Gill ventilation frequency and mortality of Atlantic salmon (Salmo salar L.) exposed to high ammonia levels in seawater

Atlantic salmon (Salmo salar L.) postmolts weighing 430 ± 73 g were exposed to six different water ammonia levels ranging from about 3 (control) to 425 mg/l TA-N (total ammonia-N) (0.01–2.78 mg/l NH3-N) in aerated seawater at mean levels of 7.2–7.4 mg/l O2, 6.4–6.6°C, 34–35‰ salinity and pH 7.5–7.6 in two replicate static 48 h tests. Likely due to CO2 excretion by the fish, water pH and NH3-N levels generally decreased with time during the tests. During these conditions, 48 h-LC50s of 59.4 mg/l TA-N (0.34 mg/l NH3-N) and 58.6 mg/l TA-N (0.24 mg/l NH3-N) were found. Gill ventilation frequency was slightly, but significantly increased at exposure to 30.0–56.2 mg/l TA-N (0.14–0.30 mg/l NH3-N) in both tests. At higher water ammonia levels, ventilation frequency was not measured due to convulsions and panic-like bursts of swimming of the acutely intoxicated fish, and a rapid onset of a mortality finally reaching 100%.