The present study was conducted in order to study the behavioral changes and histopathology of Caspian roach (Rutilus rutilus caspicus) and rainbow trout (Oncorhynchus mykiss) exposed to zinc sulfate in a 96-hour poisoning test. Experiments were carried out in the fisheries laboratory of Gonbad kavous University during 2020 to 2021. Caspian roach fish fry with an average weight of 2.5 ± 0.27 g and rainbow trout with an average weight of 6.18 ± 1.31 g were used for the experiments. In order to determine the behavioral and pathological changes, 13 juveniles of Caspian roach and rainbow trout were exposed to zinc metal concentrations of 0, 10, 30, 50, 70, 80, 90 and 23, 21, 18, 15, 10, 5, 0 mg/l, respectively. One group was considered as a control. Zinc sulfate salt was used as a source of zinc metal. The test tanks were aerated for 24 hours so that the amount of dissolved oxygen remained at saturation level. Samples of gill tissue, kidney and liver were collected and evaluated. Complete mortality (100%) was obtained at concentrations of 90 and 21 mg/l for Caspian roach and rainbow trout, respectively. The results of the experiment showed that the lowest concentration that caused losses in fish (LOEC) was 5 and 30 mg/l for rainbow trout and Caspian roach in 96 hours respectively. Also, the results showed that at a concentration of 10 mg/l of zinc sulfate, no losses were observed for the Caspian roach (NOEC), however at the same concentration (10 mg/l), 38.46% of losses occurred for rainbow trout. Based on the probit, LC50, 96h for Caspian roach and rainbow trout were 51.354 and 11.54 mg/l, respectively. The external observations of the fry in the presence of zinc generally included vertical and downward swimming, loss of balance, nervous convulsions, and staying at the bottom of the tank. Finally, the fish died with their mouths and caps open. In pathological examinations of gill organs, kidney and liver, the most lesions were bleeding, hyperplasia, hyperplasia and necrosis of gill covering cells, destruction of renal ducts, expansion of Bowman's capsule and necrosis of liver cells. However, the experiment showed that the presence of zinc in the water can increase the mortality rate of fish in contact with zinc. In general, the results of such tests will be very useful for ecological risk assessment.
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