Survival time modeling of exposure of juvenile atlantic salmon ( Salmo salar) to mixtures of aluminum and zinc in soft water at low pH

Abstract

The toxicity of mixtures of aluminum and zinc to juvenile Atlantic salmon ( Salmo salar) was studied in low ionic strength waters similar to those encountered during spring snowmelt in salmon rivers on the Canadian Precambrian Shield. Fish were exposed to environmentally realistic concentrations of inorganic monomeric Al singly (≤ 15 μM), and to mixtures of Al and a sublethal concentration of zinc (1.7 to 0.8 μM), at three acidic pH levels (pH 4.5, pH 4.8 and pH 5.2). For Al-only and Zn-only exposures, the LC-50(M) values exhibited a marked pH dependence (higher values at low pH); even when expressed in terms of the free-metal ions (Al 3+ or Zn 2+), to take into account the influence of pH changes on metal speciation, the LC-50(M z+) values were still different at each pH. Modification of the free-ion activity model, to include competition between the proton and the free-metal ions at the gill surface, can explain part of this pH dependence, but there remains some residual dependence on the H +-concentration. as yet unexplained. Fish mortality data at each pH exposure were also modeled with a Cox proportional hazards model. In all cases, the presence of Zn reduced survival times in the Al + Zn mixtures, despite the use of sub-lethal Zn concentrations (only 0.1–0.2 toxic units). Survival time modeling indicated a simple additive mode for the toxicity of the Al + Zn mixture; models with the best fit included a covariate term for Al (either as Al 3+ or as inorganic monomeric Al) and a classification code variable for Zn.