Aluminium and acid rain: Mitigating effects of NaCl on aluminium toxicity to brown trout (salmo trutta fario) in acid water∗

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Abstract
The comparison of a fish stocking experiment in a Swiss mountain lake (Lake Laiozza) with results obtained in a South Norwegian lake (Lake Liervatn) revealed contradictory results as to the toxicity of the respective acid water. This, even though the pH, aluminium concentration, conductivity, and ionic composition of the two lakes proved to be almost identical. Lake Liervatn water was less toxic and had a substantially higher NaCl concentration. In order to answer the question whether NaCl could have a mitigating effect on pH‐aluminium toxicity to fish, experiments were performed in the laboratory using “Synthetic Laiozza”, a media made up from deionized water and salts added according to the concentrations found in Lake Laiozza. Synthetic Laiozza was then enriched with 0, 0.125, 0.25, 0.5, and 4.0 meq NaCl per liter media. The addition of 0,0.125,0.25 and 0.5 meq NaCl/L had no significant effect on the survival time of the fish (all MT50’s laying between 16 and 23 hours), whereas the addition of 4.0 meq NaCl/L resulted in longer survival of the fish i.e. MT50 = 85 hours. The analyses of plasma electrolytes on the other hand, revealed a progressive reduction in electrolyte loss with increasing ambient NaCl concentration.