Effects of pH on the gill ATPase activity and blood composition of whitefish (Coregonus peled) and trout (Salmo trutta fario).

1. The effects of pH on blood composition and gill (Na+ + Mg2+) ATPase activity were studied in 142 whitefish and trout kept for 3 to 17 hr in tanks at pH 3.0-9.5 in Northern Finland. 2. pH clearly influenced all blood values, except plasma alanine and aspartate aminotransferases. Blood haemoglobin and packed cell volume of whitefish and trout were lower at pH 9.5 but higher at pH 3.0 than those of controls at pH 6.5. 3. Both fishes had lowest plasma sodium and chloride values in groups kept for 9 to 17 hr in tanks with water acidified to pH 3.0 to 3.5. Changes in the plasma chloride concentration were associated with plasma sodium (r = 0.92, n = 102, P less than 0.001). 4. The gill (Na+ + K+ + Mg2+) ATPase of trout showed high activity over wide ranges of pH (6.0-7.5) at 13 degrees C having a distinct optimum at pH 7.0. 5. Blood glucose and lactate concentrations of whitefish and trout increased after exposure to acid and base. High values at pH 3.0 to 3.5 suggested hypoxic stress due to acidaemia.