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

Anadromous whiteÞsh [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 lg liter21 for 4 days. The test waters were: (1) lake water diluted 1 : 1 with ion exchanged water (DLW) and (2) artiÞcial water (AW). The major di¤erences were: [Ca21] 0.069 mmol liter21 in DLW versus 0.034 mmol liter21 in AW, [Na1] 1.5 and [Cl2] 1.8 times higher in DLW, with no dissolved organic material in AW. The fry were sampled daily for the analysis of exchangeable body Na1 and Cl2 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 e¤ects, seen as decreased exchangeable body Na1 and Cl2 content, decreased swimming activity, and increased mortality. The e¤ects of Al were more pronounced in AW compared with DLW. In AW, the Na1 and Cl2 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 Na1 and Cl2 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 Na1 and Cl2 concentrations at which yolk-sac fry became passive; it was at approximately 0.3 lmol per fry in DLW and was slightly higher in AW. In the DLW control with moderate ionic strength, the Na1/Cl2 ratio was above 1 from the second exposure day on but remained closer to 1 in soft AW control. The Na1/Cl2 ratio was predominantly >1 in AW when the fry were exposed to moderate acidities but approached 1 at pH values < 5.0. The Na1/Cl2 ratio was mostly41 in DLW, irrespective of the exposure, indicating cation selectivity of ion leakage pathways in slightly harder water.