The Interaction of Environmental Calcium and Low pH on the Physiology of the Rainbow Trout, Salmo Gairdneri

Exposure of adult rainbow trout to low pH (pH 4.3) in soft water (Ca2+ = 223 μequiv/1) caused a substantial ionic disturbance which arose primarily because of large net losses at the gills. In contrast, renal ion losses were low initially and declined even further because of a pronounced reduction in urine flow. A net influx of H+ occurred across the gills but this was not sufficient to cause a blood acid-base disturbance or a renal response. Although branchial ion and H+ fluxes declined with time, blood ion levels did not return to normal and many of the fish died. Further reduction in water calcium (Ca2+ = 69 μequiv/l) provoked a higher mortality and a more substantial ionic imbalance. These results contrast sharply with the effects on trout of acid exposure in hard water (Ca2+≥ 1600μequiv/l), where net ion losses and mortality are reduced and H+ uptake increased. A preliminary model for the interaction of low pH and calcium is proposed and evidence for adaptation to acid stress and for the origin of acid lethality is discussed.