Ammonia and nitrite toxicity in Paralichthys orbignyanus sole
Paralichthys orbignyanus sole is a species that has great potential for intensive aquaculture, where the accumulation of nitrogen compounds can be one of the main limiting production. Ammonia and nitrite are the most toxic among these compounds, being able to induce several physiological disturbances in fish, even if at sublethal levels. Therefore, the present thesis aimed to determine the sublethal effects of exposure to ammonia and nitrite, and subsequent recovery in juveniles of sole using blood parameters, oxidative stress and histopathology as biomarkers. Two experiments were carried out, in which the fish were exposed to three concentrations of ammonia (0.12; 0.28 and 0.57 mg NH3-NL-1) or nitrite (5.72; 10.43 and 15.27 mg NO2-NL-1), in addition to a control treatment, for 10 days.Then the animals were kept for another 10 days in free water or with minimal concentrations of ammonia (0.00 mg NH3-NL-1) or nitrite (0.05 mg NO2-NL-1) to evaluate recovery. Samples of blood, gills, liver, muscle and brain were collected from 9 fish per treatment after 1, 5 and 10 days of exposure, and at the end of recovery. The results demonstrate that the effects attributed to the toxic action of ammonia and nitrite in juveniles of sole were influenced by the time of exposure and concentration, and even in the lowest tested concentrations of both compounds, there was a disturbance of homeostasis of some of the parameters. The analysis of blood parameters (glucose, Na +, K +, Ca ++, HCO3−, pCO2, pH, leukocyte profile) during exposure to ammonia and nitrite indicated different metabolic, osmoregulatory disorders,acid-base and the immune response of fish. The induction of a pro-oxidant condition in the gills, liver and muscle was also demonstrated as one of the mechanisms of toxicity of these compounds for the species. The inhibition of the antioxidant system (reduction of the total antioxidant capacity and the activity of the antioxidant enzyme glutathione-S-transferase) and / or increased levels of oxidative damage in lipid and proteins demonstrate this effect. In addition, the activation of different enzymatic and non-enzymatic antioxidant defenses was observed, indicating an adaptive mechanism to combat or minimize oxidative stress. Histopathological changes in the brain, liver and gills of fish resulting from exposure to ammonia or nitrite were not observed. After the recovery period, only the fish submitted to treatment 5.72 mg NO2-NL-1 showed all blood parameters and oxidative stress within baseline values. For the other treatments of the experiment with nitrite and in fish exposed to ammonia, this restoration occurred only in part of the parameters. As a conclusion, concentrations from 0.12 mg NH3-NL-1 and 5.72 mg NO2-NL-1 cause several physiological changes in juvenile sole, therefore they should be avoided in production systems. The 10-day recovery period was able to lead to partial or total restoration of the fish's physiological condition, depending on the compound and concentration to which they were exposed. Complete recovery was demonstrated only in fish exposed to 5.72 mg NO2-NL-1.