Atlantic salmon (Salmo salar) smolts (mean start weight 42 g) in soft freshwater were exposed to three pH ranges (two replicates in each group) for 35 days: pH 6.56.8 (control group), pH 5.96.3 (medium-pH group), and pH 5.45.9 (low-pH group). All exposures had citrate added to remove labile, toxic Al from the water. On day 35, all groups were transferred to 34 seawater and kept there for 100 days. H+ was the main stressor in the exposures because labile Al was <6 µg·L1 and the gill Al was lower than 27 µg·g dry weight1 (highest in the medium group). The exposure environments did not cause any significant changes to gill tissue structures, gill Na+,-K+-ATPase activity, mortality, and growth parameters during the freshwater period. However, haematocrit was significantly increased and mean plasma chloride was significantly reduced (p < 0.05) in the low-pH group compared with the control group. This study indicated that the H+ concentrations in the pH range 5.45.9 in water containing no to little gill-reactive Al do not impact salmon growth and physiology during smoltification. However, the reduction in blood haematocrit in the low-pH group 3 months after seawater transfer may imply long-term effects of the treatment.
