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Survival, Acid-Base Regulation, Ion Regulation, and Ammonia Excretion in Rainbow Trout in Highly Alkaline Hard Water

Timothy Y. Yesaki and George K. Iwama
Physiological Zoology
Vol. 65, No. 4 (Jul. - Aug., 1992), pp. 763-787
Stable URL: http://www.jstor.org/stable/30158538
Page Count: 25
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Abstract

The hypothesis that water $Ca^{++}$ plays a significant protective role in the regulation of ionic, acid-base, and nitrogen balance of fish in highly alkaline environments was tested in two experiments. In each experiment, rainbow trout were exposed to water of pH 10.1 in soft water ([CaCO₃], 4 mg · L⁻¹) and hard water ([CaCO₃], 320 mg· L⁻¹). The experiments consisted of a chronic experiment in which the fish were exposed to those conditions for 196 h, and an acute 12-h experiment in which the fish were subjected to four protocols, each lasting 3 h. Acid-base parameters, and concentrations of blood ions, ammonia, and urea nitrogen, were measured in each experiment. Mortality and blood cortisol concentrations were measured only in the chronic experiment, while ammonia flux rates and the effects of amiloride addition to the water on these parameters were determined only in the acute experiment. While all the fish chronically exposed to alkaline soft water died and exhibited stress, as evidenced by elevated cortisol levels, fish in alkaline hard water experienced no mortalities and showed no signs of stress. Exposure to alkaline soft water caused severe blood alkalosis: maximum of +0.314 pH units above the fish in alkaline hard water. Compared to the corresponding levels in fish under control conditions, plasma $PCO_{2}$ and [HCO₃⁻] in both soft and hard water decreased by approximately 2 mmHg and 4 mmol · L⁻¹, respectively. While Na⁺ and Cl⁻ concentrations both fell by about 25 mmol · L⁻¹ in alkaline soft water, these ion levels remained relatively constant in fish in hard water under alkaline conditions. Ammonia levels, in contrast, increased fivefold in the blood of fish exposed to alkaline soft water, whereas the ammonia levels of fish in alkaline hard water remained relatively constant after an initial doubling of resting values. The addition of amiloride nearly abolished ammonia excretion and caused blood ammonia levels to increase. These data suggest that the higher CaCO₃ levels in the water stimulated ammonia excretion through a Na⁺-related exchange mechanism, most likely on the branchial epithelium.

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