Lethal Levels of Mixed Copper–Zinc Solutions for Juvenile Salmon

Abstract

The toxicity of copper and zinc sulphates to juvenile Atlantic salmon (Salmo salar L.) was tested in very soft water at 17 °C. The salts were tested separately and in mixtures. Incipient lethal levels were 32 μg/litre of copper alone or 420 μg/litre of zinc alone.Concentrations were expressed in "toxic units" by taking them as proportions of incipient lethal levels. Compared this way, resistance-times were similar for the two metals. Experiments showed that the incipient lethal level for mixtures was attained when addition of toxic units contributed by each metal reached a total of 1.0. The lethal threshold was therefore governed by simple additive effect of the two toxicants. This result is useful for applying to pollution problems in the field.In stronger mixtures totalling 2 and 5 toxic units, fish died faster than would be expected from their resistance to the metals separately. This type of potentiation in short-term tests seems to account for more-than-additive effects previously reported in the literature.