SUBACUTE TOXICITY OF AMMONIA TO ATLANTIC SALMON (SALMO-SALAR L) IN SEAWATER - EFFECTS ON WATER AND SALT BALANCE, PLASMA-CORTISOL AND PLASMA AMMONIA LEVELS

Atlantic salmon (Salmo salar L.) postsmolts weighing about 300 g were exposed to two replicates of six ammonia levels in running seawater at 4-9 degrees C, 34-35 parts per thousand salinity, pH 7.7-7.8 and 5-9 mg/l O-2. Mean water ammonia levels ranged from 0.07 (control tanks) to 12.84 mg/l TA-N (< 1-100 mu g/l NH3-N), Blood and skeletal muscle tissue samples were taken after 2 and 5 weeks of exposure, and plasma osmolality, Na+, Cl-, Ca2+, Mg2+, TA (total ammonia) and cortisol levels and muscle tissue water content measured. Plasma cortisol was significantly increased at all water ammonia levels above control after 2 weeks of exposure, but the cortisol levels were low and did not increase with increasing water ammonia level. After 5 weeks of exposure, plasma cortisol was only significantly increased in fish from two single tanks with low and intermediate ammonia levels. No effects were found on muscle tissue water content or plasma Na+ or Mg2+ levels. Plasma osmolality and Cl- levels were significantly increased with a LOEC (Lowest Observed Effect Concentration) of 10.59 mg/l TA-N (81 mu g/l NH3-N) after 2 weeks and 12.84 mg/l TA-N (100 mu g/l NH3-N) after 5 weeks of exposure. The increases in plasma osmolality and Cl- were small, however, and all values were within the normal range. Plasma TA levels increased linearly with increasing water TA level, and the LOEC was 3.59 mg/l TA-N (28 mu g/l NH3-N) after 2 weeks and 0.82 mg/l TA-N (6 mu g/l NH3-N) after 5 weeks of exposure.