environments where calcium hardness was va ried from 10-400 mg l(-1) CaCO3. Total alkalinity was 20 mg l(-1) CaCO3. As calcium hardness increased, copper-induced catfish mortalities decreased s ignificantly from 90% at 10 mg l(-1) CaCO3 to 5% at 400 mg l(-1) CaCO3. In the second experiment, catfish were exposed to 1.25 mg l(-1) copper sulfate in environments containing either calcium or magnesium hardness, 20 and 40 0 mg l(-1) CaCO3, with total alkalinity set at 20 mg l(-1) CaCO3. Survival rates in calcium hardness treatments were consistent with those in the firs t experiment. However, 100% mortality was observed in both treatments conta ining magnesium-based hardness. These data suggest a calcium-specific mecha nism with respect to acute copper toxicity in channel catfish. (C) 1999 Els evier Science B.V. All rights reserved.