Osmoregulatory and Hematological Responses of Rainbow Trout (Salmo gairdneri) to Extended Environmental Acidification

The dose-dependent relationships of several physiological responses to acid were examined in rainbow trout (Salmo gairdneri) exposed for 22 d to water at pH 4.2–6.0. Significant increases in ventilatory and cardiac rates occurred at pH w < 4.9. Hematocrit and hemoglobin concentration increased progressively with acid stress at pH w < 5.5. Plasma calcium, magnesium, sodium, and chloride concentrations were reduced and plasma phosphate elevated in acid-exposed fish. Seasonal differences were observed in the quantitative responses to acid exposure in the relationship between hematocrit and hemoglobin concentration and in the plasma concentrations of calcium and magnesium. The fractional contribution of sodium and chloride to plasma osmolality decreased linearly with increasing hydrogen ion concentration, and the change in plasma sodium, chloride, and osmolality per unit change in pH w was 35.2 mmol/L, 39.7 mmol/L, and 47.4 mosmol/kg, respectively, in the pH range of 4.5–5.2. We concluded that the discrepancy between the reduction in plasma osmotic pressure and the combined reduction in the major plasma electrolytes is a result of the elevation in concentration of an unidentified plasma solute which offsets 40–45% of the expected reduction in osmotic pressure.