The acute lethal toxicity of mixtures of cyanide and ammonia to smolts of salmon, Salmo salar L. at low concentrations of dissolved oxygen

The survival of Atlantic salmon smolts on exposure to constant concentrations of cyanide and ammonia, singly and together, has been measured under laboratory conditions at a concentration of 5 mgl^-1 of carbon dioxide. The 24-h LC50 values of cyanide and of un-ionised ammonia, in fresh water, were 0·073 mg HCN l^-1 and 0·20 mg NH₃l^-1 respectively at a concentration of dissolved oxygen of 10 mg l^-1, and 0·024 mg HCN l^-1 and 0·08 mgNH₃l^-1 respectively at a concentration of dissolved oxygen of 3·5 mg l^-1. In 30% sea water the corresponding values were similar for cyanide but markedly higher for ammonioa. In 80% sea water the values were intermediate between those of fresh water and 30% sea water. Prior acclimation of the fish to the respective toxicant increased the resistance of the fish only slightly to cyanide, but with ammonia the 24-h LC50 was increased between 1·4 and 2-fold after acclimation for 1–3 days to between 0·2 and 0·5 of the 24-h LC50 value. Mixtures of cyanide and ammonia were between 0·6 and 1·25 times as toxic as expected, assuming simple additivity of toxicity.