Effects of sodium chloride and methylthioninium chloride on Persian sturgeon, Acipenser persicus (Borodin, 1897): A histopathological and bacteriological study

360 pieces of Persian sturgeon fingerlings (1.94 ± 0.75 g) were exposed to various sublethal doses of disinfecting chemicals commonly used in aquaculture such as sodium chloride and methylthioninium chloride in order to investigate their impacts on bacterial loads of skin, gill and surrounding water and to define the histopathological status of gill and liver tissues. The sublethal concentrations were determined after a pre-test, then the experiment was performed  in four treatments with three replicates inside the glass aquariums with stocking density of 10 fish (1-3 g) per aquarium. The treatments included 5, 6.3, 8.06 and 10.23 mg L-1 </sup>sodium chloride and  also 1, 1.56, 2.45 and 3.83 mg L-1</sup> methylthioninium chloride. A control group (with no chemical added) was set up for each experiment. After exposure to treatments during 96 h, the microbial and histopathological examinations were carried out. Hemorrhage, elongation of secondary lamellae, adhesion of secondary lamellae, hypertrophy of supporter cartilage, mucus coagulation and secretion, hyperplasia, lamellar necrosis and clubbing of gill lamellae were observed on the fish gills. Biliary depression, hemorrhage, cell necrosis and degeneration of lipid were also found in liver. The severity of these effects was  dose-dependent. Only the sublethal concentration of methylthioninium chloride significantly influenced bacterial load (CFU g-1</sup>) on skin, while other treatments of sodium chloride and methylthioninium chloride did not exhibit any significant effects. In conclusion, the sublethal doses of methylthioninium chloride and sodium chloride showed no obvious disinfecting effect on gill, skin and surrounding water of Persian sturgeon fingerling. Nevertheless, histopathological alterations were observed on fish gill, skin and also  liver of all treatments.