Effects of chronic and periodic exposure to ammonia on growth, food conversion efficiency and blood physiology in juvenile Atlantic halibut (Hippoglossus hippoglossus L.)

The effects of chronic and periodic exposure to un-ionised ammonia (UIA) on growth and food conversion efficiency in juvenile Atlantic halibut, Hippoglossus hippoglossus were examined. Fish with a mean (SD) initial weight 51.7 (13.2) g, were exposed to five treatments consisting of a control group, three chronic groups (Chronic Low, Chronic Medium and Chronic High, ambient UIA levels of 0.06, 0.12 and 0.17 mg l−1, respectively), and one group exposed to a daily peak of ambient UIA, closely corresponding to the chronic high level (max level 0.17 mg l− 1 UIA, High Pulse). The fish were reared at 12.0 °C, pH 8.0, 34 g l− 1 salinity and about 80% oxygen saturation. The fish were reared under these conditions for 62 days, followed by a 38 day period where no ammonia was added. Chronic medium- and high levels of ambient ammonia had a deleterious effect on growth performance via appetite suppression, whereas the periodic ammonia treatment conducted in the present experiment had little or no effect on growth performance. No growth reduction was seen in the Chronic Low group, thus a NOEC value (No Observed Effect Concentration) of 0.06 mg l− 1 ambient UIA is proposed for juvenile Atlantic halibut. No indication of ion regulatory or acid base regulatory stress in the fish exposed to chronic or periodical peaks of ambient UIA was found despite minor differences in measured blood parameters.