Nitrogenous Waste Excretion, Acid-Base Regulation, and lonoregulation in Rainbow Trout (Oncorhynchus mykiss) Exposed to Extremely Alkaline Water Author(s): Michael P. Wilkie and Chris M. Wood

Rainbow trout (Oncorhynchus mykiss) survived in alkaline fresh water (pH = 9.50)for 72 h, although the exposure rendered thefish more susceptible to mortality from other causes. At pH 9.50 ammonia excretion (Jm,,) was initially blocked, and total plasma ammonia levels increased. However, Jamm steadily recovered thereafter; by 48 h control rates were reestablished and plasma total ammonia stabilized at six times the control level. The initial blockade of Jamm was associated with a reversal of the blood to bulk water PNH3 gradient. Paradoxically, the continued depression ofJAmm until 48 h occurred despite the presence offavorable blood-to-water gradients for passive NH3 and NH) difusion. An increase in urea excretion helped sustain waste N excretion in the face of inhibited JAmm. A respiratory alkalosis (decreased arterial Pco2, increased arterial pH) occurred initially but was partially counteracted by a metabolic acidosis (decreased plasma HCO0), which stabilized arterial pH at about 8 0 throughout the exposure. Increases in blood lactate, without marked changes in arterial 02 tension, suggested that an activation of glycolysis occurred that was not caused by hypoxemia. Plasma Na+ and Cllevels decreased by about 7% during the first 24 h of exposure but stabilized thereafter.