Ion Regulation in Whitefish (Coregonus lavaretusL.) Yolk-Sac Fry Exposed to Low pH and Aluminum at Low and Moderate Ionic Strength

Anadromous whitefish [Coregonus lavaretus(L.)] were exposed during the yolk-sac phase to combinations of pH values of 6.0, 5.5, 5.0, 4.5, and 4.0 and nominal Al concentrations of 0, 100, 200, and 400 μg liter−1for 4 days. The test waters were: (1) lake water diluted 1:1 with ion exchanged water (DLW) and (2) artificial water (AW). The major differences were: [Ca2+] 0.069 mmol liter−1in DLW versus 0.034 mmol liter−1in AW, [Na+] 1.5 and [Cl−] 1.8 times higher in DLW, with no dissolved organic material in AW. The fry were sampled daily for the analysis of exchangeable body Na+and Cl−contents, and the swimming activity and mortality were registered. Aluminum initially protected yolk-sac fry against acidic water at pH 4.0 both in DLW and in AW. Generally, however, an increase in [Al] and a decrease in pH increased the adverse effects, seen as decreased exchangeable body Na+and Cl−content, decreased swimming activity, and increased mortality. The effects of Al were more pronounced in AW compared with DLW. In AW, the Na+and Cl−content of the fry already decreased after 1 day of exposure to a level that predicts mortalities. There was a time trend in DLW toward decreasing exchangeable body Na+and Cl−concentrations as a function of increasing acidity and [Al]. In DLW, mortalities occurred primarily at pH 4.0; at that pH value in AW, the mortality rate was even higher, and mortalities also occurred after 2 to 4 days at all other pH values. There was a threshold limit in exchangeable body Na+and Cl−concentrations at which yolk-sac fry became passive; it was at approximately 0.3 μmol per fry in DLW and was slightly higher in AW. In the DLW control with moderate ionic strength, the Na+/Cl−ratio was above 1 from the second exposure day on but remained closer to 1 in soft AW control. The Na+/Cl−ratio was predominantly >1 in AW when the fry were exposed to moderate acidities but approached 1 at pH values <5.0. The Na+/Cl−ratio was mostly ≤1 in DLW, irrespective of the exposure, indicating cation selectivity of ion leakage pathways in slightly harder water.