ACCLIMATION OF RAINBOW-TROUT (ONCORHYNCHUS-MYKISS) TO LOW ENVIRONMENTAL PH DOES NOT INVOLVE AN ACTIVATION OF THE PITUITARY-INTERRENAL AXIS,BUT EVOKES ADJUSTMENTS IN BRANCHIAL ULTRASTRUCTURE

Two strains of rainbow trout (Oncorhynchus mykiss) were exposed to sof t water at pH 4.0 for 14 days, after ambient pH was reduced gradually. Several parameters, either indicators of acid stress, or reportedly i nvolved in the adaptive response to low pH, were monitored. No mortali ty occurred during the exposure period; feeding behavior, haematocrit, and plasma protein levels were not affected. A transient depression o f leucocrit was observed. A minor, but significant, hypochloremia and perturbations in plasma glucose levels occurred in acid-exposed fish f rom one strain only. There was no evidence of activation of the pituit ary-interrenal axis in acid-exposed fish. Baseline plasma ACTH and cor tisol levels were indistinguishable from those of control fish, and th ere was no evidence of sensitization to additional stress in acid-expo sed fish. In vitro baseline and ACTH-stimulated cortisol secretion was not significantly different in the two groups. Ultrastructural eviden ce indicated an increased turnover rate of chloride cells and leucocyt e infiltration in gills of acid-exposed fish. These results suggest th at interrenal activation and catastrophic ion loss are not inevitable consequences of exposure of rainbow trout to pH 4.0 and that ultrastru ctural changes in the gills indicate locally regulated adaptive mechan isms.