Effects of acid and nitrate environment on juveniles of Cobia Rachycentron canadum
The present study evaluated the acute and chronic effect of the acid and nitrate environment in juveniles of the bijupira Rachycentron canadum. The median lethal concentration (LC50-96h) and the histopathological effects of nitrate in juveniles of bijupirá were determined. The LC50-96 h was estimated at 1829 mg L-1 NO3 −- N. Acute nitrate exposure induced histopathological changes in the gills, esophagus and brain of the bijupira. A second study was carried out to evaluate the acute effect (24 h) of the acidic environment (pH = 7.9 (control), pH 6.5, 6.0 and 5.5) on the physiological and histological parameters of the jewelery. The acidic environment induced a blood acidosis according to the pH reduction, which culminated in a reduction in the bicarbonate concentrations and oxygen saturation in the fish's blood.In response, an increase in the hematocrit and hemoglobin and glucose concentrations in the blood of the fish was observed compared to the control treatment. The acidic environment negatively affected the histology of the gills and the skin, and the data were more severe according to the pH reduction. Hyperplasia with complete fusion of the secondary lamellae were observed for all pH reduction treatments, while telangiectasia and proliferation of chloride cells were observed in fish exposed to pH 6.0 and 5.5. On the skin, hyperplasia and hypertrophy of mucous cells were observed, with the presence of focal necrosis of these in fish exposed to pHs 6.0 and 5.5. The third study evaluated the possibility of reducing the pH to 6.5,the effect of high concentrations of nitrate and the interaction between both water quality parameters during 42 days on survival, growth, physiological and histological parameters of bijupirá. Bijupirá juveniles were exposed to nitrate concentrations of 0, 100 and 200 mg/L NO3 - N at pH 6.5 or 8 and fed until satiety. Two other treatments evaluated the growth ratio due to fish food consumption at low pH or high nitrate value. These two groups were maintained at pH 8/0 mg/L NO3 - N. One group was fed the same amount of food consumed by fish maintained at pH 8/200 mg/L NO3 - N. The other group was fed the same amount of food consumed by fish exposed to pH 6.5/0 mg/L NO3 - N.The survival of bijupirá was reduced by 10% only in fish exposed to pH 6.5/200 mg/L NO3 - N. Growth was affected by nitrate, but not by pH. Chronic exposure to nitrate significantly reduced weight gain, specific growth rate and condition factor, as well as increased fish feed conversion. Food consumption was generally reduced among fish exposed to nitrate. Plasma glucose levels were reduced in fish exposed to nitrate and pH 6.5. Lactate and lysozyme concentrations were reduced in fish exposed to nitrate at both pH levels. However, there was no influence of pH and nitrate on plasma cortisol and osmolarity concentrations. Bijupirás exposed to nitrate presented hyperplasia and telangiectasis in the secondary lamellae.In this way we can conclude that the bijupira can be created at pH 6.5 with reduced nitrate values. However, at this pH, special care should be taken with high concentrations of nitrate. In addition, the concentration equal to or greater than 100 mg/L NO3 - N impairs the growth of the bijupira regardless of the pH values.