Ammonia, Na+, K+ and Cl− Levels in Rainbow Trout Yolk-Sac Fry in Response to External Ammonia

Body ammonia content of rainbow trout alevins was about 0.6μmol g−1 but increased to 4 μmol g−1 after 24h of exposure to an external ammonia concentration of 36.2 μmol l−1 NH3 (15.8mmol l−1 ammonia) at pH7 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 24h to 36.2 μmol l−1 NH3 (15.8mmol l−1 ammonia) at pH7 and 10°C resulted in a build up of body ammonia that was almost complete within 10h, whereas Na+ loss from the body was delayed and commenced after about 5h of exposure. After exposure, ammonia unloading from the body was complete in about 10h but there was a delay of about 5h before Na+ uptake commenced. During ammonia exposure, alevins lost substantial amounts of K+ (14 μmol 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.