Acid‐base balance and Na⁺ regulation in rainbow trout during exposure to, and recovery from, low environmental pH

Rainbow trout in simulated hard water were exposed to pH 4 for 4 days, followed by 3 days at pH 7, to evaluate both the effects of lowered water pH on the mechanisms of acid‐base balance and iono‐regulation, and the ability of the fish to recover from the pathophysiological consequences of this form of acid stress. Prior to the acid stress, the fish maintained a relatively constant internal acid‐base balance. Na + was absorbed from the surrounding water. At pH 4 a net uptake of H + ions from the water resulted in acidaemia combined with a significant reduction in plasma [HCO 3 − ]. Plasma [Na + ] increased for the first 24 hours of acid exposure before falling to levels significantly lower than the preacidification values. Hematocrit remained above the preacidification levels throughout the period of acid stress. Upon returning to pH 7 the normal acid‐base status was restored within 24 hours. Na + was absorbed from the water, and plasma [Na + ] returned almost to control levels within 24 hours. Hematocrit fell rapidly and remained depressed throughout the 3‐day recovery period. The results suggest that the physiological effects of short‐term exposure to low water pH are probably entirely reversible.