Abstract
Turbot juveniles were exposed to four ammonia concentrations [0·17 (L), 0·34 (M), 0·73 (MH) and 0·88 (H) mg l −1 NH 3 ‐N] for different exposure durations (28 days minimum to 84 days). Their physiological status and growth performances were compared to a control group [0·004 (C) mg l −1 NH 3 ‐N]. No growth was observed in the H group, and by day 57, mass increase in the MH group was only 15% of that in group C. During the first month growth in the L group was similar to that in control group while it was lower (33%) in the M group; afterwards the L and M groups had a similar growth (half that of controls). Accumulation of total ammonia nitrogen (TA‐N) in plasma was dependent on ambient ammonia concentrations. Plasma urea levels in ammonia‐exposed fish were lower, similar or greater than in controls (depending on ammonia concentration or exposure duration). Osmolarity, Cl – and Na + plasma concentrations were stable in the L and M groups. The increases in Na + , Cl – , K + and total Ca concentrations observed by the end of the experiment in the H and MH groups suggest that fish failed to adapt. There was an initial rise in plasma cortisol in all ammonia‐exposed groups followed by a return to basal level (1·7–4 ng ml −1 ) in the L and M groups. In group MH, plasma cortisol peaked at 42 ng ml −1 by day 14, and after a decline at c . 1 month (14 ng ml −1 ), it rose again.