Respiratory profile and gill histopathology of Carassius auratus exposed to different salinity concentrations

The aim of this study was to evaluate the chronic salinity tolerance of Carassius auratus and the effects on blood parameters, gill morphology, and survival. In the first test, nine different concentrations (0.0, 0.5, 1.0, 2.5, 5.0, 10, 15, 20, and 25 g L-1) of NaCl were used with nine repetitions for 96 h. The survival of fish subjected to 15 g L-1 NaCl was 4 h, and 5 min at a concentration of 25 g L-1. The mortality of fish with 15 g L-1 NaCl was 100%. Morphological analyses of the gills showed hyperplasia of the coated cells in the interlamellar space and hypersecretion of mucus in fish exposed to 10 g L-1 of NaCl. At concentrations of 20 and 25 g L-1, necrosis of the support collagen caused the cells to detach from the lamellar structure mucosa. In the chronic test, two concentrations were used, with four replications containing nine fish in each aquarium for a period of 21 days. Blood samples and gills from the fish were collected, and it was observed that the fish showed a decrease in the concentration of bicarbonate (NaHCO3) in the blood, indicating hypernatremia. Acute exposure of C. auratus to sodium chloride (NaCl) should be at a maximum of 10 g L-1 of NaCl, after which level there would be a loss in animal performance and/or mortality. Chronic exposure to 5 g L-1 of NaCl promotes acidemia, ionic imbalance, and pathological changes in the gills; therefore, it is not recommended.