DIVALENT-CATIONS ENHANCE AMMONIA EXCRETION IN LAHONTAN CUTTHROAT TROUT IN HIGHLY ALKALINE WATER

The Lahontan cutthroat trout lives under highly alkaline and saline co nditions in Pyramid Lake, Nevada (pH 9.4; 0.2 mmol 1(-1) Ca++; 7.3 mmo l 1(-1) Mg++). These experiments were conducted to study the possible roles of water Ca++ and Mg++ concentrations on ammonia excretion in th e Lahontan cutthroat trout under highly alkaline conditions. The basic protocol of the experiments was to determine ammonia excretion rates during the following three exposure periods (each of 3-h duration) in sequence: (a) in normal lake water; (b) in soft lake water with the di valent cation concentrations reduced; and (c) in the soft lake water w ith either Ca++ or Mg++ (or no divalent cations added) added back al t he appropriate lake water concentration. The soft-water exposure cause d a significant reduction in ammonia excretion to about half of the co ntrol (original lake water) levels. When either Ca++ or Mg++ was added to the soft water in the third exposure period, the ammonia excretion rates were increased more than twofold back to lake water levels. (C) 1997 The Fisheries Society of the British Isles.