Acidification of expired water was studied in rainbow trout exposed to water of pH9.91, 6.80 (control) and 3.88. For the high-pH and the control exposures, the water flowing over the gills was acidified because of the dominant effect of CO2 hydration. During the low-pH treatment, the water was alkalized because of ammonium ion formation and perhaps bicarbonate dehydration. Carbon dioxide excretion was not significantly affected by the high-pH and low-pH treatments but ammonia accumulated in the plasma in both cases.

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