The effects of softwater acclimation on gill morphology and respiratory gas transfer in the rainbow trout, Oncorhynchus mykiss.

Exposure of rainbow trout, Oncorhynchus mykiss, to ion-poor water or softwater elicits several morphological and physiological adaptations. This thesis examines these changes. The effects of a naturally-induced chloride cell proliferation on gill morphology was investigated (Chapter 2) by acclimating fish to softwater ((Na$\sp+\rbrack$ = 0.055 mmol l$\sp{-1};$ (Cl$\sp-\rbrack$ = 0.029 mmol l$\sp{-1};$ (Ca$\sp{2+}\rbrack$ = 0.059 mmol l$\sp{-1};$ (K$\sp+\rbrack$ = 0.007 mmol$\sp{-1}).$ Following 1, 2, and 4 weeks exposure to softwater, the results of scanning electron microscopy revealed a doubling of the chloride cell fractional area (CCFA: percentage of gill epithelium surface covered by chloride cells) due to both an increase in the number of chloride cells and the apical exposed surface area of individual chloride cells. During normoxia, ventilation frequency and opercular displacement were significantly higher in the softwater trout; opercular displacement was similar in both groups. PaCO$\sb2$ and plasma HCO$\sb3\sp-$ concentrations were significantly lower in the softwater fish and the blood acid-base status was characterized by a mixed respiratory alkalosis and metabolic acidosis such that blood pH was not statistically different in the 2 groups. During hypoxia the ventilation frequency and amplitude increased in the control trout, whereas only the ventilation amplitude increased in the softwater acclimated fish. The rate of PaO$\sb2$ reduction during hypoxia was significantly greater in the softwater fish and at the most severe level of hypoxia PaO$\sb2$ was significantly lower in the softwater fish. This thesis demonstrated that respiratory gas transfer is impaired by chloride cell proliferation during hypoxia and that hyperventilation is a compensatory physiological adjustment for softwater fish. (Abstract shortened by UMI.