Do Rainbow Trout (Salmo gairdneri) Acclimate to Low pH?

Adult rainbow trout (Salmo gairdneri) previously exposed to long-term sublethal acid stress (3 mo, pH 4.8) in artificial soft water (Ca 2+ = 50, Na + = 50, Cl − = 100 μeq∙L −1 ) were challenged with acute severe acid exposure (4.5–5 h, pH 4.0). Their response in terms of whole-body ionic exchanges and blood chemistry was compared with that of trout that had no previous history of acid exposure (naive fish). Acute pH 4.0 exposure caused significant ionoregulatory disturbances in both acid-preexposed and naive fish. Rates of net Na + and Cl − body losses were twice as large in acid-preexposed fish as in naive fish. The two groups showed similar slight net uptake of acidic equivalents. However, the dynamics of acid–base exchange differed, especially with regard to ammonia excretion which was elevated in acid-preexposed fish and inhibited in naive fish. A larger decrease in plasma Na + , red blood cell swelling, and the maintenance of high-stress indicators (elevated plasma glucose and ammonia excretion, depressed osmolality) confirmed that osmo-ionoregulatory disturbances were more intense in acid-preexposed fish than in naive fish. Thus, long-term sublethal acid exposure did not improve but rather significantly decreased the ability of rainbow trout to respond to more severe acid stress. We conclude that acclimation to acid stress does not occur in rainbow trout.