Body ammonia content of rainbow trout alevins was about 0.6 micromole g-1 but increased to 4 micromole g-1 after 24 h of exposure to an external ammonia concentration of 36.2 micromolar NH3 (15.8 mmol l-1 ammonia) at pH 7 and 10°C. During ammonia loading, the mass of alevins remained unchanged, but body ion concentrations decreased by about 28 % for Na+ and Cl- and by 35 % for K+. These effects were reduced at lower ammonia concentrations. Exposure for 24 h to 36.2 micromolar NH3 (15.8 mmol l-1 ammonia) at pH 7 and 10°C resulted in a build up of body ammonia that was almost complete within 10 h, whereas Na+ loss from the body was delayed and commenced after about 5 h of exposure. After exposure, ammonia unloading from the body was complete in about 10 h but there was a delay of about 5 h before Na+ uptake commenced. During ammonia exposure, alevins lost substantial amounts of K+ (14 micromole g-1) that were not replaced for several days after exposure to ammonia. Ammonia exposure has major effects on ionic regulation in juvenile fish and possible regulatory processes are discussed.

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