Secondary Stress Responses in Juvenile Brazilian Flounder, Paralichthys orbignyanus, throughout and after Exposure to Sublethal Levels of Ammonia and Nitrite

This study investigated the secondary stress responses of Paralichthys orbignyanus exposed to ammonia and nitrite and after recovery. Fish were exposed to 0.12, 0.28, and 0.57 mg NH3-N/L, or 5.72, 10.43, and 15.27 mg NO2-N/L for 10 d followed by the same time length for recovery. Ammonia- and nitrite-free water was used as a control treatment. Blood samples were collected after 1, 5, and 10 d of exposure and after recovery. Fish exposed to ammonia presented lower and higher glucose levels after 10 d of exposure and recovery, respectively. Ammonia induced initial and transient ionic disturbances and metabolic alkalosis. Nitrite exposure caused hyperglycemia, increased plasma K+ levels, and respiratory alkalosis, whereas metabolic acidosis was observed after recovery. Increased proportion of monocytes and/or granulocytes and reduced number of lymphocytes were demonstrated in fish exposed to 0.28 mg NH3-N/L (Day 1) and 10.43 mg NO2-N/L (Day 5) and after recovery in the 0.28 and 0.57 mg NH3-N/L treatments. Exposure to ammonia decreased the proportion of granulocytes on Day 5. In conclusion, exposure to concentrations at 0.12 mg NH3-N/L and 5.72 mg NO2-N/L provoked physiological disorders in Brazilian flounder. Nonetheless, fish exposed to 5.72 mg NO2-N/L following a 10-d recovery period showed complete resumption of homeostasis.